Musical Instrument Tube Amp Building, Maintaining and Modifying FAQ

Much of this material applies to building or re-building hi-fi equipment, as well but it was originally intended for musical instrument crazies.

Assembled by R.G. Keen,
Most recent revision level is Version 1.10, appx. 3/29/96

Thanks to the contributors who helped and taught me:

Hundreds of folks who taught ME stuff when I didn't know a triode from a Tri-Axis; I can't remember all of your names, and it all comes out as general knowledge now, but I appreciate it. A few names in that category stand out:

And people who have contributed things that I have included as part of the actual text:

Dennis O'Neill,
Nathan Stewart,
George Kaschner,
David Kohn,       kohn@SCTC.COM
Michael Edelman,
Len Moskowitz,
Brian Carling,
Eric Barbour

Special thanks to Nathan Stewart who did the bulk of the work converting this to HTML.


  2. Why is AMP building in a musical instrument building group?
  3. Where can I learn about building tube amps?
  4. Where can I find parts to build/repair amplifiers?
  5. How can I modify my amp to be more powerful?
  6. How can I extend my tube life?
  7. How do I get...
  8. Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?
  9. Output transformer questions:
  10. What is the easiest way to get tube sound at a good price?
  11. How can I modify my tube amp to ... ? (also see recommended mods, below)
  12. When should I bias my amp and how do I do this?
  13. Amplifier Modifications
  14. Tube Characteristics and Substitutions
  15. Maintenance Issues
  16. Appendix A. Tube Stuff Suppliers
  17. Appendix B. Tube Makers Producing Today (Eric Barbour news posting)
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Working inside a tube amplifier can be dangerous if you don't know the basic safety practices for this kind of work. If you aren't prepared to take the time to learn and apply the right precautions to keep yourself safe, don't work on your own amp. You can seriously injure yourself or get yourself killed. This section is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one.


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Why is an amp like a musical instrument

For electric guitars, basses, and possibly other instruments, the amp is as much a part of the final sound as the nominal instrument is, perhaps more. The instrument is relegated to a role of providing a base tone which is profoundly modified by the following effect and amplification stages. The "instrument" is properly the instrument and amp together.

Where can I learn about building tube amps? Back to the index...

Get one or more of the following references (note that these books are mostly old, and highly sought after, and so may be expensive and hard to find):

Where can I find parts to build/repair amplifiers?

Back to the index... New tube parts and supplies were steadily getting harder to find, but in the last year this has turned around radically. There are now many companies offering new parts, especially power and output transformers. It is still true that used parts are often nominal cost or free. The hard parts to find in high quality are the transformers.

If you're building, I recommend getting your transformers first. If you are getting vintage parts, they are likely to be one-of-a-kind. If you've just ordered new ones, the transformers will have a massive effect on your chassis's mechanical layout.

The easiest but most expensive source for parts is at your retail musical instrument store as "repair" parts. Other sources:

Be sure to look at Appendix A for more sources.

Premium Suppliers

Here are "more tube supply sources": George Kaschner notes that parts other than tubes and transformers can be obtained easily from Mouser Electronics (800-346-6873). I have used Mouser and they give good service and prices; $20 min order. another good source is Digi-key for resistors, capacitors, and other general electronic parts. They are not tube oriented, but are also a good general parts source.

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How can I modify my Blender Tweety Bird amp to be as loud as a Marshall Major/AC30/Tweed Bassman/SVT/etc.?

(Alternatively, how can I make my amp/A> my Blender Tweety Bird amp to be as loud as a Marshall Major/AC30/Tweed Bassman/SVT/etc.? (Alternatively, how can I make my amp twice as loud/more power/ etc.?)

You can't do this in a low power amp, at least not electronically. To put out the power the big amps put out, you need the entire power train to be as beefy as the big amps. This means bigger power transformer, rectifiers, filter capacitors, output transformer, more power tubes, bigger chassis, more ventilation to carry off the heat, lots of things. You can't just add a couple of tubes.

An amplifier is properly thought of as primarily a big power supply that has some extra junk tacked onto it to carefully let a little of the power out to the speakers under special, controlled circumstances.

You might be able to just pull a couple of tubes OUT of a high power amp to make it quieter, under some conditions. O'Connor discusses this in "The Ultimate Tone".

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How can I extend my tube life?

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How do I get...

Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?

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Output transformer questions:

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A. How can I tell if my output transformer is live or dead?

There are some simple tests you can run to quickly determine if a transformer is grossly bad. This is much simpler than determining if it will work well and sound "good" for you. The tests of relative "goodness" are also possible, but require a lot of equipment and experience to do correctly. For the quick and dirty tests described here, you'll need a means of measuring AC voltage and current simultaneously, such as a pair of VOMs or DMMs, and a 110/120 to 6.3VCT filament transformer, and either a variac (variable transformer) or a light bulb socket in series with the primary of the filament transformer to limit the power you put into the transformer under test.


Both the filament transformer and the transformer under test will have at least AC line voltage on them, an may well have much higher voltage, several hundred volts on one or more windings. You are therefore in danger of being KILLED if you are not both knowledgeable and careful about how you do these tests.


Seek experienced help if you have any question in your own mind.

The tests run like this. Identify which wires are which by color code, circuit connection, or by using an ohmmeter to find which connects to which. Label the wires. From the same ohmmeter test, write down the resistances you measured on the windings. Generally, windings with resistances over a few ohms are high voltage windings, either a power transformer primary or high voltage output, or an output transformer primary. Note that it is common for primary windings on power transformers to have from two to six wires, with the wires over two being taps to adjust for various line voltages from 110-117-120-125-208-220-240. Secondary windings on power transformers and primaries on output transformers will have either two or three leads, and secondaries on output transformers will have to to four leads.

Also note if any winding is shorted to the transformer core. Sometimes an internal shield will be deliberately connected to the core, but if a multi-lead winding is connected to the core, this is usually an internal short, and a dead transformer.

Once you have identified the windings, hook up one and only one winding to either 1/2 of the 6.3VCT or to the variac. Try to select a low voltage winding, one that has low resistance from the ohmmeter test. Make sure that no other leads are connected (or shorted together, or touching your screwdriver on your bench or... well, you get the idea). A turn of plastic tape on each wire end you're not using at the moment is a good idea. Set your voltmeter on this winding, and the current meter to measure the current through it, and bring the circuit up. The voltmeter should measure 3 volts AC, the light bulb (if used) should NOT be lit brightly, and nothing should be humming or smoking ;-). There should be little current going through the winding. If the voltage is lower than 3 volts, or you are pulling amps of current, then there is a load on the transformer, internally since you have disconnected all the leads, meaning that there is an internal short. You should try to set, then there is a load on the transformer, internally since you have disconnected all the leads, meaning that there is an internal short. You should try to select a winding for this test that is normally a low voltage winding, either a filament winding in a power transformer, or a secondary in an output transformer.

If all is well, measure the voltage that now appears on the other windings. The voltages will be equal to the ratios of the voltages that will appear on these windings in normal operations.

B. Where can I get a good replacement output transformer for my vintage DoppelBanger amp?

Dixie Sound Works, Gunthersville, Alabama has a great reputation for (re)winding quality vintage re-makes. The company that made the amp may have service parts. The quality is variable from company to company and time to time, though.

There are a number of companies that have entered the transformer market in the last year, so expect that there will be new places to get quality rewinds and replacement transformers

C. I want to make my own power and output transformers. How do I do this?/ Where can I find information about this?

Designing and hand winding transformers is not terribly difficult, but it does require information and skills that are relatively hard to find. You are unlikely to save a whole lot of money unless used or broken parts are cheaply available to you. You may want to do this if you feel that you were selected by some deity to take this on as a life work. First, take a transformer apart. A burned out tube-type power transformer will do. Do this carefully and slowly, imagining how you would have put it together in the first place to get it the way it was. This is an excellent introduction to the manual skills and materials needed to successfully produce one on your own. Learn about how transformers are designed from one or more of the following, in this order:

  1. "Transformers for Electronic Circuits", Grossner (check your library)
  2. "Radiotron Designer's Handbook, fourth edition
  3. "Audio Transformer Design Manual", Wolpert, $36, privately published, available from: Robert G.Wolpert 5200 Irvine Blvd. #107 Irvine CA 92720
  4. "The Williamson Amplifier" D.T.N Williamson, reprint available from Old Colony Sound Labs
  5. Handbook of Transformer Design and Applications by William Flanagan (second ed.)
  6. "rewinding transformers with CAD" by Hugh Wells W6WTU Ham Radio Dec '86 p.83
  7. "Fast Optimization of Transformer Design" EDN Nov '62 by Davis, J. H.
These sources will help. They are NOT a complete cookbook. Note that it is very possible to make a transformer that will operate relatively well, but may break down unexpectedly and KILL you if it is not constructed with safety in mind.

D. Should I replace my stock transformer with a new/old/vintage/purple one for better clean/grunge/grit/etc. sound?

Unless you REALLY know what you're doing and have heard the transformer you'll be swapping in and like it, no.

There are a huge number of variables in the "sound" of a transformer, and you should exhaust other means first. You might not get that magic sound after all that work unless your ears - and amp tech - are really good.

What is the easiest way to get tube sound at a good price?

Back to the index...

How can I modify my tube amp to ... ?

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(also see recommended mods, below)