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Tips, Tricks, and Modifications


Feb 17, 2017
Tips, Tricks, and Modifications

  • How to Date a Portaflex Amplifier (top)



    Some companies such as Gibson and Martin have kept meticulous records of production dates and serial numbers for their instruments. Amplifier companies tended to not fall into that category so it is harder to date their products. There are different ways to estimate the age of a Portaflex amp, they all provide part of the story. Looking at the serial number is one way to start. The earlier portaflex amps had the serial number on a label inside the cabinet. It wasn't until later that a serial number was also stamped on the left lower side on the back of the amp chassis. The next step is to look for manufacturing date codes on the components. A date code is stamped into the metal housing of the potentiometers. They were manufactured by Chicago Telephone Supply, aka CTS. Look for a code that begins with 137, for example 1376340. 137 is the EIA manufactures' code for CTS, the next two digits are the year, in this case 1963, and the last two are the week of the year, in this case 40. So this pot was manufactured between September 30 and October 4 of 1963. Older pots have a single digit for the year. If the cap can Capacitor (the metal tube next to the 5AR4) is original, there is a similar date code, the Lord shock mount has a month-year date code, as does the speaker frame. Sometimes there's a piece of paper under the transformer with a date code as well. Some Tubes have date codes but they are less reliable for estimating the age of the amp.

    When you compare the serial number date with the component codes, you find that there was a short lead time between when the components were manufactured and when the amp was built. If you take the most recent date code and add a month or so and you have a good estimate as to when the amp was built. Of course, there is always the possibility that someone sourced a replacement part with an date code that is close but newer but from the same era. For that reason, it is important to consider all the date codes when deciding. Some people go as far as examining solder joints to determine if they are original. This could indicate that a part hasn't been changed. But there are ways to fake solder joints so that isn't always useful. My best advice is to consider the amp as a whole and take everything into account when trying to come up with an estimate.

    Dolly (top)



    Although they recommend that the dolly be removed when using the amp, it isn't necessary unless the casters are vibrating.

    Excerpt from the B15N owner's manual:

    All Portaflex Models (except B-12-N) are equipped with detachable shock-mounted dollies. Important: Remove dolly while amp is in use. Because the dolly will absorb vibrations, it will also absorb sound waves. There is a definite gain of power if the amp has a Solid footing. You may seemingly get enough power with the amp on the dolly, but the speaker is working harder than it should.

    The dollies had two fixed casters and two that could rotate through 360 degrees. This prevented the amp from rolling forward on stage while performing. Otherwise a slight tug on the the instrument cable could move the amp. On smaller amps, the dolly was attached with a bolt with a handle. It is important to not over tighten the bolt. It can cause the dolly to bend in towards the cabinet and this puts a lot of pressure on the bolt. So much so that it can lock it in place and make it difficult to unscrew. Pressing the dolly towards the cabinet can relieve the pressure from the bolt.

    Gasket Foam Tape(top)


    Gasket tape is the rubber seal at the top of the cabinet. The purpose of the foam is to create an Air Tight seal around the lid. With time, the foam becomes less resilient and compresses, loosing it's sealing effectiveness. When this happens, it should be replaced because leaking Air could produce cabinet noise and vibrations.

    The tape is a closed cell dense foam ¼" X ¼". It is available from Fliptops. You may also find it at a local car parts supplier or hardware store. Depending on what you find, you might need to use a razor knife to trim it to a width that fits in channel at the top of the cabinet. On vintage amps, the foam was glued into place which makes removing it a bit of work. It is important to remove as much of the old residue as you can so that the new foam will adhere well. Start with a Wet rag and finish with alcohol or acetone on Q-Tips. Be careful not to get the area too Wet, the blue stain around the channel is water soluble and comes off fairly easily. You might need to touch up the stain if too much comes off. Water soluble stains are available at hardware and paint stores. Colors can be mixed to adjust the color.


    Cabinets that have dolly mounting hardware have a rubber disk glued over the hole on the bottom of the cabinet to prevent leaks. These are commonly missing on vintage amps. The glue fails and the rubber disk pops off and gets lost. Rubber can be found in the roofing department of a hardware store, a scrap of tire tube can be used as well. I use a 50mm (2 inch) metal washer as a template to Cut the disk with a razor knife.

    Amps that have connector and crossover panels like the B-15R have the panel sealed with a gasket and silicone sealant. Cabinets with ¼" jacks should employ air Tight jacks to avoid leaks.

    Speaker adapter - Four-pin to 1/4" (top)


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    Speaker adapter - Four-pin Extension Cable (top)


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    Diagrams compliments of Mark Gandenberger of Vintage-Blue.

    This is the wiring diagram for two extension cables for portaflex cabinets that use the four-pin connectors. There are two versions, one using Belden 8454 four-conductor cable. This cable is very close to the B-15N speaker cable that is found on the vintage amps. The specifications are four-conductor, 18 gauge, stranded wire, rubber jacketed, 300V, diameter 0.265". This cable can be difficult to source by the foot and is expensive to buy an entire roll. Normally the cable is available from Surplus Sales of Nebraska.

    An alternative cable that uses only two-conductors is shown. Hardware stores such as Home Depot sell a two-conductor power cable by the foot. It is made by Carol (General Cable), 18 gauge, rubber jacketed. The drawback of using a two conductor cable is that it defeats the safety circuit that prevents the amp from operating in the event that it is turned on without a speaker cabinet connected. This safety circuit helps protect the Output transformer and power Tubes from being damaged. If you use this circuit, be careful to ensure that the cable is properly connected to the speaker cabinet before turning on the amp. If you do, you will be fine.

    EXT AMP: Preamp Output/Power Amp Input (top)



    The Ext Amp jack serves two purposes. It's a preamp out that can be sent to another power amp or to a DI. It is also a power amp input. Here is a quote from the mid-60's B15N owner's manual:

    If you feel you need more power than your equipment is designed to produce, you might consider obtaining an additional PORTAFLEX. All Portaflex Models are equipped with an external amplifier jack. This enables you to hook two complete PORTAFLEX amps in tandem, which, depending on the model, can double your output wattage. This tandem set-up is accomplished by using a regular mike cable with a phone plug on each end, connected to the external amp jacks of any two PORTAFLEX units; or, you may plug the other end of the cable into the input of any amp. If used in this manner, adjust the volume on the PORTAFLEX first, then adjust the volume on the secondary amp, which in most cases will be a very low setting. What happens is this: The signal from the pre-amp on the PORTAFLEX is also sent into the second amp, so you get the output from both units. If the second amp has vibrato or tremolo, it will sound as if both units have it. This is another way to get more power if you ever feel you need it.

    The earlier and the later B-15's have different preamp designs. The later ones are better at driving DI units via the Ext Amp output. Some people have reported impedance mismatch problems when using the earlier models. With the DI connected to the Ext Amp jack, the power amp looses signal and the volume decreases. If your DI has a high impedance switch on the input, try engaging it and see if it helps. If it won't work, try plugging your instrument directly into the DI, then plug the thru on the DI into the amp's input. The disadvantage of this is that the DI out will not the the B-15's preamp and tone circuit as part of the signal chain.

    Mold Growing on Speaker Cone and Cabinet(top)


    Use a soft brush to remove the visible mold but that doesn't stop it from re-growing, it appears to remove it but it doesn't remove the deeply seated roots.

    Borax laundry detergent is affective at eliminating mold. It comes as a powder that can be dissolved in water. Spray or gently wipe the solution on with a soft rag or brush being careful not to allow the paper speaker cone to get too wet. Older speakers require more care as the paper can be very fragile. Let it Dry and reapply if necessary. Clean the residue away with just water. For mold on the cabinet, use a more concentrated solution with undissolved borax granules in it. The coarser borax will help to grind the mold away. Rinse with water.

    Borax has a secondary effect of removing odors. It helps get rid of that old cab smell. It can be used on the vinyl as well. Just be careful not to get too much water into the seams of the vinyl. If it penetrates deep enough it will dissolve the glue and the vinyl will lift. Then it will need to be re-glued.

    Preventative Maintenance (top)


    There can be lethal voltages stored in the power supply caps within your amp so be very careful if you do any work yourself. Even with the amp turned off, the capacitors can be still retain enough charge to melt a screwdriver. The procedure for discharging an amp is described Safety Wash which is designed for washing circuits. It is a blend of ethyl alcohol, isopropanol and ethyl acetate. Both products are fairly safe to use but be careful. Normally color band and lettering on components are resistant to solvents but not all of them are. Spot Test components to ensure that the color bands and other identification markings do not come off. Do not let these products come in contact with the chassis silkscreen lettering, they can be dissolved in a heartbeat.

    Metal surfaces can oxidize. This can affect the contacts. Deoxit is a good product for safely removing this oxidation. There are different Deoxit products, (available at Antique Electronics). For potentiometers, use D5, this is a 5% solution with a flushing agent or Fader Lube which is a lubricant. I prefer the 100% solution D100L-25C with the needle dropper for everything but the pots. This includes tube pins and sockets; input, output, and effect loop jacks. The needle dropper allows you can apply just the amount that you want with precision. It is an expensive product and this helps avoid waste. You can strip the plastic or teflon sheathing off a wire and attach it to the end of the needle to apply the liquid into hard to reach places.


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    Deoxit D5.


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    Deoxit 100% concentrate D100L-25C for metal on metal contacts.


    Apply Deoxit to all metal-on metal contacts. This includes the jack contacts including the shunts (switches built into the jack), tube sockets and tube pins. Follow the manufacturers instructions. Apply a small amount, let it sit. I then scrub with a small inter-dental brush. These brushes are available at a pharmacy. Be careful to not stretch out the contacts on the tube socket. You want them to have a tight contact with the tube pins. If there looks like there is still oxidization on the metal, repeat the process. Then finish off by applying a small amount again and don't wipe it away. The product has an oil in it that protects the metal. On amps that don't have sealed volume and tone pots like the CTS ones in the Portaflex amps, you can apply a drop inside the pot, see Cleaning and Lubricating a Potentiometer: How and Where to Apply Deoxit. There is an opening near where the solder tabs are. Just apply one drop and then move the knob back and forth several times through the Full travel of the pot to Clean and lubricate. When you tip the amp back into position, some of the Deoxit may run out. Clean it with a Q-Tip. Deoxit will clean and lubricate the pots preventing them from sounding scratchy when you turn them. The product can be used on pots and jack of your instrument as well. I don't use it on the wire wound Hum pots that are in the back of some amps.

    If your amp has a Hum pot, adjust it by ear for minimal hum with the amp on and in playing mode.

    Inspect the components and tube sockets for any sign of charring on the circuit boards. Look for a black soot. This is evidence of burning resistors or arcing problems. An ESR meter and an oscilloscope can be used to examine the health of the electrolytic capacitors. But you don't need a lot of Test equipment. If the amp has developed a hum that didn't used to be as loud, it might be time to change these capacitors. They have a limited lifetime, even when the amp isn't being used. It is a good idea to turn an amp on in playing mode (not on standby) at least every six months for a half hour or so to help reform these capacitors. It will extend their life.

    If your amp's power tubes are fixed biased, the bias should be checked at least once a year. As the tubes are used, they wear down and the bias that they require will change.

    Exterior Cleaning(top)


    Chassis(top)


    I use an automotive product called Autosol Metal Polish on the chrome chassis and other metal parts to remove rust and leave a protective coat. Any chrome metal polish will work. You have to be very careful though. Don't let any of the cleaner come in contact with the chassis lettering or paint. It can dissolve very quickly. Start with the most conservative approach and then try more Aggressive approaches as required. I start by washing the outside of the chassis with a mild soap and water solution using a light touch, dabbing over the lettering. The lettering of a vintage amp can be easy to scratch off so be very careful. Then use the metal polish on the other areas of the chassis to remove the rust and clean and polish the metal. I remove the screws, knobs, nuts and washers first so that they don't get in the way. They can be done individually.

    For badly rusted areas, I use a product called Restore Rust Removal Gel. There are other similar products available. It is a gel based rust remover that sticks to the surface and doesn't run or Dry up while it is working. You can put it on horizontal or vertical surfaces without it running too much. It helps to try to pick away and remove any Loose rust first. These rust removal products are expensive but you don't need to use a lot so they last.

    I remove the metal corners, keeping the nails. I place them in a rust removing soaking solution such as Restore's rust removal concentrate, or a product called Evapo-Rust. The nail heads and corners are polished after the rust is removed.

    It is surprising how well the metal can be cleaned up.

    Vinyl(top)

    When cleaning the vinyl, avoid getting too much water on the seams. Vintage amps used hide glue to affix the vinyl and it's water soluble. This can cause the vinyl to pull away and it will need to be re-glued if it starts to come apart.

    I like to start with the least Aggressive approach and move up to stronger cleaners in order to avoid damaging the vinyl. Again, I start with soap and water and a soft brush. Someone else's tooth brush works well. I use a medical scrub brush because it has a larger surface area and is gentle on the vinyl. Dental picks are good for getting into cracks and the ridges of the Ampeg blue check. As you can imagine, this is a tedious task.

    An excellent cleaning product is Tuff-Stuff general purpose foam cleaner. It works well not only on the tolex but on the chassis. Two other good products are Bon Ami and Murphy's Oil Soap. The Murphy's product can be used inside the cabinet as well.


    Grille Cloth(top)


    The grille cloth takes a beating with time. The best way to protect an amp is to use a slip cover. Not only does it protect the amp from dirt but it also slows down color fading due to exposure to sunlight. Vintage amps that have been stored with a cover on them can look like new after many years.

    When cleaning a grille cloth, take a conservative approach. Amp grille cloths tend to be made out of a synthetic material. Even the metalized threads have a plastic base. Some older cloths are literally a cloth made of cotton, linen, or a similar material. This applies to the synthetic grille cloths. Start with a mild liquid pure soap such as Ivory and water. Wipe the soap solution on and rinse repeatedly. You don't want to soak the baffle under the grille cloth as it can develop ripples in the wood. If that doesn't work well enough, take a more aggressive approach. There are oxi-based laundry products such as Resolve and OxiClean that do an amazing job in bringing back the colors in the cloth. Use them with a little soap in a solution of Warm water. It works even better if you remove the grille cloth from the baffle and soak it. Always test a corner before proceeding. There are no problems with the Ampeg grille cloths. I've let it sit in the cleaning solution for up to an hour but I monitor it from time to time. When clean, rinse the cloth well and let it dry before reinstalling it. If all else fails, try using a bathroom tile cleaner or spray on Easy-Off oven cleaner. These products are strong bases, the primary active ingredient is caustic soda, also called sodium hydroxide, they feel slippery, and require a lot of rinsing to fully remove them from the cloth. Follow the product instructions carefully, wear gloves and work outside if possible. When you are using these products, it is best to remove the grille cloth from the amp for cleaning.

    Smells(top)


    To get rid of smells, I start with a coarse solution of Borax laundry soap. Dissolve the Borax in water and then add some more the isn't dissolved to keep the solution gritty. The Grit acts as a mild abrasive for cleaning the tougher areas. Rinse well. Borax works well for removing mold as well. It can be used inside or outside of the cabinet.

    For more determined smells that won't come out with Borax, I use a product that is available at pet stores for removing urine, skunk, and smoke. These products use enzymes to break down organic based smells. There are many such products, I use one called Odourmute. Follow the directions and use multiple applications if necessary.

    Sometime you just can't get the smell out of the vinyl. When all else fails, the cabinet may need to be recovered.

    Those Odd Looking Screws(top)


    Ampeg used clutch head screws on their vintage amps and cabinets. Someone there must have been a fan of Batman. You need a special screwdriver to work with them. The 5/32" drivers are available at some hardware stores as well as at Fliptops. These screws and a new chassis are about the only Portaflex replacement parts that aren't readily available other than on eBay. The screws were used in some General Motors vehicles, farm tractors, and recreational trailers but finding them in the correct size can be a problem. Often on vintage amps you find that these screws have been replaced with a more commonly available part.



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    Holy Hardware Batman!


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    Clutch Head Screw


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    Cabinet Latches(top)


    Fixing a Vintage Cabinet Latch(top)


    The metal latch that holds the lid to the cabinet can fail. If the metal plate that screws to the wood comes uncoupled from the rest of the latch, it may be able to be fixed. Start by removing the metal plate from the cabinet. There are two fasteners holding it in place. The plate has a ninety degree bend on each side. There are two pins at the top end on the left and right sides that clip into holes on the plate. If one or both of these pins are broken, you'll need to buy a new latch. If the pins are in good shape, then they may have just popped out of their holes. Lie the latch down, with the outside surface against a table. Locate the pin holes on the plate. You may need to use a pair of pliers to bend the metal with the pin holes Open a little so you can easily pop the pins into the holes on the plate. Push the spring down and pop the plate pins in place. Use pliers to bend the metal with the pins holes inwards so that the pins stay in place. You don't want to bend the metal inwards too much so that the plate won't pivot freely at the pins. Be careful using the pliers. Where ever you scratch the metal, it is eventually going to rust. Polishing the latches with a wax, such as a carnuba instrument polish or Nevr-Dull, will help keep them looking good. New latches sometimes need adjusting before they are installed on the cabinet to prevent the pins from popping out.

    Another failure can occur at the claw. The claw clamps onto the keeper that is attached to the lid and holds it down on the cabinet. If the claw is not holding tight enough, the lid isn't pulled down which could result in an air leak or a vibration. With use the claw can be bent out a bit and the assembly loosens. You can use a pair of pliers to bend the claw inward a little to make a tighter contact. Cover the latch when doing this, I use a piece of rubber. You don't want to scratch the plating on the latch, it will eventually rust. This usually fixes the problem. If the contact still isn't tight enough, sometimes you can loosen the latch's back plate, position it a bit lower, and then tighten the fasteners. Another fix is to tighten the square bolt in the lid a small amount. Turning the point upward by a small fraction makes the latch fit tighter.

    If the latch is fine but the lid is Loose enough to cause a vibration, the foam seal under the lid that runs around the opening may have become Compressed. This can happen with age. Changing the foam can correct this.




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    Latch


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    Latch Detail


    Southco Latch(top)

    When Ampeg introduced the Heritage B15 and new Solid state Portaflex amps in 2011 they featured cabinets that utilized a draw latch from Southco that was much improved over the original. Details are provided in the Speaker Cabinet Technical section Southco Latches.

    Vintage Amp Buying Checklist (top)


    What to look for when buying an amp can differ depending on your needs. The cost of any vintage amp should include a checkup from a tech. A tech can look for any potential issues and bring the amp up to spec. Costs to get the amp back into shape can vary depending on the condition of the amp.

    For someone that is looking for a restoration project, you often want to look for all original parts. For someone that is looking for a player, it doesn't matter what has been changed, as long as it sounds good. Many of these amps are in pretty rough shape. Don't let that deter you. The good thing about these amps is that, except for the chassis, all replacement parts are available, even reproduction cabinets. They can be rebuilt to better than new condition from the ground up. Recently the blue check vinyl has become scarce because Loud has moved to black check. Hopefully it will be readily available again in the future. Regular maintenance, including anything that needs to be changed to keep the amp in playing condition should not lower the value of a vintage amp. This includes changing the power supply and bias capacitors, a three conductor power cord, and any components such as resistors. An amp that has recently been serviced is good because it will save you the cost of having this work performed. That should be factored into the purchase price.

    How the amp sounds should be your priority. Start with the tone controls at noon which is flat on these amps. It can be easier to evaluate the amp if you have someone else play while you try all settings on the amp and listen. Slowly move the tone pots through their entire range to ensure that they are working. Try all the inputs. If there are two channels, test both of them. Then play through the amp yourself, interact with it, dial in a good tone setting and see what you think. Keep that in mind when evaluating an amp.

    Here are some things to look for:
    • Ask what the service history has been and if there has been any issues.
    • Check if a three-conductor power cord has been installed. It should have one, this is a safety issue.
    • When were the tubes last changed and how much has the amp been used since then,
    • Have the power supply capacitors been changed. If the amp is humming loudly, it can be an indicator that they need to be changed.
    • With the tones set flat at noon, at what volume level does the onset of Distortion occur. Eleven o'clock is normal. If it is lower, the amp might need a cap job or new tubes.
    • Ask if the speaker is original or if it has be reconed. If a speaker is reconed properly, it is as good as new.
    • Inspect the speaker cabinet's connector. Check the quality of the solder joints inside the cabinet.
    • Ask if any circuit modifications have been performed.
    • Are the knobs original and do all they all look the same.
    • Is there a scratchy sound when the knobs are turned. This could simply mean that the pots need cleaning.
    • Ask if either of the transformers have been changed. It isn't unusual to find a replacement power transformer. This can be a plus because the modern replacement transformers are better than the original ones.
    • Listen for any cabinet rattles and buzzes at higher volume levels.
    • Is the logo on the front of the cabinet original and in good shape.
    • What condition is the vinyl in. When the dirt is removed, the amp will look a lot better.
    • Is there a dolly. Not all Portaflex models came with one.
    • Is there a dust cover. The best preserved vintage amps are usually the ones that have covers.
    • How much rust is there on the chassis. For a chrome chassis, the less rust, the more valuable the amp. Although rust leaves a scar on the chassis, it can be cleaned up and made to look better. A painted chassis can be repainted. Replacing the silk screened lettering is more difficult. It makes it a lot easier if you can leave those surfaces as is.

    Vintage vs Reissue B-15 Knobs(top)


    The reissue knobs are visually close to the originals but are not quite the same. The most obvious difference is that the original knobs on the early 60's amps were made of Bakelite, a hard phenolic plastic that is highly desirable, the modern replacements are lighter plastic. They feel different, the Bakelite knobs have a rougher texture and feel more solid. The reissue knobs are a darker black color that is smoother.

    There have been stories of sellers who advertise the amp with all original parts and then swap out the knobs with new ones before shipping the amp. They then part out the knobs for more profit. If you are buying online, ask for images of the knobs on the amp and ask if they are original. Make sure that you receive what you're paying for.


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    Original Daka Romar skirted B-15 bakelite knob


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    Reissue B-15 plastic knob.

    Vintage vs Reissue B-15 Transformers(top)


    It isn't unusual to find one or both of the transformers, most often the power transformer, have been replaced. New transformers aren't necessarily bad to have. The old ones develop cracks in the protective lacquer on the coiled wires as it breaks down with age, high temperatures, and being pushed too hard. This can lead to internal Hot spots and shorts. The new transformer have better specs and can better cope with the higher line voltages found today. The new epoxy potting compounds do a better job at wicking away the heat from the transformer which helps them run cooler. The transformers that Fliptops sells are made by Heyboer and are excellent products.

    How can you tell if you have an original or reissue transformer? The original transformers have a smoother texture on the metal can. The new ones have a rougher wrinkled texture. It is possible that the transformer was replaced and repotted in an original can. So if you see a smooth can, it doesn't necessarily mean that it is original but chances are it is. If you unbolt the transformer and pull it away from the chassis you can see the potting compound at the bottom of the can. Modern potting compound is usually a hard epoxy, the originals used a black wax like tar that melts if you heat it. A good restoration will use the tar but this is very rare to see.

    The new transformers have a tougher finish that chips less. The texture keeps it looking cleaner.




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    Original B-15N transformer with smooth finish


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    Reissue B-15N transformer with wrinkle finish

    How Can I Get More Clean Volume Out of My Amp? (top)


    This is an often asked question. While great for recording or use in small venues, a 25 or 30 Watt B-15 amp needs all the help it can get to optimize it's clean volume level. There are some tweaks that can help the amp is deliver all it can. It can vary, it is normal for the onset of Distortion to begin at somewhere between 10 and 12 o'clock with the tone controls set at noon. With some amps, it is later. This assumes that the volume potentiometers in the amp are original.

    • Healthy power supply electrolytic capacitors are a must. As capacitors age, their performance decreases, lowering headroom and increasing distortion.
    • The B-15's power supply capacitors varied with different revisions. In early models, the first power supply Capacitor (Node A) was 40uF but it was lowered to 30uF in later models. Using a 40uF/600V, high current capacity, capacitor helps the low end by providing a better reserve to meet the demand of the amp. This increased headroom and lowers distortion. Flashover in the rectifier tube can be an issue with a higher capacitance, so care must be taken when performing this modification.

    • Adding a choke is another way of stiffening the power supply. It also addresses the tube rectifier flashover issue mentioned above.

    • A solid state rectifier module can be used in place of the 5AR5 tube rectifier. This will avoid the classic power supply sag and distortion when the amp is pushed.

    • Pay attention to speaker specifications. A speaker with a high sensitivity over the output impedance range of the amp will be louder. Other specifications are important as well. Everyone has an opinion as to which speaker is best and Ampeg offered a few different options over the years. There are a number of modern options available. There doesn't seem to be a consensus as to which is best. The Faital Pro 15PR400 looks interesting.

    • Use a set of power tubes that are selected to provide a late onset of distortion.

    • If the instrument signal is too strong, it can overpower the first preamp tube and cause it to distort. Lowering the signal level could help reduce the distortion. A low battery in an active bass can also produce distortion. Results will vary with the bass that you use.



    Connecting a DI (top)


    A Direct Injection (DI or direct box) device allows you to convert a high impedance line level unbalanced output signal to a low impedance balanced mic level signal. It allows you to tap off your instrument signal to feed to a mixer or some other piece of equipment. The DI buffers the signal so that it can be sent over long runs while minimizing high frequency loss, noise, distortion and ground loops.

    There are different ways to connect a DI device to an amp.


    • Plug your instrument into the DI and split off the signal to go into the amp and out to the mixing console. The disadvantage of this approach is that the tone of the amp doesn't get sent to the console. This approach is best if you want the cleanest Uncolored signal send to the mixing board.
    • Plug your instrument into the amp and feed the pre-amp output into the DI. The advantage of this approach is that you have the pre-amp's characteristic sound and tone stage contributing to the DI signal. This approach is good if you want the tone of your pre-amp sent to the mixing board.
    • Plug your instrument into the amp and feed the power amp's speaker output into the DI. This is the best approach if you want the Full tone of the amp sent to the mixing board. This can only be done with a DI that is designed to be connected to an amp's speaker out. The Countryman 85 and Radial JDI can both do this.

    The last two methods of connecting the DI are often used. The drawback is that if you change your volume setting, the signal that the mixing console receives is also changed.To ensure a uniform house volume level, the mixer has to adjust the volume every time you adjust them. This can lead to problems.


    Some people prefer to mix a miked amp with a DI. This allows them to take advantage of the strengths of both techniques. The mic provides live ambience, the DI can compensate for deficiencies in the mic's frequency response.

    In the case of a vintage Ampeg amp, the EXT AMP jack on the back of the chassis is a pre-amp out/ power amp in. You can use a short instrument cable to send the pre-amp output to a DI. When you tap off a pre-amp signal, the power amp will still be working so you will continue to hear sound coming out of the speaker.

    In the case of a B-15, the revisions are not all designed the same and it is possible that you could encounter an impedance issue when connecting a DI to the EXT AMP jack. Some amps have what is called a cathode follower before the pre-amp output jack. This buffers the signal and provides enough current to drive a DI and the power amp. If you connect a DI and hear a drop in your power amp volume, you have an impedance mismatch which causes the signal loss. The degree of mismatch can vary, depending on the design of the DI. Another make of DI may resolve the problem, otherwise a DI that can connect to the speaker output will get around the impedance mismatch problem.

    The PF350/500/800 models have a balanced DI pre-amp output integrated into the amp. A nice feature to have. The PF-20T and PF-50T go a step further and have a build in balanced pre-amp output as well as a dummy load and transformer balanced line out on the power tube output. This is a feature that hasn't been seen in pervious Ampeg amps. It allows the full tone of the signal from the output tubes to be captured without having the speaker cabinet connected. In general, tube amps require a speaker load to be in place, with these amps, it isn't necessary.

    Miking a Cabinet(top)


    Proper microphone placement will help to optimize the sound and minimize the need for EQ. The EQ can be adjusted by moving the position of the mic on the cone. It can be better to get it right at the source rather than fix it electronically. It's important to experiment and find out what placement works best for your cab with the microphone that you are using. You can vary the distance between the mic and the speaker cone, where the mic is positioned on the cone, as well as the angle of the mic.

    Common positions or combinations include:
    • center close to the dust cap Edge which gives you the most high and low frequencies
    • center off axis, with the mic tilted 45 degrees or so, which gives you a darker sound
    • outer Edge off axis with the mic close to the edge of the cone (not the surround), which gives a more even Compressed sound balancing highs and lows
    • distant center position, one or two feet in front of the speaker. This works well with more sensitive mics.

    How well this works depends on the mic. They have different characteristic pickup polar patterns as well as sensitivities. The positions listed above are good starting points. Fine tuning can improve the performance.


    [​IMG]
    Cabinet miking positions (Diagram courtesy of Michal Kaszczyszyn)


    Microphone preferences vary. I like to use a Neumann U87 or AKG C414 forward cardioid pattern with the mic positioned about 60 cm (2 feet) in front of the speaker. Otherwise, despite its lack of low end, the Shure SM57 is commonly seen live. Similar to the SM57 is the Peavey PVM 45. The AKG P2 is comparable in price to the SM57 but has the advantage of not having the characteristic dip in the low end. If you use an SM57, check out the P2. I like the flat grilles on the P2 which helps with precise positioning. Some people will mix in the signal from a DI to help augment the sound.

    Popular favorites for bass include the EV RE20, Heil PR40, Telefunken M82, Sennheiser MD421, AKG P2, and Beyer M88.

    B12N/B15N Tone Modification (top)


    Here is a B12N Tone Modification. This would apply to a B-15N as well as the amp is the same, the speaker cabinet is different. They add ultra high and ultra low tone switches to a 1968 B-12N.

    How to Build a James (aka Baxandall) Tone Circuit (top)


    As the tone circuit ages, the components can go out of spec and the amp's tone can loose clarity.

    A replacement printed circuit boardfor the original PEC tone module is available. Details are presented in the Technical Amp section: Tone Circuit Replacement.

    Adam's Amplifiers shows you how to build a Baxandall tone circuit with the components mounted directly on the pots.


    [​IMG]
    Baxandall Schematic



    [​IMG]
    Layout on bass and treble pots


    [​IMG]
    Layout using stacked Concentric Pots

    They also have a simple modification for a mid frequency tone shift. It adds a mid range hump when the tone controls are set flat.


    [​IMG]
    Mid Boost



    [​IMG]
    Mid Boost Curves

    PF-115HE Port Modification (top)

    Clint Stiles performed some surgery on his PF-115HE cabinet to add a pair of rear ports to the cabinet. He reports that he is pleased with the results. Read all about how he did the modification here [2].




    [​IMG]
    A pair of ports added to the back of a PF-115HE cabinet.


    Rattles in the PF500 and PF210(top)


    Sometimes amps and cabs develop rattles and buzzes. This can be due to seasonal changes in temperature and humidity but issues can be there right out of the box.

    These rattle debugging tips come courtesy of gtmalone: #18824:

    Here are a few tips on stopping buzzes and rattles in the newer PF Series amps.

    Remove the amp from the lid if it's attached and/or set the amp away from the speaker. The idea is to break down the problem and determine if the noise is coming from the cabinet itself or if vibrations are causing something in the head to buzz.

    If you are poking and prodding in the amp to find noises be sure to used plastic or wood to avoid a trip to the emergency room and/or damaging your amp. Don't press on any of the small components on the circuit board. In general, mechanical noises are due to a vibration of fasteners, the chassis cover, cables, etc. It is possible that components such as resistors or capacitors can be vibrating but this is rare. The manufacturer normally applies goop where necessary to avoid these type of problems. Best to stay away from these areas.

    Here's what I had do to my PF500 and PF210:

    - To make it easier to find the buzzes and rattles, I used an audio sweep generator app on my computer. Run the output from the computer (or smart phone) to the amp. You may have to use the -15db pad to keep from over driving the amp. Be sure the volume is turned down before you run the app. I used frequencies from 20hz to 40hz to find the noises. You can find these apps for PC and Apple stuff. Search for "audio sweep generator".

    - Lid on cab buzzing. Used pliers to put a little more arch in the latch arm. Pulls the lid tighter on the cab.

    - Handle rattling. The clear plastic on the metal strap inside the handle wore through. Spread the metal strap Open, removed the two U-brackets, and placed a piece of rubber hose over the U-bracket. I used 5/16" fuel line I had laying around.

    - Removed the amp cover and put strips of electrical tape on the inside top of the cover. Be sure to leave gaps for the screw holes. Didn't need any tape on the sides.

    - A tie wrap on a wire bundle was buzzing against the chassis. Put a piece of electrical tape on the chassis.

    - Check all screws, nuts, bolts for tightness. Don't over tighten anything.

    Sweep generator.
    sweep generator.

    Chassis cover
    PF500 cover.

    Cable bundle.
    PF500 cables.

    Handle
    PF210 handle.
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