Jan 27, 2021

  • How Do I Know When to Change My Tubes? (top)

    For the most part, tubes enjoy a long service life. They do wear out with use, much like how car tires wear down. The harder you push then, the quicker they wear out. Tubes conduct less current as they wear out which affects how they sound. Unfortunately, they can fail in more dramatic ways.

    If your tube amp develops a problem such as a change the sound of the amp, the easiest first thing to try is to swap out the tubes. There is no substitute for a yearly preventative maintenance checkup to keep the amp in top shape. As it relates to the tubes, this would include cleaning the tube pins and sockets with a product such as Deoxit and checking the tension of the tube socket contacts.

    Although it's expensive, it is a good idea to keep a spare set of tubes. If you encounter a problem you can swap the tubes out with a known good tubes and see if the problem resolves itself. Start with the first preamp tube, and substitute the tubes one at a time down the line.

    If a power tube blows in a dramatic way with arcing and smoke, the safest thing to do is to have the amp checked out by a tech. Related components such as plate and cathode resistors could also be damaged and require replacement. In the worst case, any spikes may have damaged the Output transformer. If you simply replace the power tube, it may resolve the problem. If there are other issues, the replaced tube could blow as well so be careful.

    The rectifier tube is part of the power supply. It serves to convert the voltage from alternating current to direct current. The 5AR4/GZ34 rectifier used in most of the vintage tube Portaflex amps is a dual diode that's used in a Full wave rectifier circuit. In general, tube rectifiers last a long time and they degrade very little over their service life. There are different ways that tube rectifiers can fail. They can become intermittent causing the amp to Loose power then come back to normal power levels. If one of the diodes in the rectifier stops functioning, the amp will Loose power and become very distorted sounding. If the amp fails to power up, check the fuse first, then check the tube rectifier to see if it has failed by swapping a new one in.

    There are certain conditions that can cause a Bright arc that can be seen at the top the tube. This is called flashover. It could indicate a problem with the rectifier. There are other causes that relate to the power supply capacitors drawing too much current so it doesn't necessarily mean that the tube is bad if you see this. This only occurs when the amp is first powered up and not when it is warmed up and operating. Flashover can take a toll on the tube and can cause it to fail prematurely. So if this is seen, the amp should be checked out by a tech.

    When should you consider replacing your tubes?

    From an Ampeg user 's manual:

    Any time you notice a change in your amplifier's performance, check the tubes first. If it's been a while since the tubes were replaced and the sound from your amplifier lacks Punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise [ed. including crackling, popping, or hissing, frying bacon], loses gain, starts to Hum, lacks "Sensitivity", or feels as if it is working against you, the preamplifier tubes may need to be replaced. The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently, they almost always fail/degrade first. If deteriorating power tubes aren't replaced they will ultimately fail. Depending on the failure mode, they may even cause severe damage to the audio Output transformer and/or other components in the amplifier. Replacing the tubes before they fail completely has the potential to save you time, money and unwanted trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more than one) be replaced by a matched set. If you're on the road a lot, we recommend that you carry a spare matched set of replacement power tubes and their associated driver tubes. After turning off the power and disconnecting the amplifier from the power source, carefully check the tubes (in Bright light) for cracks or white spots inside the glass or any other apparent damage. Then, with the power on, view the tubes in a Dark room. Look for preamplifier tubes that do not glow at all or power tubes that glow excessively red.

    According to Groove Tubes, the tubes should be changed if your amp is experiencing any of the following symptoms:

    • 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.

    Depending on use, tubes can last for years. You can extend their service life by taking a few common sense precautions.
    • Avoid thermal shocks. If it is cold outside, allow the amp to acclimatize to room temperature before turning it on. If there is any visible condensation, use a small fan to blow into the amp to Dry it out before using it. Don't take a Hot amp out into the cold. Turn the amp off and let it cool down to room temperature before taking it outside.
    • Avoid mechanical shocks. Don't let the amp bounce around more than is necessary when transporting it. If you don't have a case, it can help to put the amp on a foam pad. Some people place pads between heads and cabinets to keep vibrations down. A good cab shouldn't vibrate but if it is on a soft floor, the cab can vibrate and transfer this to the amp and tubes.
    • Allow the tubes to run cool. Allow sufficient ventilation around the amp. Don't place the amp up against a wall. I sometimes carry a small desk fan that I use to blow onto the tubes. It really helps to keep them cool. I place the fan on the floor pointing up towards the tubes. Air circulation helps and really makes a difference.

    New Old Stock vs Currently Manufactured Tubes (top)

    Ampeg sourced their tubes from different manufacturers. Ampeg labeled tubes in the Portaflex amps, except for the 5AR4, were often manufactured by Sylvania in the USA. The 5AR4 rectifier tubes were typically manufactured by or for Mullard.

    RCA tubes sound great in these amps. The RCA 5691 is a special high quality, low noise version of the 6SL7GT. These tubes had a remarkable 10,000 hour service life!

    Which tubes are best? That's a heavily debated subject. It is important to understand that all tubes are different. They have different tones, noise levels, service lives, etc. Things are further complicated in that the performance of a tube is tied to the amp design, specifically the plate voltage and how the tube is biased. This is more noticeable with the small signal pre-amp tubes, but as a result, one tube might sound better in one amp than in another. So if you swap 6SL7 tubes from different manufacturers, you will hear subtle tonal differences. People that chase tone like to shop around to find ones that work the best in their amp. One make might sound richer with more Harmonics. Another might emphasize the mid frequencies. Sometimes you can dial in different tone settings to even this out but not always. Some tubes are quieter than others.

    I like new old stock tubes (NOS) because I find that they are built and screened to a higher standard, they are more reliable and last longer, and they can sound better. If you shop around, you can find some good deals on NOS 6SL7GC tubes that are close in price to the newly manufactured ones. Currently manufactured tubes have improved considerably in recent years. People liked Winged -C- power tubes but they are no longer in production. Companies are always tweaking their designs to improve them. If you find a tube that you like, stock up. Supplies of good NOS tubes keep dwindling and the prices keep going up. Again, stock up.

    There are differences in NOS and currently manufactured rectifier tubes such as the 5AR4 used in the Portaflex amps. Because of the way that they work, rectifier tubes drop a voltage across the tube. Some of the newer tubes will drop less voltage. This means that power supply voltages within the amp will be higher. Some of these amps were designed to run at 117 VAC. With the higher power line voltages used today, the internal voltages are higher than they should be. Coupling both factors together, the amp's tubes and capacitors are operating at higher than normal voltages which strains them. It helps to keep the voltages lower by using a NOS tube rectifier or by using a newly manufactured one that conforms to the spec of the vintage Mullard tubes.

    When buying a tube for a Portaflex with a tube cage, check the height to ensure that the tube will fit under the cage. Some modern rectifier tubes are taller and do not fit under the SB-12 tube cage for example.

    Whether you buy NOS or currently manufactured tubes, get whatever type that you can afford. My only caveat would be to buy your tubes from a reputable tube seller who stands behind what they sell.

    Where Are Tubes Currently Being Manufactured? (top)

    Tubes used to be manufactured around the world. Pollution regulations have made manufacturing tubes very costly. Solutions could be found but they are often too expensive to implement. With a few exceptions, production is mostly being carried out in China, Russia, Slovakia, and Yugoslavia.

    Mullard was a major British manufacturer of tubes. Their 5AR4 rectifier tubes are considered among the best. Here is a documentary entitled Mullard - The Blackburn Tubes Factory.

    Production yields add to the high cost of manufacturing tubes. Here is an interesting advertisement that Sylvania ran citing their Tight quality control specifications.

    Sylvania's adherence to Tight quality control.

    Why Do Tubes Need To Be Matched?(top)

    All Ampeg Portaflex amps are push-pull designs and require matched sets of power tubes.

    Why do tubes need to be matched? It keeps the signal being amplified balanced, minimizes Hum and harmonic Distortion, and optimizes tube life because each tube is working equally hard.

    A push-pull design requires at least two power tubes. The tubes alternate as they work, each amplifying about one half of the input signal. The plate current increases (push) on one side while it is decreasing (pull) on on the other side. By alternating, this allows the tubes to run cooler. Additional tubes can be connected in Parallel to increase the power output on each push or pull side. In a B-15 there are two power tubes so one pushes and the other pulls. In a V4B, there are two push tubes connected in Parallel and two pull tubes connected in parallel. In an SVT, the tubes are grouped in two sets of three parallel connected tubes. The biggest advantage of a push pull design is that it optimizes power output while minimizing hum and second order harmonic Distortion. You get more Clean power out of your tubes. Each side, push or pull, requires that both the amplification (transconductance) and cathode currents amoungst tubes be matched as closely as possible. This allows each tube to share the work equally and operate at about the same temperature. The most noticeable effects of unmatched tubes are hum, distortion, and in extreme cases, oscillation. Not something that you want in an amp, especially in a high powered one that's pushed hard.

    Years ago, tubes were screened to meet a tighter set of specifications than is found today. Tubes that came out of the factory were fairly closely matched. Those that didn't meet the quality and performance standards were destroyed. Amp manufacturers who bought large quantities of tubes were able to specify that their tubes be plug and play compatible with their equipment. You bought a tube, plugged it in, and the amplifier performed optimally. With the high cost of tube production today, tubes are released from factories that wouldn't have been years ago. They conform to a wider set of tolerances. Less tubes are scrapped that way. This places the onus on tube sellers to perform quality control and matching. The degree to which these services are performed varies. Consumers are demanding more because of the high prices that they are paying. They want their tubes to last and perform well. So today matching is an issue that consumers have to be concerned with.

    How Are Tubes Matched? (top)

    What services should you expect from a tube seller? There are two ends to the spectrum. A good seller's prices will be higher but competitive. You are paying for a Series of quality control tests that they are putting the tubes through to weed out duds and those that are out of spec. Be very careful when purchasing inexpensive tubes being sold on the internet. Not every seller tests tubes and matches them to the same standard. In fact, there isn't a single standard that sellers adhere to when it comes to tube matching and this is part of the problem. Some sellers take testing and matching very seriously, some just use a tube tester. The US Department of Defense does have a standard for testing electron tubes, MIL-STD-1311. Doing it right requires adhering to a rigorous testing standard, having good equipment that is properly calibrated, having a tech that knows what they are doing, and standing behind what they sell with a warranty.

    Tubes should first be subjected to a burn-in period. The time can vary but 8 to 24 hours is typical. Then, at least the following tests should be performed: shorts, gas, leakage, microphonics or noise, section balance for dual triodes, voltage drop for rectifiers and balance if applicable, current draw from the plate and screen to the cathode, and transconductance. The last two tests are most often used in matching the tubes. Some sellers perform additional tests to refine their matching process.

    When matching tubes, I think that it is best to measure the current draw and transconductance at the same plate and screen voltages that the tube will be subjected to in the amp. A good tube seller will ask you what type of amp the tubes are going into and select tubes accordingly. That way, you'll know exactly how your tubes will perform. Some larger sellers match tubes by testing them at a standard set of pre-defined operating voltages and conditions. This is more general and allows them to match and grade their tubes. Both approaches are acceptable though.

    The majority of tube testers, even the good Hickok ones that are often used, test the tubes at voltages that are lower than what is found in a typical power amp. A power tube that is tested at 250 volts may not perform the same when it is installed in an amp and operated at 500V. There are tube testers that are designed for tube amp work that are capable of performing tests at higher voltages. Some of the better companies opt to design and build their own test equipment. Many sellers do not make this sort of investment. It pays to deal with a good seller who has the right equipment and adopts standards that allow a high attention to detail when matching tubes.

    Often when you buy a match set of tubes there are numbers on the boxes. There are different standards and this classification varies. Some sellers classify the tubes with a single number or color, others will provide the transconductance (Gm) and or current draw (mA), and if you are lucky, the readings at a specified plate voltage. How close should the transconductance and current draw tolerances be for a tube to be considered matched? A tolerance of 5% is considered good. How well this can be done depends on how stable the test equipment's power supplies are. It comes down to having good equipment. An even more refined matching can be made if the tubes are matched knowing the current draw of the tube in the amp at the same bias voltage (for a fixed bias amp) that is used in the amp. This will allow the tubes to be plug and play and no re-biasing will be required.

    So now you have a set of matched tubes. But not all matched sets perform in the same way. Different sets of matched tubes will have different breakup characteristics. Why is that? Tubes that come off the same assembly line are not identical. Building a tube is a complex process and there are variations in manufacturing that make tubes perform slightly differently. To some extent, quality control helps the tubes meet a standard but a tube is classified as good if it falls into a specified range. Nothing is absolute when it comes to tube performance. Military tubes, while some are designed and built to a higher standard, others are simply screened to conform to a more stringent range of specifications. The image below depicts a population of the same model tube from a typical production run. Most of the tubes fall into a range where there is an optimal bias for that tube, around 35mA for example. For other tubes in the population, the optimal bias point is lower (30mA or lower) or higher (40mA or higher). Matched tubes can have different breakup characteristics. The tubes that idle at a lower bias will tend to have less headroom and distort sooner, the tubes that idle at a higher bias will tend to have more headroom and distort later. When ordering a matched set, you can sometimes specify what sort of breakup performance you want. If you want more distortion at a lower volume level, specify a matched set with early onset of distortion. If you want to a little more Clean volume out of your amp, ask for a late onset of distortion tube set. This is one way to squeeze a little more out of your Portaflex.

    Distortion characteristics of a tube population.

    How Do I Know If My Tubes Are Matched?(top)

    If you have a set of tubes that you want to use and you don't know if they're matched, what can you do? The proper way to check them out would be to measure the cathode current with them in you amp using a bias probe as described below and a tube tester that can measure transconductance.

    If that isn't available, you can get an idea of how hard a set of tubes are working by measuring their temperature. This can be safely done with an infrared thermometer. The heat generated by the tube is more or less proportional to the current flowing through them. If one tube is hotter than the other, they aren't matched. Below is a Fluke Model 561 infrared thermometer. The thermometer has a built in laser pointer, you simply point it at the tube's glass bottle and read the temperature. They are useful for measuring the calibration of an oven or Air conditioner thermostat or checking out your car. I've seen infrared thermometers selling for $25 and up.

    Just to be clear, you can't set an amp's power tube bias by using an infrared thermometer. It is best to measure the cathode current of each tube and use that as a basis to set the bias. The thermometer allows you to quickly evaluate how the tubes are performing. If there's an issue and one tube is running much hotter or cooler than the others, that would be a good place to start investigating with test equipment.

    As a point of reference from the tube data sheets, the design maximum bulb temperature of both a GE 6550A and a 6L6GC is 250 degrees centigrade (482 degrees F) at the hottest point of the glass.

    Fluke model 561 infrared thermometer.

    Do I Need to Re-Bias the Tubes When They are Changed? (top)

    All tubes need to be biased in order for them to work. Some are self biasing, others are fixed biased and may need to have their bias adjusted when new ones are installed. The circuits for the small signal tubes such as the 6SL7GT are self biasing. They are plug and play and do not require biasing when they are installing in an amp. The rectifier tubes such as the 5AR4 and 5U4G are dual diode tubes. The amps do not usually require any modifications when these tubes are changed.

    The power tubes are another story. The vintage B-15 amps fall into two categories, the early 25W amps that were cathode biased, and the later 30W amps that were fixed biased. Cathode biased tubes automatically bias themselves. If your amp is cathode biased, all you have to do is install a pair of matched power tubes. Fixed bias amps on the other hand, have a bias that is set in the bias circuit. A resistor in the circuit determines what the bias voltage is that is applied to the power tubes.

    This bias voltage determines the idle operating point of the power tubes. It's similar to tuning a string. Tuning a string to key is similar to applying a bias voltage to the power tube. Too much or too little voltage affects where the tube operates, like tuning a string up or down. There's a voltage that optimally sets the operating point of the tubes. Likewise, there is an optimal tension that tunes the string to key. In the old days, when you changed the tubes on a fixed bias amp, all you needed to do was plug in a new set. They conformed to the specifications of the amp. This could be done because tubes were closely screened to meet a tight set of specs. The bias in the amp was set close enough for the tubes available at the time. If you plug in a new set of tubes and they sound good, chances are the bias is fine. It's a good idea today, because of the high cost of tubes, to check the idle cathode current of the power tubes and set the bias to optimize the performance of the tubes.

    I mentioned above the when you change a rectifier tube you usually don't need to re-bias the power tubes in a fixed bias amp. If you are replacing the tube with the same make and model, it usually isn't a problem. That is, if the B+ remains the same with the new rectifier tube in place, you will not need to re-bias. Different rectifier tubes could change the B+. Changing the B+, changes the plate voltages and that affects the bias voltage. Everything is interdependent. To be safe, it is best to check the cathode current of the power tubes, in a fixed bias amp, after changing a rectifier tube to ensure that the bias is set correctly. This isn't necessary with a cathode biased amp.

    Do I Need to Re-Bias the Power Tubes if the Rectifier Tube is Changed?(top)

    The short answer is no if you have a cathode biased amp and maybe if you have a fixed bias amp.

    If you have a fixed bias amp and you replace the rectifier with one that is the same make and model that performs the same as the old one, you don't need to rebias the power tubes. It is still a good idea to check the bias though. When I say same make and model, I'm not talking about replacing a 5AR4 with another 5AR4. If you replace a Mullard rectifier with another Mullard that is exactly the same, then you will be fine. If you replace a Mullard 5AR4 with a JJ 5AR4, the voltage drop across the tube might be different. This will affect the B+ voltage and the power tubes might need to be rebiased as a result. If there is only a small difference in the B+ it isn't a big issue. Biasing doesn't have to be exact. An amp will perform well over a range of bias settings. Use your ears. If you install a new rectifier tube and the amp sounds good, don't worry about it.

    Power line voltages can vary from day to day or venue to venue. Tube amps are pretty resilient and can perform well under a range of voltages. Power line voltage variations will change the bias which will affect the performance of the amp. Some people rebias their amp at every venue because they want their amp to sound consistent. You can do this fairly easily with an amp like a modern SVT or the PF-50T and PF-20T Portaflex amps that have the bias LED's. It isn't so easy with a vintage Portaflex.

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

    See How Do I Re-bias My Fixed Bias Amp?

    That Warm Glow Of The Tubes (top)

    Ever turn you amp around and sit in a dimly lit room enjoying the Warm glow of the tubes? Much like sitting by a fireplace on a cold winter's night.

    All tubes glow. Some more than others. With some tubes you can't see the glow because it is shielded by the metal structure inside the tube. An orange glow is normal. In some cases there is a rich blue glow. This is also normal. A bright reddish glow caused by too much current through the plate is not good. A light blue glow or a white cloudiness inside on the glass is a sign of an air leak.

    The following is an extract from How Vacuum Tubes Work by Eric Barbour.

    Glass tubes have visible glow inside them. Most audio types use oxide-coated cathodes, which glow a cheery warm orange color. Thoriated filament tubes, such as the SV811 and SV572 triodes, show both a white-Hot glow from their filaments and (in some amplifiers) a slight orange glow from their plates. All of these are normal effects. Some newcomers to the tube-audio world have also noticed that some of their tubes emit a bluish-Colored glow. There are two causes for this glow in audio power tubes; one of them is normal and harmless, the other occurs only in a bad audio tube.

    1) Most Svetlana glass power tubes show fluorescence glow. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a Solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic color. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope. This glow is harmless. It is normal and does not indicate a tube failure. Enjoy it. Many people feel it improves the appearance of the tube while in operation.

    2) Occasionally a tube will develop a small leak. When air gets into the tube, and when the high plate voltage is applied, the air molecules can ionize. The glow of ionized air is quite different from the fluorescence glow above, ionized air has a strong purple color, almost pink. This color usually appears inside the plate of the tube (though not always). It does not cling to surfaces, like fluorescence, but appears in the spaces between elements. A tube showing this glow should be replaced right away, since the gas can cause the plate current to run away and possibly damage the amplifier. With a more serious leak, a telltale sign of a loss of vacuum is a light Colored powder on the inside of the glass envelope. Don't confuse this with a powder on the outside of the envelope. This can usually be wiped away.

    Fixing a Loose Tube Base (top)

    Power tubes have a base that's glued to the glass envelope. Wires extend from the glass envelope and are soldered to pins in the base. With old tubes, it isn't uncommon to find that the glue joint breaks leaving the base loose enough to wiggle around. This usually doesn't affect the operation of the tube but it can be the source of a rattle.

    Tube with base removed. The wires are soldered to the pins in the base. The glue has broken away and can rattle, as can the glass envelope against the base.

    There are several ways to fix a loose tube base. The easiest and most conservative way is to try to reuse the original glue. The dried glue can be softened with a solvent such as isopropyl alcohol or acetone. A Q-Tip, sponge, or eyedropper can be used to apply a generous amount of the solvent around the top of the base into the space between the base and the glass. Let the solvent penetrate into the glue for a few minutes and then firmly press the glass envelope and base together and hold it for at least a minute. Position the tube with the pins pointing upward and let it sit for twenty-four hours. This allows the solvent to evaporate and the glue to harden. It helps to have a tube box to hold and protect the tube at this point. This will allow the solvent to evaporate and the glue to harden. This usually works and the tube is as good as new. Some people unsolder the wires on the pins, separate the base from the glass envelope, recover the glue, reconstitute it with alcohol, then reassemble the tube. This is the best solution if you want to keep the tube as original as possible. Other glues can be tried. I sometimes use a more modern alternative, a high temperature, automotive gasket, silicone product called Permatex Ultra Copper. Others have reported using a dab of clear nitrocellulose based nail polish (heat can be an issue here), varnish, shellac, J-B Weld steel reinforced epoxy, and cyanoacrylate glue. There is a commercial tube glue available for repairing tubes from Emission Labs.

    The formulation for the glue that RCA used on their tubes has been published. It produces about a 200 pound batch.

    • coarse marble flour, 170 lb
    • orange flake shellac, 19-1/2 lb
    • Durite phenolic resin LR275-2, 7-1/2 lb
    • medium-color (grade G) rosin, 3-1/4 lb
    • denatured alcohol, 9 liters
    • Malachite green aniline dye, 10 grams

    The formulation was alcohol based and that's why the dried glue can be softened or dissolved with alcohol.

    It is important to note that an old tube should never be removed from its socket by pulling on the glass envelope. This can break the seal between the base and the glass envelope. Hold the base and gently rock it back and forth while applying pressure away from the tube socket. Reverse the procedure when installing a tube, only holding it at the base.

    Tube Diagrams (top)

    The following tube diagrams are courtesy of Antique Electronic Supply and are available on their site in the technical section. They illustrate the structure within a tube.

    The filament or heater winding is what you see glow when the amp is turned on in standby. The heater serves to heat the cathode which allows electrons to boil off into the vacuum within the tube. The current of electrons is controlled as it passes through the grid. The electrons strike the plate and are absorbed. By controlling this process, the tube amplifies a small AC input signal to produce a larger AC output signal. The plate is what has the warm orange glow within the tube when the amp is operating.

    If your tube is microphonic, look at the insulating spacers within the tube. They should be firmly pushed against the glass envelope to help stabilize the internal structure, keeping vibrations down which can translate into noise that is amplified by the tube.

    12AX7 tube diagram

    6L6 tube diagram

    Tube Data Sheets(top)

    Ampeg labeled pre-amp and power tubes used in Portaflex amps were often made by Sylvania in the USA. The 5AR4 rectifier tubes were made by or for Mullard. Below is a selection of tube data sheets used in Portaflex amps. Some are substitutes that offer better performance, others are newly manufactured products.

    Tube data sheet repositories such as Tube Data by Frank Philipse are available. Every manufacturer also published tube data books and application circuits. See Pete Millett's Tube Books.

    The following data sheets are in pdf format.

    Pre-Amp and Phase Inverter Tubes(top)

    Power Tubes(top)

    Rectifier Tubes(top)

    Tube Rectifier Comparison(top)

    The 1960 B-15 model employed a 5U4GB rectifier tube, the later vintage models had a 5AR4/GZ34 tube. As you can see in the chart, the 5U4GB drops 50 volts DC at rated current whereas the 5AR4 drops 30VDC. The voltage drop means that the power supply (B+) voltages in the amp with a 5U4GB installed will be lower. This makes the amp sound more like the amp from the 50's, softer, less headroom, easier to drive into distortion. When swapping tubes, it's important to note the heater current requirements. In the case of the 5U4GB, the heater current draw is 3 amps; the 5AR4 draws 1.9 amps. A healthy B-15 transformer is capable of supplying the additional current that the heater requires. If you have an original transformer and want to try a 5U4GB in your amp, keep in mind that old transformers can have issues such as cracked coil windings would may lead to a short within the transformer. Be careful. Start by not pushing the amp too hard and monitor the temperature of the power transformer. If the heat is higher than normal, it could lead to an issue. If you have a replacement transformer in place, there are no concerns.

    Tube Rectifier Comparison.jpg
    Tube rectifier specification comparison.
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