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Subject: Tube Amp Mini-FAQ
Date: 12 Jan 1995 01:19:33 GMT
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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, keen@eden.com
Most recent revision level is Version 1.06, appx. 1/15/95
Changes from the previous version are marked with a ">"
==============================================================================
Special thanks to the contributors who made this possible:
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 knowlege now, but I appreciate it. A few names in that category
stand out:
Tom Balon, balon@cup.hp.com
David Mourning, dam@dcs.gla.ac.uk
Mark Hammer, no longer on the net that I can find
And people who have contributed things that I have included as part of
the actual text:
Dennis O'Neill, denio@seismo.css.gov
Nathan Stewart, npstewar@eos.ncsu.edu
George Kaschner, gckaschner@ucdavis.edu
David Kohn, kohn@SCTC.COM
Michael Edelman, mje@pookie.pass.wayne.edu
Len Moskowitz, moskowit@panix.com
Tremolux@aol.com
Brian Carling, brian.carling@acenet.com
=========================================================================
INDEX
1. Why is AMP building in a musical instrument building group?
2. Where can I learn about building tube amps?
3. Where can I find parts to build/repair amplifiers?
4. 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 twice as loud/more power/ etc.?)
5. How can I extend my tube life?
6. How do I get...
- blues distortion?
- Marshall/metal/Boogie/etc. distortion?
- good distortion at low(er) volumes?
7. Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?
8. Output transformer questions:
A. I want to make my own power and output transformers. How do I do
this?/ Where can I find information about this?
B. Should I replace my stock transformer with a new/old/vintage/purple
one for better clean/grunge/grit/etc. sound?
9. What is the easiest way to get tube sound at a good price?
10. How can I modify my tube amp to ... ? (also see recommended mods, below)
- get lower hum?
- have higher gain/more distortion?
- have a smoother, less buzzy distortion?
>11. When should I bias my amp and how do I do this?
> A. What is "bias"?
> B. When should I bias my amp?
> C. How do I bias my amp?
12. Recommended amp modifications
13. NOT Recommended amp modifications
14. Tube Characteristics and substitutions
15. Maintenance (Still under construction, pretty thin right now)
16. How can I tell if my output/power transformer is good?
Appendix A. Tube Stuff Suppliers
=========================================================================
1. Why is AMP building in a musical instrument building group?
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.
2. Where can I learn about building tube amps?
Get one or more of the following references:
- "The Tube Amp Book" by Aspen Pittman, now in its fourth edition.
This contains the majority of guitar amp schematics ever made.
- "Electric Guitar Amplifier Repair Handbook" (?) By Jack Darr. Good
intro to actually making repairs as well as many schematics.
- "ARRL Handbook", preferably a late 60's or early 70's edition. Read
the sections on construction practice, safety, and tube info.
- Guitar Player Magazine's article on tube types and operation from a
year or so ago
- Glass Audio magazine, Old Colony Sound in Peterboro NH
- Mesa/Boogie will send schematics of their amps, call 1-707-778-6565;
note however, that these schematics are known to be innaccurate.
- "Vacuum Tube Amplifiers" by G.E. Valley, Jr. Part of the MIT
radiation lab series, originally published by Boston Technical 1964.
Reprints are currently available from Antique Radio Classified (P.O.
Box @, Carlisle, MA 01741, 508-371-0512)
- Amplifiers, H. Lewis York.
(Evidently part of the Encyclopedia of High Fidelity). Good basic
technical ref. Simple math, good explanations. Includes a couple
of designs (several use hard to find tubes) and tips on phyiscal
construction as well.
- Radiotron Designer's Handbook, Langford-Smith. Heavy theory, heavy
technical. Not coffee table reading, but it you want to know, it's
probably in there. (Rumoured to be divinely inspired - if not the
Bible on vacuum tubes, it's at least Leviticus.)
- RCA Receiving Tube Manual. Reprints available from several sources,
including Antique Electronic Supply & others (Old Colony?) Mostly
tube spec sheets & some charaterstic charts. The intro is a pretty
good technical primer.
- Electron Tubes, R.G. Kloeffler. little application, but a good easy
to digest explanation of charateristics of diodes, triodes, beam
power & true pentodes, with the math to go along. Worth reading if
you're trying to do modeling.
- The Audio Designer's Tube Register. Tom Mitchel. 1993, Media
Concepts. Volume 1 - Common Low Power Triodes. 144 pages of
freshly compiled tube data, some of which was not previously
published. Kinda pricey ($18 from Antique Electronic Supply) unless
you need the data. Included are plate characteristcs, tranfser
characteristics, constant current curves, mu as a function of grid
potential and plate potential, transconductance as a function of
plate current and grid potential, and dynamic and static plate
resistance as a function of plate potential and plate current.
(Tom mentions a 2nd and 3rd volume in the distant future - covering
low power pentodes & oddball tubes, and Power & Beam Power pentodes
respectively.)
- Learn about the manual and safety aspects of working on tube
amplifier circuits. Read the ARRL handbook, or better yet, get to
know a ham radio operator who will give you some guidance and
teaching. Do not skimp on the safety aspects. Tube circuits
contain deadly voltages. You can - * DIE * - if you mess up or are
careless. It is your personal responsibility to learn how to do
this safely.
- Get to know a guitar repair technician, perhaps do some free
apprentice grunt work for them in return for some teaching.
3. Where can I find parts to build/repair amplifiers?
New tube parts and supplies are steadily getting harder to find, but
paradoxically, used parts are often nomimal cost or free. The hard
parts to find are the transformers.
If you're building, I recommend building around what transformers are
available to you. And AFTER you have them in your grubby little
hands. It often happens that the transformers you THOUGHT you would
buy after you got the chassis, sockets, etc. was just sold to another
fellow, and there aren't any of those left... (Stewart)
The easiest but most expensive source for parts is at your retail
musical instrument store. Other sources:
- Musical instrument repair shops will sometimes order parts or sell
you parts out of their stock.
- Amp makers' repair parts departments. Many manufacturers will sell
their parts to "repair shops" to fix their amps. Some of them are
better than others about this, so be polite and businesslike.
- Antique Electronics Supply, Tempe AZ. They stock tubes, some
transformers, some capacitors, tube sockets, etc.
Antique Electronic Supply recently added several steel and a few
aluminum chassis boxes to their line. I was told this is to be a
continuing trend, and not just a one time buyout of a couple of
boxes.
- Mouser and SESCOM (don't have the address/phone) sell various
rack enclosures. Mouser has a couple which would house a healthy
size tube amp project.
- old, broken, or unloved equipment. This may be free, or
units-of-dollars. You get transformers, sockets, tubes, and chassis
in the deal. May require cruising garage sales or diving in
dumpsters. Trash every part except the tubes, transformers, sockets
and chassis. I got a 15 Watt mono amp/preamp intended for mono hi-fi
music for $20 at a local garage sale. Needs only some tweaking to be
a Studio .22 or an AC-15.
> Be sure to look at Appendix A for more sources.
In some issue of The Absolute Sound (a "high-end" audio mag), Steve
Melkisethian wrote an article called "Where the Tubes Are: Tube Tips
and Topics" [reference lost, sorry, I just have a copy of the article;
it was sometime in 1987-1992). The article obviously is aimed at the
high-end hifi market, not at guitarristas, but he lists some of the
following sources, with various cautions (see the article for more
info).
SM called the following "premium suppliers":
o Fender Musical Instruments - call 800-854-6230 for a list of
dealers
o Richardson Electronics - 800-348-5580 for dealer list
o RAM Tubes, 805-962-4445
o VTL, 714-627-5944
Here are "more tube supply sources":
o Triode Electronics, 312-871-7459
o Elmiria Electronics 800-847-1695
o Antique Electronic Supply, oriented to radio collectors,
602-820-5411
o Antique Audio, oriented to radio collectors, 512-467-0304
o New Sensor, mostly imported tubes (here's the source of Sovtek),
call Mike Mathews, 212-980-6748. Min. order is $50.00.
o ARS Electronics, 800-422-4250
o Department of Defense surplus auctions. DRMO-Tobyhanna Army Depot,
Building 16, Tobyhanna, PA 18466 is the gummint storage facility
for communications gear and is said to have good stuff. Also,
get "How to buy...Surplus Personal Property from the Department of
Defense", free from DOD Surplus Sales, PO Box 1370, Battle Creek,
Michigan 49016.
o Surplus electronics dealers - see the Telco yellow pages
o call everybody in the yellow pages under Electronics, TV-Repair,
Communications, and any other promising category.
o Hamfests
o SM's store, Angela Instruments, 8600 Foundry St. Box 2043, Savage,
Md. 20763, 301-725-0451.
o Tube Amp Service in San Francisco,run by Tom Balon; call
415-334-5200 PST. (O'Neill)
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.
4. 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 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. You might be
able to just pull a couple of tubes OUT of a high power amp to make it
quieter, under some conditions...
5. How can I extend my tube life?
- modify the power on switching to heat the filaments first, let them
warm up for 30 seconds, then switch on the high voltage plate supply.
- add more ventilation to the amp chassis, perhaps with a small fan.
- modify the tube operating conditions so the maximum cathode current
is not exceeded under even maximum warp drive conditions. Exceeding
max cathode current causes cumulative emission losses and early tube
death. This requires a somewhat deep understanding of the design of
tube amps to do, unfortunately.
6. How do I get...
- blues distortion? Made by overdriving preamp and power tubes a
little, enough to just start compressing the peaks of the waveforms,
and not very much high frequency content, by electronically cutting
highs or running the signal into a speaker cab that acoustically
cuts highs.
- Marshall/metal/Boogie/etc. distortion? Made by massively overdriving
preamp tubes until the original waveform is massively compressed and
clipped. Usually followed with a moderate amount of high frequency
cut to remove some of the "insect attracting" overtones generated
in the clipping process. There is commonly some output tube
overdrive in this process, too.
- good distortion at low(er) volumes? overdrive preamp tubes until you
get the clipping you want, then feed a limited amount of this to a
power amp stage to get the loudness you want. This is how master
volume controls work.
7.Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?
- ElectroVoice sells (?) makes available (?) plans for cabinets for
their speakers
- copy an existing cab
- some cabinet fitting suppliers have example plans
-- (addresses in a future posting) ---
8. Output transformer questions:
A. 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 annointed by some deity to take this 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 sucessfully produce one on your own.
- Learn about how transformers are designed from one or more of the
following, in this order:
* "Transformers for Electronic Circuits", Grossner (check your
library)
* "Radiotron Designer's Handbook, fourth edition
* "Audio Transformer Design Manual", Wolpert, $36, privately
published, available from:
Robert G.Wolpert
5200 Irvine Blvd. #107
Irvine CA 92720
* "The Williamson Amplifier" D.T.N Williamson, reprint available
from Old Colony Sound Labs
* Handbook of Transformer Design and Applications by William
Flanagan (second ed.)
* "rewinding transformers with CAD" by Hugh Wells W6WTU Ham Radio
Dec '86 p.83
* "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.
B. Should I replace my stock transformer with a new/old/vintage/purple
one for better clean/grunge/grit/etc. sound?
- under construction
9. What is the easiest way to get tube sound at a good price?
- Obtain an old piece of tube gear, perhaps intended for another
purpose, like mono hifi, at no or low cost. Modify this to duplicate
to a certain extent the circuit of an existing amplifier. Tinker to
your heart's content.
- Build a tube preamp from scratch, and use this to drive another
larger amplifier which does not necessarily have to be tube based. I
have designed things like this, so have others. Good tube sound, and
inexpensive. Really convincing tube distortion, especially if you
add some lowpass filtering to simulate the high frequency cutoff of
guitar speakers.
> This is what the Hughes and Kettner Blues Master and Cream Machine
> tube preamps did (they've been discontinued). These were entire tube
> amplifiers with maybe 2 or 3 watts output, a simulated load, and a
> line level output in addition to the speaker output. They did a VERY
> respectable job.
10. How can I modify my tube amp to ... ? (also see recommended mods, below)
- get lower hum?
* replace the defective humming tube
* replace or improve the power supply filter capacitors
* fix the defective internal ground wiring, as on a reverb tank, or
previous "improvements" and modifications
* run the preamp filaments on regulated DC, not AC, starting with the
input tubes
* rewire the grounding so the amp is star grounded, and does not use the
chassis as a ground bus
* move the signal wires around, nearer/farther from the chassis or
60 Hz AC carrying wires
* use coax cable in the signal path, at least in the early sections
where noise counts the most. Tie one end of the shield to ground
and terminate the other end with some shrink tube so it cannot
touch anything. This way the coax shield acts as an antenna and
conducts the RF to ground (as well as Faraday shielding hum out).
If you tie both ends to ground you set up some capacitance (and
the possibility of ground loops) you're better off without. The
shield should be tied to the star gound point individually, and
bypassed to the chassis locally with a good RF cap of about 0.001
to 0.01.
George notes "You may already use this in your own amps but I
thought we might share it with the rest of the tinkerers - it's
especially useful for people that are trying to add extra gain
stages. I even use it between the input jacks and the first stage
since in most Fender amps it has to traverse the width of the
board. (Kaschner)
- have higher gain/more distortion?
* install an extra gain stage by
>using an unused tube section if one exists
>adding another tube to the chassis
>using the reverb tubes as additional gain stages
>using a power MOSFET as a cathode follower to drive tone control
and volume controls for lower loss
>using a power MOSFET to replace an existing cathode follower,
freeing up that tube section for more gain
>remove the feedback on the power amp stage; newer Fenders and
other amps use feedback on the power amp to reduce distortion.
Removing this increases gain and and distortion, and makes the
distortion start at lower volumes. On Fenders, it's generally a
white wire from the 'ext speaker' jack to a 2.2k resistor. Cut
this wire, or lift it at one end. To be really slick, put in a
toggle switch. (Edelman)
* use the alternate channel for more gain, perhaps jumpering two
channels together
- have a smoother, less buzzy distortion?
* use a lowpass filter somewhere inside the amp in the signal path
to cut higher harmonics; perhaps a capacitor to ground from the
final preamp tube grid or plate -or-
* use series grid resistors to cut the high frequencies in and after
distortion stages
* use a lowpass filter after the amplifier and before the speakers to
cut out some of the higher overtones.
>11. When should I bias my amp and how do I do this?
> =================================
> A. What is "bias"?
> "Bias" in this context refers to the amount of voltage held on the
> grids of the output power tubes. This controls the amount of current
> the output tube(s) conduct exclusive of the signal current, or,
> looking at it another way, the amount of overlap where both tubes are
> conducting simultaneously.
>
> I will talk about the output tube current since the terms
> "underbiased" and "overbiased" are confusing with tube amps. A
> technician who works with only tube amps will usually refer to the
> voltage which sets the operating current in the tubes. In these amps,
> the bias is a negative voltage, so "overbiased" to such a technician
> would mean that the tubes are held in a condition of too little
> current, just backwards from the solid state terms most of us are
> familiar with. "Underbiased" would mean that the tubes have too
> little negative voltage on their grids and are conducting too much
> current simultaneously.
>
> The idle current in the output tube and the degree to which the output
> tubes overlap in conduction is what you're trying to adjust, not how
> many volts go on the grids; you just have to use the grid volts to
> change the current and conduction angle.
>
> The whole topic of bias is tied up with the "Operating Class" the
> power amp is designed for. There are only three classes useful to us
> in tube amps, Classes A, AB1, and AB2. Class A means that the output
> tubes are biased so that both tubes are always conducting. Even on
> maximum signal peaks, the tube driven most "off" will still be
> conducting some current. In both class AB's, the bias is set so that
> on a signal peak, one of the tubes can be driven completely off for
> some part of a signal cycle. In class AB1, no grid current flows into
> the grid of the tube, and in class AB2 some grid current is driven
> into the grid of the tubes. There is a class B, where both tubes never
> conduct current at the same time, only alternately.
>
> The point of all this is this: The Class of the amplifier is
> determined by how much bias current is present. If there is a lot of
> bias voltage, the grids are held 'way negative, then only the tube
> which is driven by the positive going half wave of the signal at any
> moment is conducting. This is class B. It sounds ugly because the
> point where the signal crosses over from positive to negative and
> begins to drive the other tube is not reproduced cleanly, and creates
> [surprise!] crossover distortion. You can look at the output signal
> with an oscilloscope and see crossover clearly as you make the bias
> voltage too negative for both tubes to conduct at the same time. As
> the bias voltage is made less negative and allows both tubes to
> conduct a little, the crossover notch diminishes swiftly, and you are
> in class AB2; a little less negative, and they both conduct more, and
> you have class AB1. If you go further, you get to the point where both
> tubes always conduct, making the amp work in class A, which has the
> least crossover distortion of any of these operating conditions.
>
> Too little simultaneous conduction in the output devices puts them in
> the most nonlinear region of their transfer characteristic, so
> crossover distortion is high; but as you increase the amount of
> simultaneous conduction, the power used and dissipated by the outputs
> goes up, perhaps to a disastrous degree. You are trading standby
> current and power dissipation in the output devices off against
> distortion. If both outputs are biased almost totally off at idle,
> crossover distortion is very bad. As the simultaneous conduction is
> increased, crossover goes down rapidly, until it gets smaller than the
> residual THD of the amp itself, and becomes much less audible. There
> is a fairly broad sweet spot where the crossover distortion is
> comparable to the THD and the idle current and idle power dissipation
> are reasonably low. This is the region you're looking for.
>
> Lots of bias, both tubes conduct all the time - and eat a lot of
> power, get hot, other Class A kinds of things. Little bias, both
> tubes overlap less, get less hot, put out more total power - and
> produce crossover distortion, which sounds especially unpleasant.
> Power tubes individually have slightly different DC gains, so the same
> bias voltage on two different tubes produces two different current
> levels. "Matched pairs" are two tubes selected to be close together.
> Groove Tubes grades tubes from 1 to 10 so that any two "3"'s for
> instance are close enough to sub for any other "3", so you don't need
> to rebias if you keep buying the same number from them.
>
> B. When should I bias my amp?
> You should re-bias the amp whenever you change power tubes or modify
> a circuit. Each power tube needs a certain bias current to keep it
> operating at the point where there is minimal distortion under normal
> conditions. Individual tubes vary widely in the conditions that set
> the correct bias current and amps typically provide only one
> adjustment point for bias, so it's smart to buy matched sets of power
> tubes. [unless you have modified the amp to "match" the operating
> gains of the tubes - ed.]
>
> C. How do I bias my amp?
>
> CAUTION
> CAUTION
> CAUTION
>
> Keep in mind that tube amps use high voltages, and they can *kill* you
> if you don't know what you're doing. So, if in doubt, leave the job
> to a qualified technician.
>
> How do you correctly bias an amp? There a few different approaches
> but first hook up a speaker or a passive load to the output and remove
> any input signals; tube amps need to have a load or they can sometimes
> become unstable. Check and make sure the proper size fuse is
> installed.
>
> Output Transformer Shunt Method
>
> The most common and simplest procedure is to hook a current meter from
> the plate (anode) across half of the primary of the output
> transformer; this is called the "output transformer shunt method." The
> idea here is that milliammeters commonly have a very low series
> impedance so that when placed in parallel to half of the primary,
> almost all of the current flows through the ammeter. When you hook
> things up this way, your meter is floating at the voltage level of the
> plate, which is typically hundreds of volts -- be very careful!
> You could open the wire from each plate to the output transformer and
> hook in a meter in series with the plate temporarily, but that is a
> terrible amount of work for the small gain in accuracy.
>
> Adjust the bias pot so that the current reading is the appropriate
> value for the type of tube (see the table below). Let the amp warm up
> and note if the bias changes significantly. If so, select a
> compromise bias point.
>
> Keep in mind that if your circuit uses more than one tube per side,
> the bias current you're reading is multiplied by the number of tubes
> (e.g., if you're reading 60 milliamps and there are two power tubes
> per side, if the tubes are matched each of the two are getting
> nominally 30 milliamps). Check the other side of the circuit to
> confirm that the two sides are close (within 5 milliamps) to each
> other.
>
> If your ammeter has too high a series impedance, the shunt method
> won't work because the bias current gets significantly split between
> the meter and the transformer; the meter has no idea how much current
> is going through the transformer. You'll know it's not working
> because the current values you'll be reading will be much too low no
> matter how far you adjust the bias pot, the tubes will be glowing hot,
> and when you note that you'll reach quickly for the power switch! If
> you don't reach it quickly enough, you might blow a fuse. Don't
> despair: you can use another method called the "cathode resistor
> method."
>
> Cathode Resistor Method
>
> If the circuit already has a resistor in-line between the cathode and
> ground, use it. If the circuit has the cathode hooked up directly to
> ground, insert a low value resistor (say 1 Ohm/1 Watt) [even 10 ohms
> will work well, as the currents in a tube circuit will cause only a
> volt or so max across a 10 ohm resistor, not enough to change the
> circuit operation a lot.] in between the cathode and ground. This
> doesn't have to be a permanent change to the circuit; you can make a
> little adapter that fits between the tube and its socket that runs all
> the signals straight through except for the cathode lead -- that path
> gets the low value resistor in-line. If you make the adapter, you
> don't even have to drop the chassis from the amp to set the bias.
> Just pull a tube, install the adpater, and adjust.
>
> Hook up a voltmeter across the resistor and measure the voltage. For
> a 1 Ohm resistor, if you read 30 millivolts Ohm's Law says that you
> have 30 milliamps running through it. If you have some other value
> resistor, make the approriate calculation. Easy! But since the
> current at the cathode is the sum of the bias current and some other
> leakage currents, you need to compensate the reading a bit, typically
> 5 to 10 milliamps.
>
> What's nice about the cathode resistor method is that you're not
> dealing with high voltages. The cathode sits very close to ground so
> the chance of a dangerous mistake is lessened. You're also reading
> each tube's bias current individually.
>
> Other Methods
>
> Some of the manufacturers say to set the bias voltage to some
> specified voltage, without any other measurements. Presumably some
> designer somewhere decided how much was good for you and wrote down
> "Set the bias to xx volts" as a good compromise for all the tubes s/he
> expected. This method ignores the variability of transconductance in
> output tubes, and only gives good results for matched sets that happen
> to be exactly like the "typical" ones the designer thought they'd get.
> Note that Gr@@ve Tubes tries to help by providing matched tubes with a
> bias number from 1 to 10. If you have GT's with a "4" bias number,
> and you replace with a GT "4" set, they will have selected only tubes
> that are properly biased at that level, and no rebiasing will be
> necessary. Of course, GT expects to be repaid a fair profit for this
> service to you...
>
> Another way to set bias is to use a test signal, typically a sine
> wave. Monitor the output waveform on an oscilloscope and adjust the
> bias for minimum crossover distortion. The obvious problem is when
> has it "just disappeared"? Most folks do just a bit more than "just
> disappeared" and get their outputs too hot causing shortened tube life
> and overheating. Not very accurate or repeatable.
>
> You can also use a special purpose instrument that nulls the input
> signal out of the output signal so that you can monitor just the
> distortion products. You then adjust the bias to get the distortion
> to a realistic minimum without making it dramatically less than the
> residual THD. This is the premium method, but requires a distortion
> analyzer - big bucks.
>
> These methods can be more accurate than the first two methods but they
> require expertise and tools that most folks don't have.
>
> If you are a circuit hacker, and live on solder fumes and cold coffee,
> you can modify the amp with solid state servo bias adjusters that
> twiddle the bias to each output tube on the fly on a continuous, real
> time basis to keep each tube -* exactly *- where it ought to be. Only
> recommended for real wiring fanatics...
>
> GENERAL BIAS GUIDELINES (from Tremolux@aol.com)
>
> Currents Per Tube - Class AB1 Operation (most musical instrument amps
> are designed to run in class AB1)
>
> 6L6 - 30 to 35 ma
> 6V6 - 22 to 27 ma
> EL-34/6CA7 - 35 to 40 ma., sometimes even higher!
> 6550 - 40 to 50 ma
> EL-84/6BQ5 - 22 to 27 ma
>
> Class A currents will be higher. Example is 50 ma for a 6L6. Don't
> try to run an amp designed for AB1 in pure class A, it will overheat
> and probably blow. To handle the higher idle currents, Class A amps
> usually run at lower plate voltages.
>
12. Recommended amp modifications
- put a fuse in the B+ line after the rectifier(s) and before the first
capacitor filter. This can save burning out your power transformer and
maybe your output transformer if you get a shorted filter cap, shorted
output tube, or lose bias on an output tube. It -* might *- save an
output tube that has lost bias even though it also might not.
- put a 130 or 150 Volt MOV surge protector across the AC line at the
power transformer primary to absorb spikes from air conditioners and
motors turning on and punching through the primary insulation.
Recent articles say that 130V MOV's will eventually short, recommending
only 150V MOV's.
- get rid of all two wire line cords and line switching arrangements.
Refit with three wire cords, tieing the safety ground to the chassis.
You'll love this the next time you touch a mike or stand while holding
a guitar. No shocks. Oh, yeah. Do it to ALL your equipment to be safe.
- consider putting a small fan in your amp to cool it. Try a 240vac fan
running from the 120 vac line supply, which will run much slower and
quieter than a 120vac one.
- install small cathode resistors and independent bias adjustment for
each output tube to make biasing easy.
- open the feedback from the power amp output to it's input for more
power amp gain, more and earlier distortion. Or better yet, put in
a spst switch and you can pick the characteristics on the fly...
> - for the adventurous, add a separate filament transformer/rectifier/
> filter capacitor to make 9-12VDC at several amps and then use a
> three terminal rectifier to regulate this down to 6.3VDC, and feed
> this to your preamp tube filaments. Hum from filaments will drop
> right through the floor. Lotsa work, though.
13. NOT Recommended amp modifications
- using a variac to run it at a higher or lower line voltage. This
might be OK except that running it higher can overdissipate parts
and burn them up or overvoltage things like filter caps, which can
short and burn out your -* expensive *- output transformers, as well
as burning out your tube filaments by putting too much current
through them; and running it lower starves the filaments for
current, so they can't put out enough electrons, and any remaining
gas in the tube bombards the cathodes, poisoning the electron
emitting materials on the cathode surface, and wearing the tubes out
early.
- adding massive amounts of capacitance to the power supply filters to
reduce hum. Probably OK with solid state rectifiers, but in amps
with tube rectifiers this can cause current spikes in the rectifiers
that exceed the instantaneous current rating of the rectifier and
wear it out quickly.
Nathan points out "I seem to recall one of my Tube Amp Mentors
telling me that this is pretty much only the case with the first
filter cap after the rectifier, and that the impeadance of the power
supply was high enough that you could dump hundreds of uf worth of
filtering on latter stages (though the only place it's of much
benefit is at the power tube plate supply point.)
>14. Tube Characteristics and substitutions
> Under construction. To contain quick and dirty subs and some tube data
> such as recommended bias current and appx voltages. These subs are all
> taken from the Tube Substitution Handbook sold by Antique Electronics
> Supply.
>
> A (short) catalog of tubes you are likely to see in a guitar amp:
> 12AX7[A, WA] and substitutes - preamp and driver tubes
> 12AT7, 12AU7 and subs, preamp and driver tubes
> 12AY7 - driver tubes
> 6EU7 - dual triode used in some older amps for preamp tube
> 6L6 types - power output tubes, up to 50 watts/pair, a mainstay of Fender
> EL34 - Euro power pentodes, up to 50 watts/pair, many Marshalls
> 6V6 - smaller, lower power cousin of the 6L6, 10-14 watts per pair; used
> in smaller Fenders
> EL84 - fits a 9 pin socket like a 12AX7 but twice as tall; miniature
> power pentode, good for 12-18 watts per pair; used in smaller Vox
> amps, and a quad of these drives the Vox AC-30 for 30 watts.
>
> Substitutions:
> * means appropriate for parallel filament circuits
> # means may not work in all circuits
>
> Preamp and driver tube substitutions:
> 12AX7 (high gain dual triodes with pinout 9A)
> 12AD7* 12DT7 7729
> 12AU7# 5751* B339
> 12AU7A# 5751WA* B759
> 12AX7 6057 CV4004
> 12AX7A 6681 E83CC
> 12AX7WA 6L13 ECC803
> 12BZ7* 7025 ECC83
> 12DF7 7025A M8137
> 12DM7* 7494
>
> 12AU7 (moderately high gain dual triodes with pinout 9A)
> 12AU7[A,AW,] 6189 7730
> 12AX7* and subs 6670 ECC186
> 5814[A,AW]* 6680 ECC802
> 5963 7316 ECC82
> 6067 7489 M8136
>
> 12AT7 (medium gain dual triodes with pinout 9A)
> 12AT7[many suffixes] 7492 E81CC
> 6201 7728 ECC801
> 6679 A2900 M8162
> ECC81 B152 QA2406
> 12AZ7[A]* B309 QB309
> 6060 B739
> 6671 CV4024
>
> 12AY7 (low gain dual triodes with pinout 9A)
> 12AY7(and suffixes) 6072
> 2082
>
> Power tube substitutions:
> 6BQ5/EL84 (miniature pentode with pinout 9CV)
> 6267 7189 EF86
> 6BQ5 7189A EL84
> 6BQ5WA 7320 N709
> 6P15 E84L Z729
>
> 6L6 (beam power tube with pinout 7AC)
> 6L6(many suffixes) 7581(A)
> 5881 WT6
> 5932 EL37
>
> EL34/6CA7 (power pentode with pinout 8ET)
> EL34 12E13
> 6CA7 KT77
> 7D11 KT88
>
> 6550 (power pentode with pinout 7S)
> 6550[A] 7027A#
> 7D11 KT88
> 12E13
>
> Cautionary Tubes - these are very hard to find
> 7591/7591A - rumor has it that these otherwise excellent tubes were
> all bought up by an oriental buyer who toured the USA paying cash
> for all of them he could find, then disappearing. You are likely
> to only find used ones or the odd pair in some out of the way
> place. Dealers will in general not have them. These were used
> a lot in old Ampegs. They are very small and high gain for their
> physical size, so there may not be a lot of room in a chassis for
> a larger replacement. The 5881 will work in some circuits, but
> has significantly lower transconductance.
> 7199 - combination pentode/triode used as a one-tube voltage amp/phase
> inverter/driver for a pair of output tubes in some Ampeg amplifiers
> Note: These were once popular, but are now getting rarer and more
> expensive. There are a number of other pentode/triodes that can be
> substituted, but the pinouts are different and this will require
> require rewiring the socket for the tube. Examples are the 6AN8 and
> the 6U8. There is a Russian tube that is labeled 7199 which may
> work, although this is new.
> 7027/7027A - this is a high power tube similar to a 6550. The supply
> of these is very poor.
> 7189/7189A - a higher power/voltage version of the 6BQ5/EL84. Hard
> to find. A stock 6BQ5/EL84 may work if the power and voltage
> conditions in the amp are not right out at the limits of the tube
> design.
15. Maintenance (Still under construction)
Lots of good info from Jack Darr's Electric Guitar Repair Book and
The Tube Amp Book.
- retensioning tube socket contacts
- carbonized arc paths
> - checking for capacitor leakage
16. How can I tell if my output/power transformer is good?
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.
> CAUTION
> CAUTION
> CAUTION
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 knowlegable
and careful about how you do these tests. DO NOT TRY THIS IF YOU DO
NOT HAVE THE KNOW-HOW AND EXPERIENCE TO WORK SAFELY WITH THESE
VOLTAGES. IF YOU HAVE ANY QUESTION IN YOUR MIND WHETHER YOU CAN DO
THIS WORK SAFELY, YOU CAN'T. 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'r 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 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.
> Appendix A. Tube Stuff Suppliers
>
> The following article appeared in rec.radio.swap. A lot of this may be
> more oriented to radio than audio, but there is a large cross over in
> many tube types, as well as in the power transformers and power supply
> components.
> ======================================================================
>
> Gang: The enclosed is a bit long but it's the most complete list
> of tube sellers I've seen presented on the internet. You'll want to send
> this list to your printer! I didn't edit it down like I usually do
> since that would have made it quit difficult to read. // Jeff NH6IL
> *****************************************************************
> Article: 18193 of rec.radio.swap
> From: brian.carling@acenet.com (Brian Carling)
> List of suppliers and sources for finding vacuum tubes:
>
> Adkins:Charles P. N8QXP (313) 382-0272
> 1821 La Blanc.
> Lincoln Park,MI 48146 Tubes
>
> Allied Electronics: 800-433-5700 Located in 36 states, 83 branches.
> 7410 Pebble Drive Call for nearest branch. Call for Catalog.
> Fort Worth, Tx. 76118: Electronic components and tubes. Min.Prepaid
> order $25. Min. Credit Card order $50 as well as COD is $50. Min.
>
> Antique Audio 512-467-0304
> 5555 N. Lamar, Bldg. H-105
> Austin, TX 78751 Tubes, parts, books, kits
>
> Antique Electronic Supply Co. 602-820-5411
> 6221 S. Maple Avenue
> Tempe, AZ 85283 (Tubes & other components)
>
> Antique Radio Classified 508-371-0512 Write for free sample.
> P.O. Box 802 Magazine. You'll find almost ANYTHING here
> Carlisle, MA 01741 for older radios, obscure parts, tubes etc.
>
> Arlen Supply Company (610) 352-9311 / -9388 = FAX
> 7409 W. Chester Pike
> Upper Darby, PA 19082 Tubes. 1 million stocked. Minimum $150.00!
>
> A.R.S. 602-820-5411
> Arizona Need address Tubes
>
> David Ask No phone number given
> RR 2
> Houston, MN 55943 Tubes 4, 5 and 6 pin tubes. Send $1 for list
>
> Mel Brooks, K5DJB No phone number given
> 932 Macklyn Lane
> Bartlesville, OK 74006 Tubes, parts, schematics (incl. antique)
>
> Bauman:Jeff, WB5KZW. 313-435-9922: 313-661-0202 Jeff had 1500 Tubes
> 6647 Stonebridge East, for sale as of December,1993
> West Bloomfield, MI 48322
>
> Cable:John, 619-258-7931
> Address needed. Tubes from 1941 to 1960, new.
>
> CeCo Communications. 800-221-0860: 212-646-6300
> 2115 Avenue X Vacuum tubes
> Brooklyn, NY 11235
>
> C & N Electronics 800-421-9397: 612-429-9397
> 6104 Egg Lake Road FAX 612-429-0292
> Hugo, MN 55038 Buy & sell tubes
>
> Daily Electronics 800-346-6667: 206-896-8856: FAX 206-896-5476
> 10914 N.E. 39th Street
> Vancouver, WA 98682 Tubes, all types.
>
> Davilyn Corp. 800-235-6222 xct.CA: 818-787-3334 CA: FAX 818-787-4732
> 13406 Saticoy St. Electronic Tubes. Good Prices, Call for Catalog
> North Hollywood, CA 91605-3475 Also Surplus Electronic Gear.
>
> DH Distributors 316-684-0050
> P.O. Box 48623
> Wichita, KS 67201 Tubes, radio, TV, industrial
>
> Electron Tube Enterprises 802-879-0611
> Box 311
> Essex, VT 05451 Tubes
>
> Electronic Bits 'N Pieces 303-361-6530
> P.O. Box 31654
> Aurora, CO 80041 Tubes, transistors, diodes, chips
>
> William Erickson, W4UIL No phone number given
> 3905 Cherrywood Lane
> Annandale, VA 22003-1901 Tubes, older radios etc.
>
> E.S.R.C. (407) 735-3397
> P.O. Box 1192
> Delray Beach, FL 33447-1192 Buy, sell, swap tubes of all kinds
>
> Fair Radio Sales. 419-227-6573:419-223-2196: FAX 419-227-1313
> 1016 E. Eureka Box 1105 Parts, transformers, power supplies,
> Lima, OH 45802 Surplus and gov't surplus radios etc.
>
> Fala Electronics (No number listed) send S.A.S.E.
> P.O. Box 1376-1
> Milwaukee, WI 53201 Vacuum tubes
>
> Melvin Heineken, K5MNJ No phone number given
> 2204 Spruce Needle Rd. N.E.
> Rio Rancho, NM 87124-6308 Tubes. New unboxed. Other parts.
>
> Henry Radio Co. (310) 820-1234
> 2050 S. Bundy Drive
> Los Angeles, CA 90025 New tubes
>
> International Components Corp.800-325-0101: FAX 503-336-4400
> 1803 NW Lincoln Way Cabinets, components & vacuum tubes
> Toledo, OR 97391
>
> Jolida Tube Factory 800-783-2555
> 10820 Guilford Road Vacuum tubes
> Annapolis Junction, MD 20701
>
> KB5QOH (No name given) No phone number given
> 667 Nine Mile Hill Road
> Fairbanks, AK 99712 Tubes, parts, used amateur gear
>
> Kirby No number listed
> 298 W. Carmel Drive Tubes, new up to 90% off
> Carmel, IN 46032
>
> Cliff Kurtz, N6ZU No phone number given
> 6727 N. Pershing Avenue
> Stockton, CA 95207-2522 Tubes. Minimum order $10.00
>
> Robert Lang AA2EO (212) 877-0980
> 120 W. 70th Street Apt. 7-A
> New York, NY 10023 Tubes, vacuum variables, xfmrs etc.
>
> Madison Electronics (800) 231-3057
> 12310 Zavalla
> Houston, TX 77085 Tubes, meters etc.
>
> Rex Mason (704) 392-0359
> 100 Honeywood Avenue
> Charlotte, NC 28216 Tubes, antique parts, amateur, TV, VCR
>
> New Sensor Corp. 800-633-5477: 212-529-0466: FAX 212-529-0486
> 133 Fifth Avenue. Vacuum tubes galore! Call for list.Min.Order $50.
> New York, NY 10003 Std. test= $0.75/Tube. Premium Match $2/Tube
>
> No name (SHY?!) No phone number given
> 5150 Merritt Road
> Black Hawk, SD 57718 Tubes. S.A.S.E. for list
>
> P.E.M. Tubes (916) 383-9107
> 7392 French Road Tubes, radio, TV, transmitting, CRT
> Sacramento, CA 95828
>
> Pride Tubes 800-638-3925: 205-650-5522: FAX 205-880-8077
> 8200 South Memorial Parkway (800) 456-5642 100% RF Tested Tubes
> Huntsville, AL. 35802
>
> Rauchwerger, Lawrence 217-352-6195
> 1610 1/2, W. Union St
> Champaign, IL 61821 Tubes. S.A.S.E. list
>
> R.F Parts. To Order 800-737-2787:619-744-0700 or 0750 for Tech info
> 1320 Grand Avenue FAX 619-744-1943
> San Marcos, CA 92069 Diamond Antennas, RF Power Transistors & Tubes.
>
> Richardson Electronics (708) 208-2200 / (800) 235-2143
> 40 W. 267 Keflinger Road
> La Fox, IL 60147 Tubes, RF parts
>
> Steinmetz Electronics 219-931-9316
> 7519 Maplewood Avenue Tubes
> Hammond, IN 46324
>
> Svetlana Electron Devices Co. (415) 233-0429 / - 0439 = FAX
> 3000 Alpine Road
> Portola Valley, CA 94028 Tubes, RF power etc.
>
> Turner Electronics No number listed
> 16701 Main Street Suite 121
> Hesperia, CA 92345 Tubes, capacitors, S.A.S.E. list
>
> Unity Electronics No number listed
> P.O. Box 213 Vacuum tubes
> Elizabeth, NJ 07206
>
> C. Verderber No phone number given
> 2266, Route 9G
> Rhinebeck, NY 12572 Radios & tubes
>
> Carl R. Warren, W0KWS (417) 869-4738
> MPO Box 567
> Springfield, MO 65801 Tubes & parts. Also repair service
>
> Wayne (no last name given) (301) 963-4619
> No address given
> Gaithersburg, MD Tubes, equipment, parts, books
>
> Westgate Co. (800) 213-4563
> Need address! Tubes & transistors
>
> This list was prepared by AF4K, Brian Carling
> Please send additional sources for inclusion in this list.
> If you go to a hamfest and see someone selling tubes, get a card
> please and send me their name, address and phone number.
> AF4K @ W3INK
> (301) 990-6070
> brian.carling@acenet.com
>