Technical - Amplifier

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Nov 8, 2021
Technical - Amplifier
  • Contents


    Amplifiers potentially contain lethal voltages stored in the capacitors even when they are turned off and unplugged. Do not undertake work on the amp unless you understand the safety procedures required to work on an amp. If you aren't comfortable working inside the chassis, take it to a qualified technician. You don't want to be the recipient of a Darwin Award.

    B-15N Tube Layout (top)

    B-15N: V1 to V6 from right to left in the image below. The tube positions and names are shown on the schematic, the tube names are silk screened under the tube sockets on the back of the chassis.

    Position Tube Function
    V1 6SL7GT Channel 1 pre-amp
    V2 6SL7GT Channel 2 pre-amp
    V3 6SL7GT phase inverter
    V4 6L6GC power amp
    V5 6L6GC power amp
    V6 5AR4 power supply rectifier

    1964 B-15N V1 to V6 right to left.

    1964 Tube Chart that is inside the speaker cabinet.

    B-15N Circuit Layout (top)

    The various stages of the B-15N schematic are illustrated below. Channel 1 and Channel 2 consists of the pre-amp and tone control stages. This is the first stage where the low level signal from the instrument is amplified and filtered by the tone circuit. The channels are mixed together and fed in the phase inverter stage of the power amp. The phase inverter creates mirror image positive and negative signals which are fed to the power Tubes where the pre-amp signal is amplified. The power Tubes in this amp operate in a Class AB push-pull topology. Think of two lumberjacks cutting down a tree with a two-handed saw. One pushes while the other pulls. Each is doing a little more than half the work. In a Class AB push-pull amp, each tube is doing a little more than half the work, each tube amplifying either the positive or negative signal from the phase inverter. This is more efficient. The signal is amplified by the power tubes and passed to the Output transformer and speaker. The power supply takes the line voltage and converts it to a high voltage (B+) that is modulated by the other stages as part of the amplifying process. The power supply also has a circuit that generates the power needed by the tube heaters that make the electrons fly and the tubes function.

    1963 B-15NA circuit stages.

    Tone Circuit and PEC Modules (top)

    The vintage portaflex amps utilized Printed Electronic Circuit (PEC) modules in the tone circuit. The passive tone control network, on which the one used in these amps is based, was first proposed James [1] in 1949. Peter Baxandall [2] extended the design to an active circuit in 1952. He had better marketing people so today we tend to call it a Baxandall tone circuit.

    PEC modules were manufactured by Centralab. They were intended for applications were miniaturization was important. They were widely used in radio and TV sets at the time and were a time saver in manufacturing. The encapsulated modules consisted of a ceramic substrate that held ceramic capacitors and carbon composition resistors with circuit leads attached. The tolerance of the resistors were +/- 20%, 1/5 Watt. The capacitors tolerance was +50% and -20% in the 1000mmF range, and +/- 20% in the 100mmF range. The working voltage of the capacitors was 450 VDC, 800 VDC flash. These tolerances and the effects of time causes the components to drift out of specification, accounts for slight differences in the tone of one Portaflex amp to the next. It isn't unusual for the two channels on a B-15 to sound slightly different because of this, even though the circuit designs are the same. Players use this variation to their advantage.

    These tone modules were used in other Ampeg amps of that era such as the SVT and the V4B.

    B-15N tone circuit wiring.

    B-15 tone circuit schematic diagram with leads numbered from 1 to 7.

    PEC tone circuit module.

    PEC tone module schematic.

    Tone Circuit Replacement (top)

    The PEC modules are put together with carbon composition resistors and ceramic disk capacitors. With time, these components drift out of spec. When you measure the values of the components of a Vintage tone module, it isn't uncommon to find that they are either out of spec or borderline. This affects the tone of the amp. New old stock replacements can be found but because of their age, they can be equally out of spec. One solution is to install a replacement module with new parts. A new tone module restores clarity to the amp's tone.

    Below is a rendering of the top of the board, it is a two sided board, meaning that there are copper traces on both sides. There is also an image of the board next to a PEC module.

    The board can be ordered from: Oshpark. The bare board costs about $10 delivered anywhere in the world for three copies. The part numbers for the components are listed the site. Metal film resistors can be substituted for the carbon composition ones which can lower the noise floor. I used the carbon composition resistors because I thought that they sounded better. With film resistors a larger wattage such a 2W or higher can be quieter. Whatever selected, they have to fit on the board. Which type of resistor to go with is a subjective decision. The board is wired from pads one to seven to the amp with 22 gauge bus wire leads, Belden 8021 or equivalent.

    Tone module printed circuit board.

    Below is a replacement printed circuit board next to a PEC module. I wasn't concerned about duplicating the dimensions, I selected components that sounded good. The circuit board is designed so that the components can be mounted on either side of the tone module board. This allows for different mounting options depending on the amp.

    Vintage PEC Module (note the staggered lead lengths) and printed circuit tone board.

    The same tone module board can be used in other vintage Ampeg amps such as the SVT and V4B that use PEC modules. In some cases such as in the V4B, it is desirable to mount the board piggyback on top of another circuit board. I didn't feel comfortable with just the bus wire holding the board in palace so I added a hole for a 4-40 screw to act as a foot if necessary so that the board can't come in contact with something that it shouldn't and cause a short circuit. A nylon screw and nut work well because they also serve as insulators. I used a nylon hex standoff and screw Cut to the length that I needed. I used silicone goop to hold the standoff's base to the board. The nylon standoff that I use is made by Keystone, 1902B, the 4-40 nylon screw is made by Multicorp SPC13374. If the hex standoff isn't used, a nut can be used to lock the screw in place, Multicorp SPC21811. These parts are available from electronic parts suppliers such as Mouser and Newark. The nylon parts were sanded down to the needed length which depends on how long the bus wire leads are. Rather than use a standoff, a bead of silicone goop on the bottom of the tone module would help to ensure that the board doesn't come in contact with anything under it.

    As an example of how to install the board, here are some images of the tone module board in a V4B that illustrate how to attach the bus wire. The tone module is installed piggyback on the back of the V4B's tone board.

    First install bus wires in main circuit board. I used a small loop of wire (second image) to ensure a good contact on the pad on the Ampeg circuit board pad. This isn't needed on the other end at the tone module board. It has plated through holes and pads on each side so there is no problem making a good contact with the wire. Staggering the lengths of the leads helps when stuffing the wires into the tone module board, one goes in at a time. You can see in the image above of the PEC module that Ampeg did this when installing the board.

    The same applies to an later B-15N installation where the board is going onto a circuit board. On the early vintage B-15N, the module is connected on the back of the tone pots. The bus wires would simply be soldered to the board and wired.

    Connecting wires installed on the amp's circuit board.

    Wire loop used to ensure a better contact with the circuit board's pad.

    Here the board is tilted up to illustrate how it goes together. Note the silicone goop that holds the hex standoff onto the board. I used a bit too much.

    Bus wires soldered to tone board.

    The installed board. I trimmed the goop a bit afterward. The leg prevents the board from bending down and coming into contact with the board below. A bit of goop on the bottom of the board would do the same thing.

    Tone module installed in V4B amplifier.

    Tube vs Solid State Rectification (top)

    The vintage tube based Portaflex amps all, except for the B-15NA and NB, have a 5AR4 tube rectifier.

    The rectifier is part of the amp's power supply. Its purpose is to convert the high voltage AC to DC. Although the rectifier is not in the instrument's signal path, it does affect how the amp sounds. The 5AR4/GZ34 is a Full wave rectifier capable of delivering DC current of up to 250mA to the reservoir capacitors in the power supply. It requires a heater voltage of 5 VAC and consumes around 1.9A of current from the power transformer. One of the advantages of this particular tube is a slow turn-on at power up. When you turn on the amp, this allows the capacitors to more slowly ramp up to a fully charged state. This helps extend their service life.

    We've all heard of sag in an amp when it is pushed. Sag is a voltage drop that can occur in the power supply, the rectifier, and the power supply capacitors. Think of the 5AR4 as being made up of two diodes tied together at one end and are connected to a variable resistor. As the current being drawn through the tube increases, the resistance within the tube increases which results in an associated voltage drop. This lowers the amp's high voltage power supply, often called B+. This is partly responsible for the famous sag that tube rectified amps are known for. Decreasing the B+ voltage shifts the operating points of the tubes and makes them perform differently. One consequence is decreased headroom, more Distortion, and increased compression (which is also a form of Distortion). The amp becomes dynamic in this way which some players love. As I mentioned, the transformer and capacitors also have their own sag and contribute to the performance of the amp as well.

    Not all tube amps have tube rectifiers, some employ Solid state rectification. Most higher power tube amps have solid state rectifiers. This is because these amps have a higher current demand, beyond what tube rectifiers can supply. Some amps use multiple tube rectifiers to help boost the current capacity of the power supply but they are limited in what they can deliver when you balance available real estate and tube heater power supply capacity. Having a solid state rectifier does not mean that the amp is less tube sounding, it is just provides a different type of performance.

    For some amps with tube rectifiers, it is possible to install solid state Full wave rectifier modules in place of the rectifier tube. This increases the available voltage within the amp over that with a tube rectifier, provides more headroom and less distortion when the amp is pushed. It's nice to have this option available should you desire it. A side benefit of solid state rectifier modules is that they don't draw heater current from the power transformer like the tube rectifier does. In the case of the 5AR4 tube rectifier, the 5 volt heater draws 1.9A or (5V X 1.9A = 9.5W) 9.5 Watts. This allows the power transformer to run a little cooler which is a good thing. The disadvantage of a solid state module used in place of the rectifier tube is that the higher voltages can put a strain on the power supply capacitors. At idle, with a line voltage of 117 VAC, with a NOS 5AR4 installed in the B-15N, I've measured that the rectified B+ voltage drops by about 22 volts. The voltage drop with a solid state module installed is minimal so the B+ of the amp is increased over that with the tube rectifier in place. This hasn't proven to be a problem with Portaflex amps provided that the capacitors are healthy. If you are in any doubt, consult with your tech.

    Most solid state replacement modules are encased diodes mounted on an octal socket. Weber's Copper Cap Rectifiers are interesting in that some models are designed to emulate the function of a real rectifier tube. The WZ34, which is a 5AR4 replacement, has a built in thermistor, an in-rush current limiter which ramps up the voltage at turn-on to protect the power supply capacitors, as well a resistor to emulate the voltage drop of the tube rectifier. The advantage of the WZ34 is that it is a cost effective alternative that emulates a 5AR4 by providing sag and slow turn on without taxing the power transformer. Does it function exactly like a real tube rectifier? No, but is does approximate the function. The ramp-up time of the in-rush current limiter is shorter than that of a 5AR4. Still, it offers some Capacitor protection. If you want to avoid tube rectifier sag, the WS1 rectifier module has the diodes and the thermistor but it doesn't have the dropping resistor. As a result of less sag, the power supply is a bit stiffer when the amp is pushed. I've used both the WS1 and the WZ34 in my B-15N and they performed well. At the very least, having one on hand is good insurance if your tube rectifier needs to be replaced in a pinch.

    Plug-in solid state rectifier module that substitutes for a tube rectifier.

    Typical solid state rectifier module schematic.

    How Do I Discharge the Capacitors In My Amp Before I Work On It? (top)

    The electrolytic capacitors in an amp can retain their charge even after it is turned off and unplugged. These capacitors need to be drained or discharged prior to performing any work. They are found in the power supply and, in some amps, power tube bias circuit. Electrolytic capacitors are polarized and are identified by either a (+) or a (-) adjacent to a lead. The cylindrical cans mounted on top of the chassis are multi-section electrolytic capacitors. Plus and minus are indicated but, in some cases, the can itself serves as the negative lead. This is the case with the Capacitor below from CE Manufacturing, which are commonly used in vintage amps.

    Electrolytic Capacitors

    Multi-section capacitor can.

    Sprague Atom capacitors such as these are used in many amps.

    Some amps are designed to discharge on their own over time when the power is turned off. Not all amps do this and can retain a charge in their capacitors over an extended period of time. Portaflex amps are in the latter group. It is best to always assume that the capacitors are charged and need to be discharged before working on the amp. An easy first step to discharge the caps is to hit a low E note on your bass and turn off the power. Don't put the amp in standby first. You will hear the note fade out and distort. Why play a low E note? The lower notes put more of a demand on the power supply which in turn drains the caps more. This won't drain the caps completely but will make it safer to perform the next step which is using a discharge tool. A discharge tool, also commonly called a discharge stick, is connected across the capacitor leads and allows it to slowly discharge through a resistor. They are easy to build but also inexpensive to buy. It can be as simple as a wire with an alligator clip at one end connected to a resistor with another alligator clip at the other end. Below is a Snuffer Stick available from Weber. Built into the handle, are two 180K Ω, 2 watt resistors that are connected in Series with the nose probe and ground wire. The handle is an insulator and makes doing the work safer.

    Weber Snuffer Stick for discharging capacitors.

    Unplug the amp's power cord. One end of the discharge stick is clipped onto the chassis which is a ground. The other end is held against the (+) lead of the capacitor. The tool provides a path for current to flow to ground which discharges the capacitor. The size of the resistor in the tool dictates how fast this occurs, usually a few seconds. To be safe, wait 30 seconds. The number of seconds needed to discharge a capacitor can be calculated if you know the resistance, the capacitance, and the voltage across the capacitor and then apply the RC time constant formula. The larger the resistance, the slower the discharge, the safer the process. Without the resistor the discharge would be fast and sparks would fly. There is enough energy stored in a capacitor to melt a steel screwdriver so be careful. Discharge all the electrolytic capacitors in this way. Don't forget the electrolytic capacitor(s) in the power tube bias circuit if the amp is fixed bias and any others that may be in the amp. These are smaller sized capacitors relative to the ones in the power supply and are easy to overlook. In a fixed bias amp, the bias capacitor has the (+) side of the capacitor connected to ground. This is opposite to what is seen in the power supply. So connect the discharge tool from the (-) side of the capacitor and ground.

    After discharging the amp, set a voltmeter to read DC volts (on the most sensitive scale if the meter is not auto ranging), connect the volt meter in the same way that the discharge tool was connected, and test across each capacitor to ensure that they read zero volts. If not, repeat the discharge process. When all the capacitors are fully discharged, the amp is safe to work on.

    The B-15N Transformer Set (top)

    One of the attractive features of the B-15 design is the distinctive potted power and Output transformers that sit atop the chassis. The main purpose of a potted transformer in a metal case is to dissipate heat more efficiently than an Open frame transformer can and to reduce noise by shielding the amp from electromagnet fields generated by the transformer. At the time that the B15 was designed, potted transformers were seen in military, aviation, amateur radio, and high end Hi-Fi (high fidelity) equipment. Potted transformers are still widely used today in electronic equipment. In musical instrument amplifiers, potted transformers are considered an expensive option but they can they can be seen in the current Ampeg B-15 amplifiers as well as in some applications that use toroidal transformers.

    Vintage B-15N Transformer Specifications and Color Codes (top)

    The early 25W B-15 amps had a model PT-108 power and OT-208 transformers. The later 30W models had a OT-214 output transformer.

    The PT-108 specs are primary: 117VAC 50/60Hz, secondary: [email protected], [email protected], [email protected] The measured DC resistance of one half of the PT secondary high voltage winding is 55.5 ohms. The measured DC resistance of the primary winding where the line power connects is 2.5 ohms.

    There are a number of options available when choosing a replacement power transformer. In North America, ideally you want a 120VAC or 125VAC 60Hz primary; the secondary should be rated with at least the same current capacities but the voltages should be the same. Of course, the transformer should be one that fits in the B-15 transformer can and lends itself to potting. Some manufacturers do not recommend that their transformers be potted.

    The OT-214 output transformer has the following specs: primary 6.5K impedance, DC resistance 290 ohms, secondary: 8 ohm tap, 1 ohm DC resistance, 16 ohm tap, 1.4 ohms DC resistance. An OT-214 transformer is a suitable replacement for the older OT-208. The specs for the OT-208 are similar but with a lower wattage rating.

    As a general rule, paired wires that carry AC should be twisted together to help reduce induced noise. When installing a transformer, the power supply wire pairs, black, yellow, red, and green should be twisted together and run along the chassis. If they have to cross any wires carrying DC, the wires should be at ninety degrees to each other and separated from each other by lifting the DC wires over the AC ones. Again, this helps to reduce noise.

    B-15N power and output transformer color codes

    Repotting A B-15N Transformer (top)

    The early B-15's used a tar based compound to pot their transformers. It had a lot in common with the smelly roofing tar that is used by builders. Today, modern alternatives that are polyurethane, silicone, and epoxy based are used for potting, although tar and wax based formulations are still available. The advantage of using a tar or wax compounds is that if you apply enough heat, they will melt. This allows for repotting if necessary. The modern potting compounds don't allow this. Unless you machine out the old transformer from the can, you can't repot them.

    One of the services that Jess Oliver offered was transformer repotting. Basically, this requires heating the old transformer container in order to melt the tar, draining the tar and removing the transformer, removing the tar residue, if necessary, repainting the metal container, installing a new transformer and adding potting compound. It isn't as easy as it sounds. The procedure is labor intensive, messy, and because of the fumes, can be hazardous to your health. In the past, look-alike replacement transformers were not available so repotting was necessary if you wanted to preserve the look of your amp. Unless you are performing a restoration and want to preserve as much of the original amp as you can, given the cost of buying a new one, today it makes more sense to purchase a replacement.

    If a new transformer is needed, the easiest option is to buy a replacement, designed specifically for the B-15, from a company such as Fliptops or Mercury Magnets. Both offer excellent products, Fliptops uses Heyboer for their B-15 transformers, Mercury Magnetics is a transformer manufacturer. For those performing a restoration, both companies will repot a new transformer in a vintage Ampeg metal can if you supply them with one.

    Below is a Series of photos showing how Jess Oliver unpotted a B-15 transformer in his workshop. He adapted a toaster oven by adding a door on the top. He built a tool that allowed him to lower and raise the transformer into and out of the oven. The transformer was placed in the oven, bottom down, onto a pan. The tar would heat and liquify, running out into the pan. Eventually, the transformer would be released and fall into the pan as well. The tool would then be used to remove the Hot can. It is important to remove as much of the tar as you can while it is Hot. When the tar hardens, the only way to remove it is to grind or scrape it out or use a solvent to dissolve it. With a solvent you run the risk of removing the paint on the can which is more work if you have to repaint it. It is important to completely remove the tar if another type of potting compound is going to be used, otherwise it will not adhere to the metal can.






    As was mentioned above, although the original tar based compounds are available, there are a host of modern products available that not only are engineered to do a better job, but they are safer to work with. These include polyurethane, silicone, and epoxy based compounds. The problem with most of these products is that they aren't available in small affordable quantities. You would think that samples, sufficient to do one transformer, would be available but they seemingly aren't. They want to sell these products to manufacturers, not individuals. One option is the epoxy based 832 series of products by MG Chemicals, such as 832TC. They also offer a high temperature epoxy and silicone based potting compounds. High temperature waxes have long been used to pot coils in communications systems. Cary Audio Design, who sell audio products, uses a wax to pot their transformers as shown in this set of photos.

    Jess has said that he used a black wrinkle paint from Harley Davidson to paint the transformer cans. The replacement transformers use a tougher wrinkle paint that stands up better than what was used on the original cans which were prone to chipping. For comparison,here are images of an original and a replacement transformer. The original had a smoother surface as opposed to the highly textured surface on the the replacement can. The highly textured surface is an improvement in that it stands up better. I find that epoxy based paints Dry hard and stand up well to abuse. Some high temperature paints remain soft and don't fully harden which makes them damage easily. Look for a deep wrinkle paint with a hard durable finish such as VHT's Wrinkle Plus.

    When potting the transformer, wooden shims are used to position the transformer in the center of the can. The use of a vacuum chamber for potting is optional, although using one is recommended. A vacuum chamber allows the potting compound to penetrate as deep as possible into the transformer windings and to expel Air bubbles which would create local hot spots and impede efficient heat transfer away from the transformer. A localized hot spot could lead to a premature breakdown of the coatings on the transformer's coil windings and the possibility of a short or arcing within the transformer. A vacuum chamber can be built by attaching a vacuum pump to a pail with a Tight fitting lid or to a pressure cooker. The potting procedure and curing times are provided in the product literature.

    Connecting to a Speaker Cabinet(top)

    Speaker Connectors (top)

    The earliest Portaflex amps employed an octal speaker cabinet connector. The speaker cable had a female eight-pin plug and the speaker cabinet had a male mating connector mounted in a metal cup. The female plug was used on the cable to avoid any accidental contact with the dangerous voltages and currents carried in the speaker cable. Octal connectors like this were typically used in combo and hi-fi amps at the time, interconnecting pre-amps, power amps, and power supplies. Like on octal power tubes, there was an index pin in the center to ensure that the plug could only go in one way. With the introduction of the B-15NB, Ampeg switched to interlocking Amphenol four-pin connectors. Switchcraft C4M and A4F are the modern equivalents. With the octal cable, it didn't take much force to pull the speaker cable plug from the cabinet. The four-pin locking connectors were a step up and avoided accidental disconnections.

    The standard Ampeg speaker wire colors were green and black. Green was connected to the speaker "+" or dot, black was the return.

    Speaker Connectors

    Octal speaker connector wiring diagram.

    Octal cup with male connector on cabinet.

    Octal cable plug showing indexing pin.

    Octal connectors and speaker cable plugged into B-15N speaker cabinet.

    Four-pin connector circuit diagram.

    Four-pin Amphenol cabinet connector.

    Four-pin connectors on cabinet.

    Switchcraft A4F female cable connector.

    Switchcraft C4M male cabinet connector.

    Combo-Speakon Connectors(top)

    Some people modify their cabinets to convert them from the 4-pin connector to a ¼" jack. Others add a second connector so that they have both a 4-pin connector and a ¼" jack. Extension B-15 cabinets often have ¼" jacks on them.

    Neutrik makes a speakon connector with an integrated ¼" jack. This allows you to plug in either a speakon or a ¼" plug. For anyone wanting to add a speakon-combo connector to their B-15 cabinet, there are two circuit boards available that can be used with speakON Combo - Neutrik connectors. These connectors are printed circuit board mount only, they don't have screw terminals like other speakons have. The circuit boards allow you to more easily connect the wires from the speaker to the connector.

    The speakon combo connector as an alternative to a ¼" jack on an extension cabinet. The speakon combo allows for a little more versatility in that either the ¼" plug or the speakon can be used. Because the speakon connector is sealed, there are no issues with Air leaking through the connector.

    The four-pin connector used on the vintage B-15 cabinets requires a .75" hole, the speakon combo barrel is .937" so a larger mounting hole in the cabinet is required.

    The printed circuit boards are designed to be used with the Neutrik NLJ2MD-V that has vertical terminals: NLJ2MD-V Neutrik | Mouser. There are two boards, a Round one that matches the barrel size of the speakon combo connector, and a 1" square one.

    The Round board costs $4.10 for three copies delivered: OSH Park ~ Neutrik Combo Adapter Round.
    The square board costs $5 for three copies delivered: OSH Park ~ Neutrik Combo Adapter Square.
    The speakon combo connector must be purchased separately.

    The boards have two 1+ and two 1- pads. One set of pads goes to the speaker, 1+ to [+], 1- to [-]. The other set of pads are intended to be connected in Parallel to another speakon, 1+ to 1+, 1- to 1-. So you can have two combo speakon connectors on your cab if you wish to accommodate daisy chaining. This feature was added to the boards to make them more versatile in other applications. People don't daisy chain B-15 cabinets because the amp requires them to be 8 ohms.

    Both boards mount flush against the speakon. It is important that the wires be soldered to the boards before the boards are soldered to the speakon. On the round board, the soldered wires need to be trimmed flush with the back of the board so it will sit flat against the connector. These wires should be soldered first, before the board is soldered to the connector. On the square board, which is a bit larger, the solder pads clear the barrel which makes connecting the wire easier. The wires can go through the pad and there is space for it to wrap onto the pad, a better connection. You can see the pads in the attached images.

    The round boards were designed to be used with an Ampeg B-15N cabinet where the connector is mounted on the cabinet wall which is ¾" Thick plywood. The board and wires can be installed on the speakon and pushed through the mounting hole in the cabinet wall.

    If you have the option in terms of space, I recommend that you go with the square board. The square board will work with a B-15, it requires that the board is soldered to the connector inside the cabinet. The round board works better but the position of the wire soldering pads makes working with it more difficult.

    Round speakon circuit board.

    Square speakon circuit board.

    Power Amp Safety Circuit(top)

    If a speaker only requires two wires, why did they use a four conductor speaker cable? Ampeg designed a safety circuit which would disable the amp if it was turned on without the speaker cabinet properly connected, which could damage the amp. Two wires were used for the speaker, the other two were used for the safety interlock. This safety feature was used in other Ampeg amps that had four-pin speaker connectors, such as the the SVT. How the feature was implemented varied.

    In the early amps, the center tap on the high voltage (B+) winding was routed out to the speaker cabinet connector where, if the cable is plugged into the cabinet, is routed back to the amp to complete the B+ ground connection. This is dangerous in that if you are grounded and happened to touch the wrong pin, you could potentially have the high voltage pass though you. According to Jess, because of an objection from the regulators, Ampeg redesigned the safety circuit so that it was carrying a safer low voltage. The redesign involved disabling the power amp at the phase inverter. This was first seen in the B-15NC revision dated 3-65 (see the schematic section, compare the 2-65 and 3-65 wiring of the four-pin connector). Later they eliminated the four-conductor speaker cable and added a ¼" speaker jack on the the back of the chassis. A safety feature was still in place. They added a resistor across the output transformer that acted as a load and protected the amp in the event that the amp was turned on without without the cabinet connected. This is an elegant solution and it can be found in many modern designs. Note that this resistor is intended to protect the amp in the short term until you realize what you have done. It is not intended to be used as a dummy load. This last revision to the safety circuit was a welcome change. Since the amp now used standard 1/4" speaker cables, any eight ohm speaker cabinet could be used with the amp. Prior to that, adapter cables that converted from four-pin to 1/4" plugs were required.

    A common modification is to remove the safety circuit and the speaker cable and install a 1/4" jack on the chassis. Ampeg enventually did this with models that had the four-pin speaker connector. Basically, the safety circuit loop that goes out through the speaker cable is removed and replaced with a connection inside the chassis. I prefer using insulating washers, Switchcraft S1028 and S1029, to isolate the speaker jack from the chassis so that the ground return from the speaker is not on the chassis. The hole in the chassis needs to be slightly enlarged to fit the isolating washers. Avoiding using the chassis for grounds can help keep the amp quieter. Noise can come into play when multiple chassis grounds are used in the amp and they interfere with each other and signals on wires close to the chassis. Multiple chassis grounds can be used in an amp and, if they are done carefully, they don't create a problem. If an amp does not have the safety resistor across the output transformer, it is important to use a shorting jack for the speaker output, Switchcraft 12A. With the insulating washers, the 12A just fits on the chassis. Switcraft makes a jack with a longer barrel called the L12A that accommodates the washers better. This jack has a shunt or switch which can be wired to short the speaker output to ground when a cabinet is not plugged in. It's not as good as the safety load but better than an Open output if you turn the amp on by mistake.

    Switchcraft long threaded collar L12A jack. The shunt in contact with the tip can be seen at the right side of the image.

    The evolution of the various safety circuits used in the B-15 amps is outlined below.

    [*]B-15 Power Amp Safety Circuits

    1960 - 1965: This circuit was used with both the octal and four-pin connectors.

    1965: This safety circuit was introduced in the 3-65 schematic.

    1968: Safety resistor across the transformers output.

    In the 1960 - 1965 circuit, as shown in red, the ground path runs from the high voltage center tap, through the standby switch, to pin-2 of the connector in the speaker cable. When the speaker cable is plugged into the cabinet, a connection is made to close a loop, via a jumper wire on the speaker cabinet's connector, from pin-2 to pin-3. This completes the circuit from pin-3 to pin-4 and on to a ground point. As mentioned earlier, this puts a high voltage through the speaker connectors and there are safety issues as a result. If you are grounded and touch pin-2, the high voltage of the amp, depending on the amp over 450VDC, will go through you. It protected the amp but not the player. It's ironic that the safety circuit is unsafe!

    In the 1965 circuit, the safety circuit now serves to disable the phase inverter. This prevents signal from traveling to the power tubes and essentially shuts down the power amp. When point F in the schematic is lifted (not grounded), the phase inverter is disabled. When the speaker connectors are mated, pin-2 and pin-3 are shorted and the ground is completed.

    In the 1968 circuit, a 250 ohm / 10 Watt resistor is placed across the highest impedance tap on the output transformer to ground. If the speaker is not connected, the resistor acts as a load that, in the short term, prevents damage occurring. As was mentioned earlier, this resistor is not intended to serve as a dummy load. It is a 30W amp with a 10W resistor connected across the output. It won't last that long if the amp is turned up all the way. Why doesn't this resistor affect the impedance of the amp? If you place a large resistor in Parallel with a small resistor the combined resistance is about equal to the small resistance. In this case 16 ohms parallel to 250 ohms is about 15 ohms. Close enough. Since normally the 8 Ω tap is used with the speaker cabinet, the safety resistor doesn't come into play. This safety circuit is commonly found in modern amps, including Ampeg's reissue B-15 models.

    SB-12 Speaker Connection(top)

    Like the B-15, the SB-12 went through a number of speaker connection revisions. The connection evolved from a unique connection through the latch to a four-pin connector. Let's take a look at the latch connection.

    The earliest SB-12's used the two lid latches, one on each side to connect the speaker to the amp. Not using a more traditional connector allowed the company to save on the cost of a cable, four-pin connector or 1/4" jack. In the earliest model, the chassis was attached directly to the speaker cabinet lid, the next revision adopted an amp tray with shock mounts like they used on the B-15. Two speaker wires are fed from the amp chassis through holes in the lid or amp tray, depending on the revision. These wires connect to the square head bolts, one on each side of the lid that the latch mates to. In the cabinet, the speaker wires run from the speaker, one to the left, the other to the right along the bottom of the cabinet, and each up one side to the latch where they are attached. With the lid in place and the latches closed, the signal runs through wires from the chassis, through the square bolts, then the latches, and back on wires to the speaker.

    This is how Jess Oliver described the connection method used to get wires attached to the square-head lag screws in the lid:

    "As to connecting the wires to the lag screws we Cut strips of the tin-plate we used for shielding on the trays. The strips measured 5/32" X 3-1/2". With a 1/2" end mill we'd bore a hole, centered between the two shock mount centers on each end of the inside of the cabinet top. We would feed the metal strip into the inside of the pre-drilled hole where the lag screw goes. When we screwed the lag screw in, it would engage the strip. We would fold the end of the strip and solder it, one to a green wire and the other to a black wire. BTW, the trays for the B15N and SB-12 were identical since the centers for the shock mounts are the same."

    This approach is kind of complicated and prone to many problems. There is the possibility of forgetting to buckle down one or both of the latches which would mean a disconnected speaker. The wire connections at the bolts and latches could come Loose. The latch and bolt contact could become Dirty or oxidized which impedes the electrical connection. The latch could get bent out of shape which weakens the connection. The foam seal around the top of the speaker cabinet that comes into contact with the lid can become less resilient and Compressed with time leading to a weaker contact between the bolt and the latch. All affect the quality of the speaker connection. There is also a safety issue, you don't want to touch both latches while someone is playing the amp. This could result in an electrical shock. It is important to note that the early SB-12's had no speaker safety circuit in place. The later ones did. They had a 250 Ω 10W resistor across the 16 Ω output transformer tap like the later B-15's had as shown below. This protection feature could be added as a reversible modification to an early SB-12 model should you want to have some insurance.

    Turning on an early SB-12 without the latches connected properly could damage the amp. Most amps have at least a shorting jack connected to the output transformer. This shorts the transformer if nothing is plugged in. A short is better than an open circuit so it can help in the short term. The SB-12 has nothing. It is an open circuit if the amp is turned on with a bad speaker connection. The amp isn't going to instantly die if you turn it on without the speaker connected. If you stand there playing and wonder what is wrong, then turn up your volume and keep playing trying to get it to work, you might damage the amp. The tubes can arc, damaging them. The spikes from the tube arc'ing can damage the tube sockets. The spikes can also Punch holes in your power transformer, again causing damage. All this is worse case, but possible. If you turn on your amp with no instrument connected and then notice that it isn't connected, chances are you won't have a problem. The potential for damage is there though.

    Some owners leave the latch connections in place. If the latches and their wire connections are inspected from time to time to ensure that they are in good shape, the amp will be fine. Some choose to add a speaker out jack on the chassis so that the latches are out of the speaker circuit. The modification is not that dramatic but it does involve drilling a hole in the chassis. A jack will provide a much better electrical connection.

    SB-12 1966 single latch per side.


    SB-12 black and yellow speaker wires coming out of the chassis attach to the lid wires. Handles cut outs in the tube cage can be seen.

    SB-12 lid and amp tray.

    SB-12 amp tray mounted on the lid, the speaker wires connect inside the amp.

    SB-12 speaker cabinet wiring from the latch to the speaker.

    SB-12 speaker connection 1969 with safety resistor in place from the 16 Ω tap to ground.

    On the later SB-12 models, starting in 1969 they had four-pin speaker connectors, the jumper from pin-2 to pin-3 that was used in the early B-15 models was not included. It isn't necessary because there isn't a loop back circuit running from the amp to the speaker cabinet and back to the amp.

    The Plastic Strain Relief Chassis Bushings Used on the Power and Speaker Cables(top)

    The original strain relief bushings are still being manufactured by Heyco, model number SR 6W-1, part number 1184. The same bushing was used in other Ampeg amps such as the SVT. Although you can install the bushing with a pair of pliers, there is a special tool that makes the job a lot easier and prevents maring the plastic bushing. Heyco strain relief bushing pliers, Model number No. R-29, part number 0022. These parts are available from Amazon as well as other electronic supply shops.

    Heyco 1184 strain relief bushing.

    Heyco R-29 bushing pliers.

    Where Can I Obtain a Replacement Four-Conductor Speaker Cable? (top)

    Belden 8454 is the same diameter as the original cable. It will fit in the original hole with the Heyco strain relief bushing. Specs: Belden 8454 cable, UL/CSA Types: SVT, SO, 0.272" O.D., 18 AWG, 4-conductor rubber jacket, 300V, 60oC, paper tape separator, color code: black, white brown, red. Try Fliptops or Surplus Sales of Nebraska.

    Installing a Three-Conductor Power Cord On a B-15N (top)

    For safety reasons, it's a good idea to upgrade the wiring of older amps that have two-conductor power cords. It's a fairly straight forward procedure and it is fully reversible. The electrical code currently requires that any appliance with a metal chassis that can come in contact with a user must be connected to an earth ground. This prevents the user from being shocked if voltage presents itself on the chassis. The strings are connected to the chassis through the instrument cable so if you are grounded, whatever voltage is on the chassis is going to go through you. A safety ground will trip a line power circuit breaker and prevent this from happening.

    In order to retain the original look, it is preferable to use a cable that is approximately the same diameter as the original so that you can use the same Heyco SR 6W-1 1184 strain relief bushing and not have to enlarge the chassis hole. A good product to use is the following: General Cable, Carol Brand, part number 02602, SJOOW, #18/3, 300V, 10A, UL/CSA, rubber jacket, 0.3" outer diameter, copper stranded conductors, color code: black, white, green/yellow, made in the USA. Equivalent cables are available. Look for a rubber jacket, 18 gauge, of the same outer diameter. This cable is inexpensive and has been available by the foot from hardware stores such as Home Depot. The white product lettering on the outside of the rubber jacket can be removed with acetone. I soak a rag in acetone, wrap it around the cable, and pull it through. This cable requires that you add an outlet plug on the end.

    A three-conductor power cable wiring diagram is presented below. The colors of the wires will vary depending on where you live. In this case, black is hot, white is neutral, and green is the safety ground. It is important that the green wire be longer than the other two. The reason being, if the cable is pulled out of the chassis, the black and white will break first, leaving the chassis ground intact to prevent shocks. If the hot wire comes in contact with the chassis, it will trip the circuit breaker in the electrical panel. Code requires that he green chassis wire should have a ring terminal on its end and be connected to the chassis with a dedicated fastener close to where the cable enters the chassis. It should not share the same ground point with any other grounds. On a B-15, I like to use the tube cage threaded rod for the earth ground. The chassis contact point should be Clean with no rust between the ring terminal and the chassis. A star washer and a nylon locking nut can help make a firm connection between the ring terminal and the chassis. Here are some instructions for attaching a wire to a ring terminal.

    Notice that the chassis fuse has a heavy duty terminal at the end. This is where the hot (black) power cord wire connects. The fuse side terminal connects to the power switch. The other switch terminal connects to either one of the black transformer primary wires.

    The white neutral power cord and the other black primary transformer wire should be twisted together and the wires connected. There are different ways of doing this. A crimpable wire terminal can be used as shown in the images below. They are compact and have an outer plastic covering that insulates the connection. An alternative is to use a wire nut and a plastic anchor to hold the assembly down. If the wire nut is not anchored, it could be knocked around during transport and possibly come Loose leaving an exposed power connection. The image below shows wire nut in place in a B-15. If you are removing the death cap and ground switch connections, another alternative is to use one of the terminals on the ground switch as a tie point and connect the wires there.

    Having a ground switch on the amp can be useful if you have a noise issue. Rather than remove the ground circuit and panel switch, you can replace the death cap with a Type Y safety cap. This is detailed in the next section. If you decide to remove the ground circuit, the ground switch can be used for some other purpose such as a cathode/fixed bias selector. Notice in the Installed Power Cable image, I used a short length of heat shrinkable tubing to hold the power wires together. It helps keep the layout a little neater. If your power cable does not have an integrated plug, you will need to add one. The images below show how to wire a 120 VAC outlet and plug connecter for use in North America. Be sure to not mix up the hot and neutral connections.


    Wiring diagram for transform primary with fuse and power switch

    Chassis fuse: power goes in the end terminal and out the side terminal.

    Ring terminal: the green safety ground wire is crimped to this connector.

    Nylon locking nut.

    Closed end wire terminals: the wires are crimped to lock them inside the cover.

    Wire terminals.

    Wire nut: the cap is screwed on over the twisted wires.

    Wire nut detail showing it anchored to the chassis with a cable clamp.

    Installed power cable.

    Duplex Receptacle: hot connects to the short spade terminal, neutral to the longer one, ground connects to the D terminal.

    Outlet Plug: Leviton 15A, 120V, NEMA 5-15P, straight blade.

    Installing a Three-Conductor Power Cord On an SB-12 (top)

    Here's a procedure for installing a three-conductor power cord on a 1966 SB-12. This amp doesn't have a polarity switch like is found on the B-15 which makes it a bit easier.

    The power transformer has a primary side and a secondary side. The primary side is on the right in the image below, it's the input where the AC line voltage is connected. The secondary side has three windings, 5VAC heater for the rectifier tube, a high voltage winding, also called B+, and 6.3VAC heater for the power and preamp tubes. The wiring on the primary side of the power transformer needs to be changed to install the three-conductor power cord.

    The primary side of the power transformer has two black leads, they are interchangeable so either can be used when making the connections.

    The 0.05uF 1000V line filer capacitor shown in the first image should be removed. It's located on the eyelet board. Any orphaned wires that run to the eyelet board and connect to the capacitor should also be removed.

    Install the power cord as described in the section above. The black wire is attached to the fuse end terminal, the white to the power switch, install a ring terminal to the end of the green wire. The green wire should be the longest of the three power cord wires so that it will break last if the power cord is pulled out of the chassis. Connect the ring terminal using a lock washer to a transformer bolt. Clean the contact area well of any paint or rust so that it makes a good chassis ground. The lock washer ensures that it won't loosen.

    Additional details, including pics and a video are included in this thread Shocking Ampeg SB12 story! Amp repair..

    Remove the line filter cap.

    Uncomplicated SB-12 power cord wiring.

    The Ground Switch, The Death Cap, and the Three-Conductor Power Cord (top)

    For safety reasons, three-conductor power cords that ground an exposed metal chassis are required by electrical codes. It's a good idea to upgrade the two-conductor cords on a vintage amps. Often when this is done, techs remove the so called death cap and the ground, also called polarity, switch circuit. Many amps built today don't have a ground switch. Some that do, like on the Fender '59 Bassman are installed on the chassis but not connected to anything. It's there to retain the classic look but serves no function in a circuit. Other manufactures retain the ground switch and related circuit.

    The ground or polarity switch is part of a filter circuit that can help minimize noise in the amp from the power line. The capacitor is used to create an AC ground that shunts high frequency noise such as radio frequency interference that is in the power line to ground, helping to keep it out of the amp.

    Because of the reactance of typical capacitor used in these applications at the 60Hz or 50Hz power line frequency, the AC current on the chassis is limited. In the case of a 0.05uF capacitor, the reactance is about 53K ohms (reactance of the capacitor = 1/(2 * pi * F * C; F=60 Hz, C=0.00000005 F). With 125 VAC power, the AC current when the capacitor is connected to the hot line is limited to 2.4mA on the chassis. This assumes that the capacitor is healthy.

    The filter circuit is on the primary side of the power transformer and connects a capacitor from either the hot or neutral power line to the chassis ground. The capacitor, often called the death cap, got its name because when it deteriorates you can get a shock from the amp. Depending on how the ground switch is set, a leak caused by a lower reactance can result in a mild shock, a shorted capacitor can put the full line voltage and current on the amp chassis which is unsafe. Remember, your instrument cable is connected to the chassis so if you touch your strings you are connected to the chassis. If you are grounded, you can be shocked. Grounding can occur if you touch anything that is grounded such as a mic or another piece of equipment. To avoid this, techs often remove the capacitor and disconnect the ground switch when installing a three-conductor power cord. Many modern amps don't have a ground switch and capacitor.

    Installing a three-conductor power cord does not negate the need for a power line high frequency noise filter. On some amps, they keep both the filter and the ground switch. This is a useful feature as long as the power outlet is wired correctly.

    Some amplifiers have an EMI (electromagnetic interference) power line filter. This is more comprehensive than a simple capacitor. These filters employ components that serve to supress differential and common mode interference.

    Take a look at the back of a modern SVT. Traditionally Ampeg used a two position polarity switch. Some modern amps, such as the SVT, use three position switches. The two outer positions work exactly like the vintage ground switches, either the hot or neutral power lines are connected to a capacitor to ground which serves to shunt radio frequency noise to ground. In the middle position, the capacitor is disconnected from the power lines. This is the same as removing the capacitor and ground switch from the amp. This is illustrated in the image below. What makes this safe to do is using what is called a safety capacitor.

    Safety capacitors are specifically designed for power line applications. They do not short in a way which is unsafe. Here is an explanation as to how they work. There are different types of safety capacitors, Type X which is used across two lines, such as hot to neutral, and Type Y which is used from line to ground. In an amplifier ground circuit you want a Type Y safety capacitor. To complicate matters, there is also a Type XY safety capacitor which can also be used as a noise filter capacitor in an amplifier. So if you see a capacitor that is market Type Y or Type XY in your amp, it's a good thing. When sourcing these capacitors, many of them have short leads that are intended to be used with printed circuit boards. These can be difficult to retrofit in a vintage amp. There are products with long leads that can be found if you look. For a B-15N which uses a 0.047uF capacitor (close enough), a Kemet PHE850EB5470MB14R17 will work. These are available from Digikey.


    What Type Of Light Bulbs Should I Use In My Vintage Portaflex (top)

    The early vintage amps use two #1847 bulbs inside the chassis to illuminate the lucite logo. This is a common bulb that goes way back. It's interesting that 1847 is the year that Thomas Edison was born. Technical specs: Miniature Bulb, T-3-1/4 Miniature Bayonet BA9S Base, 6.3 Volt, 0.15 Amp, 0.945 Watt, 0.38 Mean spherical candlepower (MSCP), with the C-2R Filament Design, the service life is 5,000 Average Rated Hours, dimensions: 1.19" (30.20mm) Maximum Overall Length, 13/32" (10.0mm) Maximum Outer Diameter, 0.78" (19.80mm) Light Center Length (LCL).

    Later revisions of the amps had neon pilot lights mounted on the chassis. Neon lamps have a relatively long service life. Often the entire lamp assembly will need to be changed when the bulbs do expire. Fortunately new old stock lamps as well as modern replacements are available.

    Modern LED replacement bulbs for the #1847 are available from eLite. They are offered in a variety of colors.

    A super Bright OP-MAX LED is available in a variety of colors here: Comet LED Optix Maximus. The pair in the image below is the blue color, #44/#47 Bayonet.

    Blue LED OP-MAX.jpg
    Courtesy Chris Ramlar.

    How Do I Re-bias My Fixed Bias Amp?(top)

    Below is the schematic for a 30W B-15N that has a fixed bias circuit. The bias circuit is outlined in orange. The purpose of the bias circuit is to provide a negative voltage that is used to set the steady state or idle operating point of the tube with no input. This is much like tuning an instrument's stings to pitch. If the string is tuned too low or too high, it doesn't perform well.

    The bias circuit takes an AC voltage that is tapped off the high voltage secondary of the power transformer, a limiting resistor (R36) drops the voltage, it is rectified by a diode (D1) and converted to a DC voltage with the help of a smoothing capacitor (C15), a voltage divider resistor (R34) helps set the voltage level that is applied to the power tubes. Some amps have a potentiometer that allows you to adjust the bias voltage, others, as in the case below, have a resistor (R34) that needs to be changed to fine tune the bias. A schematic for an adjustable bias circuit is provided should you want to install a potentiometer under the chassis.

    There are different procedures used to bias an amp, they are summarized by Randall Aiken here. Their effectiveness varies and some are safer to follow than others. One way to optimally bias an amp is to connect a dummy load to the output, inject test signals at different frequencies into the input, and set the bias voltage based on the cathode current. Test equipment such as an oscilloscope is used to monitor crossover distortion, a spectrum analyzer to monitor Harmonics, and a distortion analyzer are used to help minimize distortion while setting the bias. An audio analyzer will measure frequency response, power output and distortion over the frequency range of the amp. This allows you to optimize power output while minimizing distortion. It sounds complicated but it really isn't. You just have to have the equipment. That's why folks go to a tech. Another way to bias an amp is to simply set the bias voltage, in the case of the B-15N to -50VDC with a 120 VAC line voltage. These are two ends of the spectrum. Not surprisingly, many manufacturers follow the latter procedure. They select a bias voltage that allows the amp to perform well with the tubes that they are using and, at the same time, optimizes the tube service life. It's a conservative approach and saves time in manufacturing.

    The bias point is not a hard number written in stone, it can vary. When fine tuning the bias voltage, you can use your ears to determine when the amp sounds the best. With some experience, you can set the bias by ear without a lot of test equipment and then measure the cathode current at the end to confirm that the tube is functioning within it's specified safe operating parameters.

    The bias procedure for a fixed bias B-15N Portaflex amp is outlined below. I like to use the plate/cathode method as described by Aiken above. The cathode current is measured with a tool called a bias probe. Bias probes such as the are Bias King are commercially available. If you want to build one, kits are available such as this one from Hoffman. The kit requires that you have a volt meter that can measure current. The commercially available bias probe units come as a complete package with a built in meter and there is no assembly required. The User Manuals show you how to use the probes. Basically you remove a power tube and plug it into the probe, then the unit goes into the amp. A meter reads the cathode current and you adjust the bias to a level that is specified in the manual. This is a relatively safe procedure, minimizing ones exposure to shock hazards.

    The first step in adjusting the bias is examining the components in the bias circuit. When the electrolytic capacitors in the power supply are changed, the components in the bias circuit should be tested and changed if necessary. Electrolytic capacitors have a limited lifetime because of the nature of their construction and eventually need to be replaced. In the case of the B-15N bias circuit, R36, the 100K 2W resistor get quite hot during normal operation and is prone to drifting with time. If it is replaced, use a flame proof wire wound or metal oxide resistor, a higher wattage such 3W to 5W is good. Just make sure that it fits on the circuit board, higher wattage resistors are usually bigger. I like to mount these resistors a millimeter or so off the circuit board to protect it in the event of a catastrophic failure. C15, the 100uF 100V electrolytic capacitor needs to be changed with age and because of the elevated operating temperatures and heat under the chassis. If you have an ESR meter, it can be used to help determine the health of the capacitor. When you are paying a tech for bench time, capacitors are so inexpensive, it is often easier to simply change the components than to spend time diagnosing issues. In this amp, the bias is adjusted by changing the value of R34, the 47K 1/2W 5% resistor. You can always use a larger wattage resistor, such as a 1W. Resistors are available in standard EIA values. It might help to piggyback parallel combinations to get the right value so buy a selection. This is where having a potentiometer in the circuit can help make things easier.

    The next step is to set the bias current using the bias probe. The bias voltage is adjusted to provide a cathode current as per the instructions in the probes user manual. These manuals are available for downloading which will help in understanding what is going on. A 6L6GC power tube has a maximum plate dissipation of 30W. Exceeding this will shorten the service life of the tube and it could result in damaging it. The plate voltage should be measured. The plate voltage used in Portaflex amps can vary depending on the revision so check your amp. On this schematic, the plate voltage is indicated as 450V with a line voltage of 120 VAC. The plate voltage in your amp will be higher or lower if the line voltage is higher or lower. It is important to measure the actual plate voltage that is in the amp and use that value when calculating the bias voltage as described below. So substitute that number in for the 450V that is in the formula. Also, it is important to be aware that as the bias voltage is changed, the plate voltage will change. Measure the plate voltage after setting the bias. The health of the power supply capacitors also affects the plate voltage which could explain why your readings may not match the reference voltages on the schematic.

    The tube industry has adopted a 70% plate dissipation standard at which a push-pull class AB amps are ofter set to operate at. So since power equals voltage times current (P=V*I), for a 70% plate dissipation, I=(P/V) * 0.7 = (30/450)*0.7= 47mA per 6L6GC tube. The total cathode current comes from the plate and the screen. So to be more precise you should subtract the screen current to obtain the actual plate current. If you assume that the cathode current equals the plate current, this gives you a more conservative estimate of what the plate dissipation will be. A rule of thumb is that the screen current is typically 3-5mA for the 6L6GC. If your amp has a screen resistor you can easily calculate the current by measuring the voltage across the screen resistor and using ohm's law, I=V/R. Then subtract that from the measured cathode current to obtain the plate current.

    Earlier it was mentioned that the amp's internal voltages, including B+ and the bias voltage will change with line voltage. If you set the bias at 120 VAC, it will be lower or higher if you are at a venue where the voltage is 115VAC or 125VAC or higher. This will make the amp sound different. In a recording studio, sometimes they want to check the bias based on the studio's AC voltage to ensure that the amp will sound its best.

    The bias should be set with all the components warmed so that they reach what is called a steady state. At this point, the readings are stable and not drifting. Before setting the bias, let the amp Warm up in playing mode (standby off) for at least 20 minutes.

    After setting the bias, let the amp run for an hour or so and recheck the cathode current to check for any drift. Each time you set the bias, measure the line voltage and record it. Since it affects the bias, any drifting of the line power will affect the setting to some extent. Don't worry about getting it perfect. With some experience you can roughly set the bias, then listen for what sounds optimal and then check the bias to ensure that it is within a safe operating range. So it can be done without a lot of fancy test equipment. Some amps sound better set at 70% plate dissipation, other amps sound better when the bias is set to drive that tubes with a higher or lower plate dissipation. THis is where experience with the amp comes in handy.

    Schematic For 1968 B-15N Amp With Fixed Bias

    Isolated Bias Circuit Schematic

    Adjustable Bias Circuit Schematic

    Adding a Bias Pot to a Fixed Bias 1973 B-15N(top)

    The following describes a bias pot modification that was performed by sjrash on his 1973 B-15N.

    Let's start by looking at the inside of the amp. The 1973 B-15 circuit is built on a circuit board. Not all fixed bias B-15's are the same as this one. The components on the earlier fixed bias amps are on an eyelet board. The same approach can be used in those amps. How the pot is mounted will differ and a little creativity will be required to adapt what was done to this amp.

    B-15N 1973 circuit boards.

    This is the schematic for this B-15N revision. Note that the bias circuit design has two 10uF capacitors that smooth the rectified DC bias voltage. Other revisions such as the one in the previous section utilized a single 100uF capacitor. Different ways of designing the bias circuit.

    B-15N 1974 schematic.

    Let's go back to the circuit board and examine the bias components.

    The bias components are within the red circle.

    The bias components are located at the lower left side of the main board. The large gray wire wound Tubular resistor, R39, is not part of the bias circuit. R42 is mostly obscured and C19 is covered by R39.

    B-15N 1973 labeled bias components.

    Below is the schematic for the bias circuit. A raw AC high voltage is tapped off pin-4 of the tube rectifier. It is fed to R35 and R36 which act as a voltage divider to reduce the AC voltage. This is rectified by D1 and the ripply DC voltage is smoothed by the network which consists of C15, R42, and C19. R34, the 47K Ω resistor is what fine tunes the bias voltage.

    B-15N 1973 bias circuit.

    To change the voltage, R34 has to be unsoldered and a new resistor of a different value soldered in its place. We are going to remove the resistor and replace it with the circuit below. R2 and R3 are fixed resistors connected in parallel; R1 is a variable resistor. R2 and R3 allow you to get close the the target resistance of 47K, R1, when set in the middle of the pot's travel, allows you to adjust the total resistance +/-. Why not just use a pot? The series resistors, R1 and R2, set the bulk of the resistance and are there as a safety measure in the even that the pot shorts and so that the pot can't by turned down to zero. Why the parallel combination of R1 and R2? It adds flexibility when finding the proper resistor is a problem. A single resistor can be used and the other one not placed. Two ¼W resistors can be used instead of a single ½W resistor.

    B-15 bias pot circuit.

    Now that we know what to do, we have to figure out how to retrofit the circuit in a fully reversible way. This isn't always easy. You want to use the existing pads and not modify the main printed circuit board. That will allow you to return the amp to it's original condition should you ever wish to. The solution is to design a circuit board and install it piggy back on the main board.

    This is a rendering of the designed board and the layout that we designed. The board is tiny, about one square inch in size. Remember, the board is intended to replace only the bias resistor, R34. It is removed from the main board, freeing up the mounting pads. The pads marked DC and GND are for bus wires that run to those pads. Rather than just relying on that connection to support the board, there are four addition mounting holes on the board that are sized for 4-40 screws. One or any combination of these mounts can be used to ensure that the board is stable. You need some unused space on the main board for the feet to rest. If the top two mounting holes are not being used, they can be clipped off. A section was taken out of the board, the black area at the top center, to allow for cooling and heat dissipation for R36 which is a 2W resistor and does generate some heat.

    Rendering of the B-15 bias pot board.

    As you can see in the circuit layout, the DC voltage comes in to the board, goes through the parallel combination of the resistors, R1 and R2, the voltage goes through the pot and to ground. The board is designed for a Bourns multi-turn pot. This allows the pot to be less sensitive to changes. A large rotation is needed to change the resistance and the bias a small amount. With single turn pots, a small change can make a big difference in the bias setting. This means that if the amp get knocked around and adjustment screw turns a bit, there is less of a chance that the bias will change.

    Circuit layout, bias pot circuit board.

    This is a front and back view of the unpopulated printed circuit board that was designed.

    Bias pot circuit board.

    This is the populated board installed in sjrash's B-15N. Note that there is clearance for R36, the resistor on the left. Bus wire, 22 gauge Belden 8021, is used to attach the board at the DC and GND pads on the right where R34 was mounted. Two 4-40 nylon screws and hex standoffs sanded down to size were used in the mounting holes. The nylon standoff is a Keystone part number 1902B, the 4-40 nylon screws are Multicorp SPC13374. If the hex standoff isn't used, a nut can be used to lock the screw in place, Multicorp SPC21811. A small amount if silicone goop is used to anchor the standoffs to the main board.

    The bias pot board can be mounted onto an older fixed bias eyelet based board. The bias resistor will mount the same way with the bus wire extending to the eyelets. The mounting holes on the board will be used in the same way to ensure that the board is firmly held in place.

    The following components were used on the board:

    (2) 75k ohms ¼ watt metal film resistors, a single 36K ½W resistor could have been used.
    (1) 20k Ω 5% pot is a Bourns 3296W-1-203 Trimmer pot. This is a 25 turn ,½W, linera pot. Alternately, a 3296W-1-253 25K pot could be used.

    The two 75k ohm resistors in parallel total 37.5k ohms. With the 20k Ω trimmer connected in series, the adjustable range is +/-10k ohms from the original 47k ohms.

    The US made boards are available, 3 boards for under $6 with shipping included anywhere in the world. They can be ordered here: OSH Park ~ B-15N bias pot V1.2. The components have to be purchased separately from a supplier . These were obtained from DigiKey Electronics.

    Sjrash's B15N with the bias pot board installed. It looks pretty good.

    Ref: Official Ampeg Portaflex Club - adjustable bias

    Converting a B-15N from Cathode Bias to Fixed Bias(top)

    There are differences in the performance of cathode bias vs fixed bias amps. A cathode biased amp will react differently to a sudden hard note. A cathode biased amp will react by sagging the plate voltage and then recovering. This affects the Attack of the note. A fixed bias amp doesn't do this. The advantage of fixed bias is that you can get a few more watts out of the amp. The cathode biased B-15's were rated at 25W, the fixed bias amps at 30W.

    If you have a B-15N with cathode biased power tubes and want to convert it to a fixed bias, it can be done. Most, but not all of the circuit board revisions are fixed bias, so we are predominantly talking about converting an eyelet based model to fixed bias. The early 1965 circuit board models are also cathode biased. It is important to carefully examine your amp to determine of it is cathode or fixed bias. The telltale sign of a fixed bias amp is a diode close to where the cathode bias components normally are. Let's look at some circuit Schematics and amp examples.

    This is a B-15NA amp schematic that has cathode biased power tubes. There are two components, a 250 Ω resistor and a 50uF 50V capacitor connected in series. One end connects to pin-8 of each of the 6L6GC power tubes, the other end connects to ground. The resistor is 10W so it is large and stands out on the eyelet board.

    B-15NA cathode bias.

    This is a B-15NF schematic that has fixed bias power tubes. The two 6L6GC power tube cathodes, pin-8, are tied together and connected to ground. The bias circuit is tapped off the AC high secondary high voltage winding of the power transformer and feed through a 100K 2W and 56K voltage divider which reduces the level, it is rectified by the diode, the 47K resistor sets the bias voltage, and the 100uF100V capacitor smooths the now rectified DC voltage. The bias voltage is applied to the input, grids, of the power tubes.

    B-15NF fixed bias.

    The next step is to look inside the chassis and determine of the amp is cathode or fixed bias. In an amp with an eyelet board, the cathode resistor and capacitor are often situated close to the middle of the board neat the lamp frame. The components are labeled so they are easily identified. As you can see, this resistor is labeled 250 Ω, the capacitor is 50uF 100VDC.

    Cathode bias components.

    This is circuit board based, cathode biased 1965 B-15NC. The copper metal capacitor and the large resistor next to it are the cathode bias components. In this case the resistor has color bands which identify it as 250 Ω. The fourth band is the tolerance, gold is +/- 5%.

    Circuit board based B-15N, cathode bias.

    Here are two examples of fixed bias circuits. In both cases, you can see the black diode which is a clear indicator that the circuit is fixed bias. Of course, it is always a good idea to trace the circuit using the schematic as a reference to verify that the amp is fixed bias.

    Eyelet board with fixed bias circuit.

    Circuit board with fixed bias.

    There are two options to implement a fixed bias circuit in a cathode biased amp, copy the cathode bias circuit layout or install a printed circuit board. Copying the circuit layout will require rearranging the components on the eyelet board to match what Ampeg did. This approach is usually easier. If a bias printed circuit board is used, you will have to find a way to mount it. How this is done will depend on the revision of the amp.

    The first step is to remove the cathode bias circuit components, the 50uF capacitor and the 250 Ω resistor.

    A Fixed Bias Circuit Board(top)

    A stand alone board has been designed which implements the bias circuit based on the 1973 B-15N that was described in the previous section. In this case, it is the entire bias circuit rather than just adding a bias pot. The board can be mounted in different ways.

    As with the previous boards, the mounting holes accommodate 4-40 nylon machine screws (Multicorp SPC13374), hex standoffs (Keystone 1902B), and nuts (Multicorp SPC21811). If necessary, longer lengths for the standoffs and machine screws are available, check with the supplier. The fasteners were ordered from DigiKey. The 4-40 nylon screws and hex standoffs are sanded down to size were used in the mounting holes. A small amount if silicone goop is used to anchor the standoffs.

    B-15N 1973 bias circuit.

    Bias circuit schematic.

    Bias circuit layout.

    Bias circuit board rendering.

    Top and bottom faces of the bias printed circuit board. This is a prototype board, the resistor labels are different from those of the rendering above.

    The dimensions of the board are 38mm (1.5 inches) square. Three printed circuit boards cost $11.25 US delivered anywhere in the world: OSH Park ~ B-15 bias V1.2. A 20K or 25K pot can be used. The 25KL potentiometer is a Bourns 3296W-1-253. All the resistors are ½W metal film with the exception of the 100K resistor, R36, which is 2W. R34B can be 36K or 38K. The capacitors, C15 and C19, are 10uF, at least 100V. The circuit board capacitor lead spacing is 5mm, outline radius is 10mm. Any general purpose radial aluminum electrolytic capacitor that fits can be used. In general look for a 160V or 200V to fit the lead spacing and diameter requirements. One example is a Nichicon UPW2C100MPD1TD, available at DigiKey Electronics.

    Cathode-Fixed Bias Switch (top)

    There's a difference in the sound and performance of cathode and fixed bias amps. Fixed bias allows the designers to squeeze a few more watts out of the amp. In the case of the B-15N, the early 25W amps have a cathode biased power amp, the 30W amps are fixed bias. These amps are capable of a little more output power but those are the assigned ratings.

    At lower volume levels, the amps sound pretty much the same. At higher volume levels, they sound a little different. With cathode biased amps, the Attack on a note can sound different than that of a fixed bias amp when you suddenly dig in hard. Cathode biased amps are self biasing. When you push an amp, the power supply dips which lowers the plate voltage. At the same time the plate to cathode current increases. This changes the cathode bias voltage. The bias is self regulating and equilibrates. It takes a short time for the power tube bias to adjust to the sudden change in cathode current. This affects the sound in a softer, subtle way that is different from that of a fixed bias amp.

    Imagine having the option of switching between cathode and fixed bias in your power amp. Boutique amp designers have been offering this feature in recent years. This feature is available on the newer Heritage B-15 amps. There is a switch to select 1964 cathode bias or 1966 fixed bias. If you have a fixed bias amp and want to add a cathode bias option, it can be done. If, when a three-conductor power cord was installed the death cap and ground switch wiring was removed, the ground switch chassis hole can be used for a bias switch.

    If you have a fixed bias amp, you will need is a DPDT switch, a 250 ohm 10W resistor, a 50uF 50V capacitor, and some wire. Toggling the switch selects either the cathode or fixed bias circuit. It also grounds one or another part of a circuit. Two circuits are switched in each toggle position. More on this below. If an amp is cathode biased, it can be modified by adding the fixed biased circuit components and DPDT switch. Both options require a small amount of real estate on the eyelet or circuit board for the components.

    It is important to note that switching between cathode and fixed bias should only be done with the amp power off.

    DPDT (double pole, double throw): A DPDT switch routes two separate circuits, connecting each of two inputs to one of two outputs. A DPDT switch has six terminals: two for the inputs, two for the A outputs, and two for the B outputs.

    Below is a sub-section of the B-15 schematic showing the fixed bias - cathode bias circuit. The switch selects either the cathode bias circuit or the fixed bias circuit. There are two sides to the DPDT switch, each is serving a different purpose.

    Power amp sub-section showing fixed bias - cathode bias circuit

    In a cathode bias circuit, the power tube cathodes (pin-8) are tied together and connected to ground through a parallel resistor/capacitor pair. In the image below, you can see the cathodes tied together and they are connected to the 250 ohm resistor and the 50uF capacitor which in turn are connected to ground. The switch in the cathode bias position needs to do two things: switch in the cathode bias circuit (traced out in blue), and ground the junction of the two 270K ohm resistors (traced out in green).

    Cathode bias is selected.

    The fixed bias circuit is shown below. The switch in the fixed bias position does two things: switches in the fixed bias circuit (traced out in red), and grounds the junction of the two power tube cathodes (traced out in green). The fixed bias circuit starts with a connection to the 360VAC tap on the power transformer. The voltage is reduced, rectified, and filtered to convert it to a negative DC voltage. One side of the switch connects this voltage to the grids of the power tubes through the 270K ohm resistors. The other side of the switch connects the cathodes (pin-8) that are still tied together but this time they are connected directly to ground. The cathode resistor and capacitor are switched out of the circuit. This is traced out in green.

    Fixed Bias

    Installing a Choke in a B-15 (top)

    A choke is an inductor which can be used as a filter in a power supply. It acts as a buffer, resisting changes in current flow and thereby stiffening the power supply. This reduces the power supply's ripple voltage and Hum in the amplifier. When Loud was developing the Heritage B-15, Jess insisted that the amp have a choke in the power supply. He felt that a stiffer power supply would make the amp perform better so they put one in.

    The red trace shows the power supply voltage after it is rectified and filtered with the capacitors. A stiffer power supply with an inductor will have less ripple, it will be flatter, and will sag less when a demand is placed on it.

    The easiest way to install a choke in a B-15 is to replace the resistor between the first and second power supply capacitors (Node A and Node B). This is illustrated below in the B-15NC power supply segment. The 1K 10W resistor is removed and replaced with the choke. An added value of the choke is that it permits the power supply to use higher valued capacitors than the 40uF ones shown. The choke should be rated at around 120mA, the inductance used typically varies between 5H and 15H, a 9H Heyboer HY027707 is a good choice. These chokes look like small transformers with two leads. Mounting the choke inside the chassis is a bit of a problem. Typically two holes are drilled on the side or top of the chassis at the end with the power transformer. A further modification that some designers like to incorporate is to use a DPDT switch to select either the resistor or the choke in the circuit. If you removed the ground switch as part of a three-conductor power cord installation, you could install the switch there.

    B-15NC power supply

    B-15NC power supply with choke

    Heritage B-15 with choke at the bottom right corner.

    Lowering The Line Voltage (top)

    In North America, vintage Portaflex amps were designed to run at 117 VAC. Some older amps were designed to run at 110 VAC. It isn't unusual to find line voltages of 120-125 VAC today. These higher voltages cause proportionally higher voltages in the amp, raising the B+ and heater voltages. This can stress old components such as capacitors and the transformers. One solution is to lower the line voltage with a device called a Variac. A Variac is a variable transformer, that allows the line voltage to be lowered or raised by turning a dial. Some guitar players use them to attain distortion at lower volume levels, the so called brown sound, because they are browning out or lowering the voltage in the power supply with the Variac. Variacs can be expensive though. They are also heavy to lug around to gigs. Also, if the dial is set too high or low, the amp can be damaged.

    A less expensive alternative is to use a bucking transformer. A bucking transformer reduces or bucks the voltage by a fixed amount to a lower level. The idea is to hold a fixed voltage across a transformer, thereby reducing what is delivered to the line outlet that the amp is plugged into. By using a center tap heater transformer for example, the voltage can be lowered by 6.3 or 12.6 volts. Two designs are presented below.

    It is important to note that the bias on fixed bias amps is dependent on the line voltage. If the amp is biased with a line voltage of 122VAC in a tech's shop and the line voltage is lowered to 117 VAC at a gig, the amp may need to be re-biased. This isn't a problem with cathode biased amps as the bias self adjusts. A lot of clubs have bad wiring and the line voltage can be low, 115VAC and lower. If you notice that your amp sometimes sounds different at gigs, it might be because the low voltage is affecting not only the voltages within the amp but also the tube bias. Voltages that are too low can damage the amp so be careful. It's always a good idea to carry an outlet tester and volt meter. An outlet tester checks if the outlet is wired properly, this helps to prevent shocks. The volt meter can be used to measure the line voltage at gig before plugging your amp in.

    Staco Variac

    Bucking transformer schematic diagram.

    Outlet tester: a useful tool to carry in you gig bag.

    PF-20T Rewiring the Line Voltage(top)

    The Portaflex tube amps come with a universal power transformer that can be wired for 100VAC, 120VAC, or 220-240VAC operation. It requires rerouting some spade connectors on the printed circuit board. This task is best performed by an authorized Ampeg service center. Doing it yourself is dangerous, both to you and the amp, and will void your warranty.

    The fuse also must be changed in accordance with the line voltage. This is specified on the back of the amp. Lastly, a proper IEC power cord is required.

    After the conversion, the power tube bias should be checked. The procedure is described in the user manual.


    PF-50T Rewiring the Line Voltage(top)

    The Portaflex tube amps come with a universal power transformer that can be wired for 100VAC, 120VAC, or 220-240VAC operation. They’ve made the line voltage conversion much easier in the PF-50T, simply pull and replug into the correct molex connector as shown in the image below. This task is best performed by an authorized Ampeg service center. Doing it yourself is dangerous, both to you and the amp, and will void your warranty.

    The fuse also must be changed in accordance with the line voltage. This is specified on the back of the amp. Lastly, a proper IEC power cord is required.

    After the conversion, the power tube bias should be checked. The procedure is described in the user manual.


    My Amp Is Humming (top)

    Well, at least it isn't picking up radio stations.

    Aluminum shield in amp tray.

    If your amp is humming, you need to determine if the source of the noise internal or external to the amp. It could be picking up external noise such as from the house wiring, fluorescent lights, light dimmers, a fridge or air conditioner motor, a local ham radio station, or even a fault in a power transformer out in the street. There are many possibilities. Interference has a way of getting into the circuits of an amp and causing noise. The amp chassis serves to shield the circuits. If the bottom of the chassis isn't shielded, the interference can get in that way. In a similar way, instrument cavities and the inside of the pickgard on an instrument's body are lined with aluminum tape or conductive paint to shield the pickups and electronics. If the bottom of the amp's chassis is not shielded, it should be. Some companies use sheets of aluminum to do this, others use aluminum or copper tape. The B-15N's amp tray has a sheet of aluminum lining it. Usually a schematic is pasted onto it. Rolls of aluminum can be found in the roofing department at hardware stores. The aluminum sheet is Thin enough to cut with heavy scissors so it is fairly easy to work with.

    The amp could be humming because of a problem within the amp. The first thing to check is the Hum balance pot on the back of the chassis. There are two types of hum balance pots. Early SB-12 amps, for instance, had a hum pot that would balance the pair of output tubes. By adjusting the pot, you could compensate for unmatched tubes. The other type of hum pot is found on many amps including the B-15N. It is used to balance the heater circuit. In both cases, the pot is adjusted for minimal hum by ear with the instrument plugged in, standby off, and the tones set at noon. Sometimes these hum pots fail and the amp can exhibit a loud hum that doesn't change when the pot is adjusted. It might mean that the pot needs to be changed. I use a replacement pot made by CTS. It is wire wound, 100 ohm, 5W, 10%, 300 degree rotation, short slotted shaft that you use a screwdriver to adjust, DigiKey part number 026T419S101A1A1-ND. In a pinch, when a replacement is not available, the pot can be removed and replaced with a pair of 100 ohm 1/2W resistors as shown blow. The circuit is called an artificial center tap.

    Artificial center tap can be use in place of a hum pot. Both can not be used at the same time.

    Layout of an artificial center tap.

    Hum Pot: 100 Ω, 5W, wire wound, CTS 026T419S101A1A1

    Another possibility is that the power supply and bias circuit electrolytic capacitors need to be changed. These capacitors have a lifetime that is based on age, heat exposure, operating voltage, and storage. If fact, using the amp regularly helps promote a longer service life for these capacitors. A telltale sign of needing to be replaced is a constant hum. Depending on the amp design, this can be a 60Hz or a 120Hz hum. Prior to the amp humming, as the capacitors age, the amp will loose low end, will run out of headroom and distort sooner than it used to. The changes are subtle and difficult to notice, then one day the amp starts to hum louder than normal. Sometimes this occurs when the amp has been sitting unused for a long time. It is a good idea to power up an amp in playing mode (standby off) at least every six months, to keep the electrolytic capacitors healthy. Exercising them causes a chemical reaction within the capacitor that reforms a protective insulating layer on a substrate that is in a roll within the capacitor.

    How long do electrolytic capacitors last? It varies depending on the make and model of capacitor. Some capacitors are rated for a long service life at high temperatures and will perform better than others. If you hear your amp humming louder than it normally does or if there is less headroom or an earlier onset of distortion, it could mean that the caps are on their way out. A tech can determine if a capacitor needs to be changed with an ESR meter and oscilloscope. With a gigging amp, some players find that it's worth having them changed before they start to deteriorate as part of a preventative maintenance program.

    Amp Hum Checklist (top)

    It doesn't hurt to try a few simple tests before taking the amp in to a tech for a checkup. If you document any tests that you do, it may save on some bench time charges.

    Let's look into some possibilities:
    • Did this come on gradually or all of a sudden?
    • Have you tried a different bass and cable into the amp?
    • Plug the amp directly into the wall, not through a power conditioner.
    • Eliminate any external sources of noise. Try the amp on a different power circuit, one that is on a different circuit breaker. This might mean moving to a different room. Try the amp at a friends house. External sources of noise include fluorescent lights, light dimmers, motors, fridges or air conditioners, power line wiring issues, a power transformer on the street, a transmitter in the neighborhood, even a cell phone.
    • Swap out the tubes one by one with known good ones, starting at the first pre-amp tube V1 and moving down the line to the rectifier tube. The power tubes should be swapped out with a match pair.
    • Are the nuts on the input jacks Tight? Check them with your fingers just to be sure. Same goes for the pots. Wiggle the knobs to ensure that nothing is loose.
    • When you adjust the hum balance pot, do you hear another hum of a different frequency that gets louder and softer as you turn the pot back and forth? If yes, chances are it is not the hum balance pot.
    • Despite the hum, does the amp go as loud as it normally does?
    • Do you have a spare rectifier tube or SS module that you can try?
    • The ext spkr jack on the back should have a fiber isolation washer on it. Any chance has it has gone missing?
    • The next step involves swapping tubes. Try running the amp with just two pre-amp tubes. Pull V2. Take two 6SL7 tubes that you feel are the best. The idea is to try just channel 1, then just channel 2.
    • Put one tube in V1 and the other in V3. See what happens.
    • Then move the V1 tube to the V2 position. See what happens.
    • Plug your bass into the B-15. Connect an instrument cable into the ext amp jack. Plug the other end into the power amp in on one of your other amps. Is the hum present? If yes, possibly there is an issue in the pre-amp.
    • Take a pre-out from another amp (or VT pedal) and connect it to the B-15's ext amp. See if the hum is present. This will test the power amp.

    It is a good idea to give the amp a general servicing. A lot of details need to be checked. A tech may start with the following:
    • perform a careful inspection,
    • check the power tube bias circuit,
    • in the case of a fixed bias amp, ensure that the bias is optimally set,
    • scrub the sockets and front and back jacks with Deoxit,
    • check if the sockets need re-tensioning,
    • check the shielded wires and layout that go from the input jacks to V1 and V2,
    • check the shielded wires and layout from the tone pots,
    • there is always the possibility of a bad solder joint,
    • it is possible that the capacitor can has developed a problem even if it isn't that old. It should be checked to rule it out.

    Of course, there are always other possibilities and the hum can be tracked down with test equipment.

    My Amp Is Picking Up Radio Stations(top)

    An amp is designed to amplify whatever signals find their way into the circuits. Unfortunately this can include different types of high frequency noise which includes radio stations. There are different measures that can be taken to eliminate or minimize this problem but there isn't single solution. If you are close to a transmitter that is powerful enough to overwhelm whatever measures you put in place, the radio signal is going to get into the amp.

    In general, amps are designed to minimize noise pickup. Some amps do a better job at doing this than others. This is accomplished through circuit design and layout, shielding wires in low level signal stages, shielding the chassis, and lead dress or where wires are placed and run within the chassis.

    It is important to have a resistor called a grid stopper, which is placed in series with the input of at least the first tube stage of each channel. It serves several purposes, one of which is that in conjunction with the input capacitance of the tube, it creates a low pass filer which is tuned to eliminate or at least reduce radio frequency interference. Basically it controls the high frequency roll-off of the input stages of the amplifier. The value of the resistor determines what the roll-off frequency should be. Targeting frequencies 20KHz and above will reduce radio frequency interference.

    Below is a schematic showing the inputs of channels 1 and 2 of a 1963 B-15N. Some amps have grid stoppers, some don't. Those that do usually have the resistor mounted at the input jack for convenience but some have them mounted at the tube socket of the tub's input or grid. This is most preferable. Having the grid stopper at the tube input takes care of any interference which could be picked up in the shielded cable between the jack and the tube. Channel 1 has two input jacks, each with a 120K Ω resistor connected to the grid, pin-1, of the 6SL7GT tube. This is the grid stopper. Note that Channel 2 doesn't have a grid stopper which can lead to channel 2 picking up radio stations in these amps.

    B-15N input stages

    The wire running from the input jack tip to the tube socket input terminal should be shielded. The effectiveness of the shielding on cables can vary with how tight the braid is so it is important to use a good quality cable. No cable will offer 100% shielding. The input from the instrument is a very low level signal and, because of this, it is susceptible to picking up noise. The shielded cable within the chassis acts like an extension to the shielded instrument cable. In some amps, I've found that the shielding has rusted. Rust is actually a good insulator but along with the rust comes brittleness and the shielding can break if the wire is moved and can even crumble when subjected to vibrations which means that the braid isn't acting as a shield. If there are any signs of corrosion or if you are in doubt, change the cable. Only one end of the outer shield should be grounded, otherwise you can introduce hum caused by a ground loop. I prefer to ground the shield at the input jack rather than at the tube because it is easy to ground on a jack terminal. The center signal wire that extends beyond the shielding at the ends should be as short as possible to avoid exposure to noise. The resistor lead wire that connects to the tube socket should be as short as possible. The first image below shows the original gray shielded wires in a B-15N. The second image shows a 100K ohm, 1/2 Watt grid stopper resistor added to the end of the shielded cable connecting to the channel 2 input (pin-1 of V2). Heat shrink tubing was used to insulate the solder joint and prevent any shorting both to the shield and anywhere else. The third image shows replaced shielded cables. The new better quality wires helped make a quiet amp even quieter. If you replace any of these cables, note how the original ones were run and position the new ones along the same path. The position of the wires, so called lead dress, can determine how quiet the amp is. Poor lead dress, in the worst case, can also cause oscillations to be induced in the amp.

    Original gray shielded wires.

    Grid stopper resistor, 100K ½W, connected to pin-1, the tube input to reduce radio frequency interference.

    Replaced shielded cables.

    The value of the resistor will determine what the high frequency roll-off will be. A frequency range of 20KHz to 25KHz is a good target. In the B15N they used a 120K ohm resistor. In the example above, I used a 100K ohm resistor. There is room for variation. You can calculate what resistor to use. R = 1/ (2 * PI * f * C). C is the input capacitance of the tube in farads, f is the low pass cutoff frequency in Hz, PI is constant 3.14, R is in ohms. The input capacitance C = CGK + (CGA * A) where A is the voltage gain of the stage, CGK is the grid-to-cathode capacitance and CGA is the grid-to-anode capacitance of the tube. The latter two values are found in the tube's data sheet.

    For a 12AX7 with a gain stage designed with A to be 60, C is about 103 pF. Let's say you want to cut off everything above 20 kHz. R = 1/(2 * 3.14 * 20000 * 103 * 10-12), which is about 80 K ohms. In the case of a B15 N with a Sylvania 6SL7 GT they used a 120 K ohm resistor. CGK = 3.0 pF, CGA = 2.8 pF, so C is about 73 pF, A is around 25 for this amp. The high frequency roll-off would be around 18KHz.

    If the grid stopper doesn't work the radio frequency interference might be simply too strong to contend with. There are other measures, depending on the offending frequencies, such as employing ferrite beads or RF shunt capacitors at the input of the amp that can be tried.

    How Do I Connect More Than One Portaflex Amp Together? (top)

    In order to connect more than one amp (run a second amp as a slave amp) you can connect an instrument cable from the "ext amp" jack on the back of the B-15N to the "ext amp" jack on another portaflex. In this scenario, you will be using the pre-amp of the amp your instrument is plugged into to control the power amps on both amps. Only the volume and tone controls on the amp that your instrument is plugged into will be effective as you are bypassing the slave amp's pre-amp.

    If you connect an instrument cable from the "ext amp" jack on the back of the amp that your instrument is plugged into, to the input of any another amp, you are serially daisy chaining the pre-amps. The tone controls and power amps on both amps will be working. In this case you have to be careful with the gains. If the first pre is set too hot, the second pre will distort.

    You can also plug your instrument into channel 1 input 1 and plug another instrument cable from channel 1 input 2 to the input of the second amp. Here you are splitting the input, similar to what was mentioned above. Some amps have two inputs that are slightly different in terms of input attenuation, other amps have what is called a normal and Bright input on the channel. The bright input passes the signal through a high pass filter so it sounds brighter. One concern in general, depending on your amp. If input 2, for example, is a bright input, and the bright signal is the one chained to the input of the other amp, you will have to boost the low end on the second amp if you want them to sound the same.

    No issues if the amps are not the same. No issues if you are running a second cab on either amp.

    Is There a Way That I Can Use Just the Pre-amp and Not the Power Amp? (top)

    There are different ways to do this, they all involve silencing the power amp. Several Ampeg amps have a jack on the back labeled EXT. AMP. This is a pre-amp out/power amp in jack. Some amps have two jacks, they are often simply wired in parallel but not always. You connect a (shielded) instrument cable to the EXT AMP jack, the other end can go into a power amp or a DI. The problem is, the power amp is still functioning so sound is going to come out of the speaker. With tube amps require that a load is connected, The options are, disconnecting the speaker and connecting a dummy load in its place, or disabling the power amp. The power amp can best be disabled by removing the power tubes or by removing the phase inverter tube, this is V3, the last tube before the power tubes. Tubes should only be removed or reinstalled with the power off. Later revisions of the B-15N, that were introduced in early 1965, has a safety circuit that disabled the phase inverter and shut down the power amp if the speaker cable was not connected to the speaker cabinet. With these amps you can simply disconnect the speaker cable and connect to the pre-amp out. The safety circuit changed again around 1968 and you can't do this. It is best to check with a tech to verify which B-15N revision you have and how your amp is wired before doing this to avoid damaging your amp.

    What Is the Input Impedance Of a B-15N (top)

    Most pickups will do just fine when plugged into a B-15. Players with some Piezo transducers installed in the basses often use dedicated preamps that can plug into the B-15N amp. Having the proper impedance at the input of the amp will ensure that the full bandwidth that the pickup delivers will be amplified and that there will not be any high frequency roll off.

    Impedance is a complicated issue. So what is the input impedance of the B-15N? The early B-15N has three impedances, depending on which input you plug into.

    The instruments pickup, tone controls, and instrument cable form an output impedance that the input of the amp sees. The amp has an impedance at the input. All this is frequency dependent. At low and mid frequencies, the amp behaves one way. At higher frequencies, the input capacitance of the tubes comes into effect, like adding a capacitor in parallel across the input, and this affects the frequency response of the amp.

    How do you figure out the input impedance? Let's look at the B-15NC input stage. The top jack is channel 1 input 1, the bottom jack is input 2. There's a 120K resistor at the input and a 5.6M resistor across the tube input. The input resistance effectively sets the impedance that the instrument sees. But it gets more complicated. Things happen when you plug in because of the resistors in the input circuit.

    B-15NC channel 1 input stage.

    Rearrange the input 1 circuit and you get this.

    The 120K and 1M resistors are in series so they add.


    Now you have two resistors in parallel, this works out to be about 1M. Put a big resistor in parallel with a small resistor and the total resistance is about that of the small resistor. The input impedance of the tube is much bigger. So the total input impedance works out to be about 1M.

    At low and mid frequencies, the input impedance that the instrument sees will be about 1M. Turns out that this is a good value and is used by a lot of amps. Above 1M and resistors get noisy so keeping it around 1M or less is optimal.

    The 120K resistor does a few things. It is called a grid stopper. It prevents oscillations within the tube, limits distortion, and very importantly, at high frequencies it combines with the input capacitance of the tube to form a low pass filter that limits radio frequencies from getting into the amp.

    Plug into input 2 of channel one and the impedance changes. Now you have the 120K input resistor and 5.6M across the input of the tube. Now the total impedance is around 5.6M. There will be a bit more noise as a result. So the B-15 has a low and high impedance input on channel 1.

    What about channel 2? There is no grid stopper resistor so that channel is more susceptible to picking up AM radio. It has a 2.2M input impedance.

    The B-15 is a versatile amp with 1M, 2.2M, and 5.6M input impedances. Now you know one reason why there are three different inputs. Different pickups will be more suited to different inputs. The impedances will affect the frequency response of the amp. If the input impedance of the amp is not well suited to the pickup, the amp will not deliver the full range of frequencies that the pickup is supplying.

    A 1M input impedance is pretty standard in the industry. I haven't looked into it but it wouldn't be unreasonable to assume that Ampeg would have wanted inputs for both their PEG pickup and an accordion to make the amp as versatile as possible. I think that the impedances were chosen for very specific reasons.

    Here's an excerpt from an early B-15N manual, I assume that when they refer to the diagram, they are talking about the lettering on the amp:

    Now plug your instrument into an appropriate input on the panel. All Portaflex Bassamp models are two channel amplifiers, each having its own set of volume and tone controls. While more than one instrument can be played through an amplifier at the same time, lets consider its use with a single instrument for the present. The illustration of input designations should help you select the proper input for your particular instrument. Electronically, these inputs are designed to favor certain instruments, i.e., some, such as the accordion, require a stronger bass response, especially on the left hand, where a mellowing, organ-like sound is desired. Many guitarists desire a crisper sound. The diagram is merely a guide. Experimentation will determine your preference.

    Clearly, they were trying to market the amp for different instruments with different requirements.

    How Do I Connect an Extension Cabinet and What Impedance Should It Be? (top)

    Connect an 8 ohm cabinet to the EXT SPKR with a speaker cable.

    The images below illustrate how it works. There are two taps on the output transformer, 8 and 16 ohms. The main portaflex cabinet has an 8 ohm speaker and it is connected to the 8 ohm tap. The signal path is shown in the second image below. The signal runs through a switch on the output jack. This is commonly found in a lot of amps and can lead to problems if the switch contact is oxidized or Dirty which could affect the sound of the amp. Keep those contacts and jack shunts (switches) clean by applying Deoxit at least once a year. When you plug in to the EXT SPKR jack, the main and extension speakers are connected in series and switched to the 16 ohm tap on the output transformer. The signal path is shown in the third image.

    [*]Output Speaker Connections

    Jack shunt is circled in green.

    It is important to ensure that the jack shunt contact is not oxidized. When a plug is not inserted, the signal flows through the shunt on its way to the speaker as shown in yellow below. Oxidation can be like adding a resistor in the signal path. Too much oxidation and it can cut off the speaker completely. Cleaning the contact can be like lifting a blanket off the cabinet. It restores clarity.

    When the B-15 is connected to the standard cabinet, the signal flow is shown in yellow.

    When an external eight Ω speaker is connected to the ext. spkr. jack, the two 8 Ω speakers are connected in series and switched to the 16 Ω tap.

    Can I Use a 4Ω Cabinet With My B-15N?(top)

    Yes, but there will be an impedance mismatch. Normally, the speaker installed in the combo cabinet is eight ohms. The B-15N has an external speaker jack which is intended to by used with the main speaker. In order to keep the impedance properly matched, an extension cabinet should be rated eight ohms as well.

    You can connect a four ohm speaker cabinet in place of the combo cabinet. This will result in an impedance mismatch but people do this with no apparent harm to the amp. It will affect the tone a bit, the output transformer will run a bit hotter, and long term the power tubes will not last as long. All this depends on how hard the amp is pushed.

    One concern is that if the output transformer is original and time has taken a toll on it, running it hotter could lead to problems. The coating on the wires inside the transformer could be cracked which might result in internal shorts. Use your hand to gauge the temperature and common sense to determine if the can is getting too hot. If you have a modern replacement output transformer in your amp, there shouldn't be a problem.

    To connect a four ohm cabinet to the amp, you will need a four-pin to 1/4" plug adapter cable. The adapter will need to short pin-2 and pin-3 together to allow the amp to come out of standby; pin-1 connects to the 1/4" tip, pin-4 connects to the sleeve. Pre-made cables are available from fliptops.

    Protecting the Rectifier Tube From Flashover (top)

    If you ever see your 5AR4 rectifier flash brightly when you turn on your amp, you could be damaging or shortening the service life of your expensive tube. This is called flashover and is due to arcing within the tube, never a good thing. The 5AR4 rectifier tube is basically two diodes. The tube is used in a circuit that converts AC to DC. When the tube conducts current in the reverse direction, flashover occurs when the inverse peak voltage applied to the tube exceeds its critical or inverse breakdown voltage. This can occur when the current demand from the power supply capacitors exceeds what the tube can deliver. This can sometimes be seen when new capacitors with a high current capacity are installed in the amp or when the value of the capacitors are bumped up. It tends to occur when the amp is turned on when the power supply capacitors are drained. This is when the current demand is highest. When the amp is warmed up and operating, you shouldn't see any flashover. At one point Ampeg lowered the value of the first power supply capacitor in the B-15N from 40uF to 30uF. This was to protect the rectifier tube. But a higher valued capacitor helps in the low end. There are always tradeoffs.

    There's a simple solution, increase the peak inverse voltage spec of the tube. This can be done by adding a diode in series with the plate of the rectifier tube. Peak inverse voltage is additive so the diode's peak inverse voltage spec is added to that of the tube. A schematic and wiring diagram are provided below. Be sure to tap off the voltage for the bias circuit from a point that is before the diode as indicated in the schematic. The diodes can be wired to unused terminals on the tube socket which makes mounting them easier. I use UF5408 diodes but a 1N4007 will also work.

    Rest assured that this does not convert your amp to one with a solid state rectifier. The tube rectifier will still do its job and sag the way that it did. The diodes are in the power supply and not in the signal path. The diodes only serve to boost the function of, and help protect an expensive rectifier tube by increasing the peak inverse voltage spec. This modification has been around for a long time and Fender uses this in some of their current tube amps so you can consider the design well tested.

    It should be noted that flashover within the tube can also be a sign of a bad tube. It can sometimes be seen when the rectifier tube needs to be replaced.

    Diodes in series with the tube rectifier plates.

    Diode wiring diagram. Common 1N4007 diodes can be used and the relatively Thin leads are easy to wire onto the socket.

    An alternative to the diodes as a means to prevent flashover, would be to install a choke in the B-15's power supply. The choke acts as a buffer resisting change, decreasing the ripple current and lowering the noise in the amp. It helps stiffen the power supply.

    Amp Problem Troubleshooting(top)

    There are many different noises related to the operation of an amp. They can be electrical and generated within the amp or external and picked up by the amp. They can also be mechanical caused by something vibrating in the room or with a speaker or cabinet. Tracking down these problems can drive you nuts! Cabinet issues will be dealt with in the cabinet section. Let's examine a few possibilities.

    Tube Related(top)

    Tube service life depends on usage, not their age. With light and even moderate use, tubes can last many years. If an amp is cranked up and used often, tubes will wear out faster. How long they will last will vary. Some players will change their tubes every one to two years on a regularly gigging amp as a preventative measure. Others insist on a fresh set of tubes when going into the studio. The performance of tubes will change with time. Some amps sound great with new tubes in them and then slowly deteriorate with time losing, for example, clarity and tone. Other amps are less affected this way. It depends on the design of the amp. Signs of tube wear include:

    • loss of tone, clarity, Sustain and harmonic richness
    • inconsistent output level
    • lack of mid-range Punch and Definition
    • rattling, whistling or humming
    • feedback or metallic sound on certain notes
    • Weak sound and power loss.

    One thing to note is that the bias required by the tube in fixed bias amps will change as it ages. With new tubes, I like to check the bias after about three months of regular use. Bias in a gigging amp should be checked at least once a year. Bias will also change with the line voltage so if your amp sounds off, it could be attributed to that. It could also be related to the acoustics of the room, the humidity, or how swollen your ears are.

    See the tube section for additional details .

    Amp Voltages (top)

    It helps when trouble shooting an amp problem, to compare measured voltage readings with known reference voltages. Some Schematics have reference voltages on them, not all do. If you don't have a set of reference voltages for your particular revision, you can gain insight by examining voltages on a schematic that is close to yours.

    Below is a set of reference voltage readings taken from a B-15NA that was converted to a B-15NC. The B-15NA reference voltages are provided as well as voltage readings with a NOS Mullard 5AR4 tube rectifier installed as well as Weber WS-1 (no sag) and WZ34 (5AR4 emulator) solid state rectifiers.

    B-15NC Voltage Readings with NOS Mullard 5AR4 Rectifier
    Tube Pin-1 Pin-2 Pin-3 Pin-4 Pin-5 Pin-6 Pin-7 Pin-8
    V1 6SL7GT. 111 1.21 . 177 2.25 6.3 VAC 6.3 VAC
    V2 6SL7GT . 111 1.28 . 176 2.26 6.3 VAC 6.3 VAC
    V3 6SL7GT. 277 3.00 . 270 2.75 6.3 VAC 6.3 VAC
    V4 6L6GC. 6.3 VAC 420 408 .. 6.3 VAC 32.6
    V5 6L6GC. 6.3 VAC 418 408 .. 6.3 VAC 32.6
    V6 5AR4 rectifier. 5.0 VAC . 353 VAC . 354 VAC . 428

    B-15NC Voltage Readings with WZ34 Rectifier
    Tube Pin-1 Pin-2 Pin-3 Pin-4 Pin-5 Pin-6 Pin-7 Pin-8
    V1 6SL7GT. 112 1.23 . 178 2.30 6.3 VAC 6.3 VAC
    V2 6SL7GT . 112 1.30 . 178 2.30 6.3 VAC 6.3 VAC
    V3 6SL7GT. 279 3.00 . 273 2.78 6.3 VAC 6.3 VAC
    V4 6L6GC. 6.3 VAC 427 414 .. 6.3 VAC 33
    V5 6L6GC. 6.3 VAC 425 413 .. 6.3 VAC 33
    V6 WZ34 rectifier. 5.0 VAC . 358 VAC . 358 VAC . 433

    B-15NC Voltage Readings with WS1 Rectifier
    Tube Pin-1 Pin-2 Pin-3 Pin-4 Pin-5 Pin-6 Pin-7 Pin-8
    V1 6SL7GT. 118 1.27 . 186 2.38 6.3 VAC 6.3 VAC
    V2 6SL7GT . 117 1.35 . 185 2.39 6.3 VAC 6.3 VAC
    V3 6SL7GT. 292 3.17 . 286 2.90 6.3 VAC 6.3 VAC
    V4 6L6GC. 6.3 VAC 441 429 .. 6.3 VAC 34.5
    V5 6L6GC. 6.3 VAC 440 429 .. 6.3 VAC 34.5
    V6 WS1 rectifier. 5.0 VAC . 359 VAC . 360 VAC . 453

    B-15NA Voltages from Schematic
    Tube Pin-1 Pin-2 Pin-3 Pin-4 Pin-5 Pin-6 Pin-7 Pin-8
    V1 6SL7GT. 110 1.40 . 185 2.50 6.3 VAC 6.3 VAC
    V2 6SL7GT . 110 1.40 . 185 2.50 6.3 VAC 6.3 VAC
    V3 6SL7GT. 292 3.35 . 309 3.10 6.3 VAC 6.3 VAC
    V4 6L6GC. 6.3 VAC 470 450 .. 6.3 VAC 36
    V5 6L6GC. 6.3 VAC 470 450 .. 6.3 VAC 36
    diode rectifier. . . . . . . 470

    All voltage measurements are DC unless otherwise noted. The measured line voltage varied between 117-117.8 VAC which accounts for some slight variations in the readings.

    The B-15NA readings were taken on a modified amp. The following changes and enhancements were made to convert the B-15NA to a B-15NC:
    • removed rectifier diodes and added an octal socket to the chassis to provide option of tube or solid state rectification
    • first power supply node has a high current capacity 41uF capacitor, Panasonic HA series, 400V, 105 degree C
    • added UF5408 or 1N4007 diodes from high voltage secondary to each tube rectifier plate to prevent flash over
    • removed safety interlock loop to the speaker cabinet, hardwired high voltage center tap ground, moved standby to after rectifier
    • rewired power and ground bus to use a single piece of wire rather than separate wires to jump from eyelet to eyelet
    • rewired speaker output return to ground point of phase inverter
    • added 3-prong power cord
    • added 120K 1/2 Watt grid stopper resistor to channel 2, V2, pin-1

    PT-108 specifications:
    • 750 VCT (375-0-375) secondary, 150mA, 5 VAC @ 3A with 117 VAC primary. Measured: 744 VAC (372-0-372) unloaded
    • DCR average of one half of PT secondary winding: 55.5 ohms (measured)
    • DCR PT primary winding: 2.5 ohms (measured)
    • PT secondary average high voltage to center tap voltage / PT primary voltage: 372 / 117 = 3.18 (measured)

    Total impedance presented to rectifier by power transformer:

    RS = RSEC + N2RPRI + RA


    RS is the plate supply resistance per plate,

    RSEC is the DC resistance of the high voltage secondary per section,

    RPRI is the DC resistance of the primary,

    RA is the DC resistance per plate of the added series resistance,

    N is the transformer step-up turns ratio per section.

    Therefore, RS = 55.5 + (3.18 * 3.18) * 2.5 = 80.8 ohms per plate.

    The 5AR4 tube data sheet (1959 GE) specifies that at 372 volts, RS should be around 112 ohms for a capacitor-input filter (as opposed to an inductor-input filter). So limiting resistors in series with each anode (plate) of around 27 ohms @ 5-10 W should be used. Limiting resistors are intended to protect the tube rectifier and are seldom seen in musical instrument tube amps. These are not used in the Portaflex amps.

    For reference, here is the original B-15NA schematic.

    B-15NA Schematic

    Static, Scratchy or Popping Sounds(top)

    Static, scratchy or popping sounds can be due to a number of issues that may or may not be related. When debugging any problem, it's a good idea to break it down into subsections that can be examined one at a time. It is best to try the simplest, easiest things first.

    Plugging into another amp would eliminate your instrument and the cable as the cause of the noise. If you can't do that, try changing your instrument cable.

    Now you've narrowed the problem down to either the amp or a source of noise the the amp is picking up. Moving the amp to another location or another room on a different circuit breaker will help. This can help you determine if some external influence is being picked up by the amp and is being amplified. Sources of external noise can include faulty appliances, air conditioner fans or compressors, fridges, heaters, fan motors, washers, driers, wiring in walls, junction boxes, circuit breaker boxes, fluorescent or other lighting transformers, light dimmers, even a power line transformer out in the street, the list goes on. You can use a portable AM radio tuned between stations and use it as a noise detector. Move it around the room, close to outlets, junction boxes, any electrical panels, lighting fixtures, etc., and listen for noise to identify the source.

    Let's assume that external influences have been eliminated and the problem is within the amp. The first thing to do is to change the tubes. Start at the pre-amp end and swap in and out a replacement one tube at a time and test the amp. Again, changing the tubes is a simple thing that can easily be done.

    In the case of a B-15, there are three 6SL7 tubes in the first three positions, V1, V2, and V3. V1 is channel 1, V2 is Channel 2, V3 is the phase inverter. If you don't have any spare tubes, pull V1 and plug your instrument into Channel 2. If the noise goes away you know that the tube in V1 has a problem. Otherwise reverse what you did and pull V2 and plug your instrument into Channel 1. If the problem goes away you know that the tube that was in V2 was the problem. Of course, there is the possibility that both of these tubes have a problem. In that case you are no further ahead. If you can assume that the V2 tube is good, try it in the phase inverter, V3, position and plug your instrument into Channel 1. This allows you to test the first three tubes simply by swapping.

    The next step is to give the amp a general checkup. Clean the inside. Dust and dirt can act like tiny resistors and capacitors, bridging circuits and causing shorts. Apply Deoxit to all metal-to-metal contacts. This includes input, output, and effect jacks, pre-amp out/power amp in jacks, tube pins and sockets, and the pots. Follow the directions, do not use more than is necessary and leave a protective coat behind. Sometimes it takes more than one application to scrub the oxidization away so look closely. Examine all the components and look for discoloration, charring, or something leaking from them. This is a sign of a bad part. Don't forget to apply the Deoxit to your instrument jack and volume and tone pots as well. They are susceptible to the same type of oxidation and dirty potentiometer problems that amps are.

    Some tubes have pins that are larger than others. If you have been tube swapping, it is possible that the tube socket contacts were stretched out. If this occurs and you install a tube with smaller diameter pins, the contacts will not be good and intermittent connections can cause a crackling static type of noise. If this occurs, the tube sockets need to be re-tensioned. This is a simple process that involves removing the tubes and pinching the socket terminal contacts closed. The metal can be brittle so do this carefully. I use a pair of small forceps, a dental pick, or a small screwdriver to pinch in the contacts. You want to close the contact evenly. Here is the process explained. Keep in mind that if you bend the metal too much, it could break. Changing a tube socket can be time consuming. Re-tension only if you need to. You can tell fairly easily if the tubes are making a good contact with the tube sockets. If they feel loose when inserting or pulling the tube, they need to be re-tensioned. If the contact feels solid on ll the pins, it's fine.

    I mentioned cleaning the pots with Deoxit, this is discussed in the next section. If the scratchy problem persists when you move the knobs, there may be a leaky coupling capacitor. DC voltages in the circuits, can cause a scratchy sound. This is especially noticeable in the pre-amp and tone stages. It is often easier and more cost efficient to simply install new capacitors, one at a time, and see if this resolves the problem. A capacitor tester that operates at high voltages can be used to test a capacitor for leaks. This requires removing the capacitor from the circuit. Here is a DIY capacitor leakage tester.

    Another source of pops and static is a bad solder joint. Examine all the solder joints looking for solid shiny connections. Check component leads and wires at joints to ensure that they can't be pulled out. Give them a gentle tug with a pair of small needle nosed pliers or tweezers. If necessary, use a soldering iron to re-flow any suspect solder joints. Look for any solder bridges that shouldn't be there. Even a single strand of a multi-strand wire in the wrong place can cause a problem. Use a dental mirror and light to examine under the boards as well. This is especially important to do with the eyelet boards that are found in older amps.

    Next examine plate, screen, and cathode resistors in the power amp. Any old resistors, especially carbon composition ones that run hot can go off spec. If in doubt, change them.

    A bad transformer is rare. The worst case scenario is intermittent shorting within a transformer. This can cause pops and other types of noise. If the transformer is not potted (unlike the B-15), look for signs of black carbon marks, like pin pricks, in the fish paper that is against the windings. If there is a short near the surface, you will see evidence of it. Shorts deeper inside the transformer coils are more difficult to detect. Sometimes you can hear a popping sound from the transformer. If you are in doubt, Mercury Magnetics will test your transformer if you pay for the shipping.

    Cleaning and Lubricating a Potentiometer: How and Where to Apply Deoxit(top)

    If a potentiometer sounds scratchy when you rotate it, it could be that it needs to be cleaned and lubricated. Deoxit is often recommended by manufacturers and techs for accomplishing this. See Preventative Maintenance for additional details.

    There are a few simple but important details to keep in mind when cleaning pots. Apply the Deoxit D5, this is a 5% cleaning solution with 75% mineral spirits for flushing, where it will be effective, don't apply too much, and use it only when necessary. Chemicals can rejuvenate a pot but they can also destroy it. Deoxit also makes a Fader Lube, F5 which will lubricate clean surfaces. Follow the recommendations of the manufacturer.

    The attached image shows where to apply the deoxit and where not to. Not all pots have an access port where the deoxit can be applied, some are sealed. With sealed pots, you can sometimes bend back the tabs and remove the back case to gain access to the resistive element and wiper. With CTS pots like the ones used in the vintage B-15N, it's best to apply the Deoxit where the solder terminals are. Apply a drop or small spritz inside the pot and work the pot back and forth to spread it. It helps to use a small plastic tube so that the Deoxit can be better deposited inside the pot. Follow the instructions and do not apply more than is required.

    The knurled adjustment shaft has lubricant at the top where the it enters the body and at the opposite side where it indicates not here in the image. DO NOT apply Deoxit in either of those places. If the lubricant is dissolved by the Deoxit and migrates onto the resistive element or wiper, it can destroy the pot.


    As the pot is used and with time, oxidation, dust, and other gunk build up and make the pot noisy and rotation sluggish. The following links show the anatomy of a pot: The Secret Life of Pots and Repairing old potentiometers. The links show you where the surface of the resistive element and the wiper are. There are the parts that need to be cleaned. When cleaning a pot, visualize where these parts are so you can apply the deoxit where it is needed.

    When a pot can't be cleaned from the outside, you can carefully disassemble the pot and do a better job of cleaning it from the inside and sometimes save having to change the pot.
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