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 1.10, appx. 3/29/96
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Contributors 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:
* 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
Eric Barbour ebarbour@netcom8.netcom.com
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Special thanks to Nathan Stewart who did the bulk of the work converting
this to HTML.
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INDEX
Contributors
Back to the Effects Page
1. *** SAFETY WARNING ***READ THIS FIRST!!!!!
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...
o Blues distortion?
o Marshall/metal/Boogie/etc. distortion?
o Good distortion at low(er) volumes?
8. Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?
9. Output transformer questions:
o How can I tell if my output transformer is live or dead?
o Where can I get a good replacement output transformer?
o I want to make my own power and output transformers. How do I do
this?/ Where can I find information about this?
o Should I replace my stock transformer with a
new/old/vintage/purple one for better clean/grunge/grit/etc.
sound?
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)
o get lower hum?
o have higher gain/more distortion?
o have a smoother, less buzzy distortion?
12. When should I bias my amp and how do I do this?
o What is "bias"?
o When should I bias my amp?
o How do I bias my amp?
o Matched output tubes - do you need them?
13. Amplifier Modifications
o OK/Recommended amp modifications
o NOT Recommended amp modifications
14. Tube Characteristics and Substitutions
15. Maintenance Issues
o Cap Jobs
+ What's a cap job?
+ Do new caps need to be formed?
+ How do I "re-form" electrolytic caps?
o Sockets
o Dirt
o Why do my tube have a blue glow inside them?
o Other Issues
16. Appendix A. Tube Stuff Suppliers
17. Appendix B. Tube Makers Producing Today (Eric Barbour news posting)
Back to the index...
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*** SAFETY WARNING ***
READ THIS FIRST!!!!!
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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.
BASIC REQUIREMENTS
* UNPLUG IT FIRST Pretty self explanatory. Do not, ever, ever, leave the
equipment plugged in and start work on it. Leaving it plugged in
guarantees that you will have hazardous voltages inside the chassis
where you are about to work. This is like setting a trap for yourself.
* LET IT DRAIN If the amp has been turned on recently, the caps will
still have some high voltage left in them after the switch is turned
off. Let it sit for five minutes after you turn it off.
* SUCK IT DRY When you open up an amp, you need to find a way to drain
off any residual high voltage. A handy way to do this is to connect a
shorting jumper between the plate of a preamp tube and ground. This
jumper will drain any high voltage to ground through the 50k to 100K
plate resistor on the tube. To do this successfully, you will need to
know which pins are the plate pins. Look it up for the amp you're
going to be working on. You'll need to know this for the work anyway.
Leave the jumper in place while you do your work ( high voltage
electrolytics caps can "regrow" voltage like a battery sometimes.
Really. ) Remember to remove it when you finish your work.
* TEST IT Take your multimeter and ground the (-) lead. Probe the high
voltage caps and be sure the voltage across them is down, preferably
to less than 10V.
* BUTTON IT BACK UP FIRST Take the shorting jumper out. Put the chassis
back in the cabinet, making sure all of your tools, stray bits of
solder, wire, etc. are out of it. You don't have to actually put all
the screws and so forth back in if you believe more work might be
needed, but make sure that the chassis is sitting stably in the
cabinet and won't fall out. At the end of a listening test, either
continue buttoning up if you're done, or go back to UNPLUG IT FIRST.
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Back to the index...
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.
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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):
* "The Ultimate Tone" by Kevin O'Connor. This the best book on guitar
amps I've found. It assumes you know some electronics to start with,
so is not a beginner's book. Published by Power Press, which now has a
web page at http://www.wwdc.com/~power/ .
* "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 inaccurate.
* "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 physical construction as well.
* Radiotron Designer's Handbook, Langford-Smith. Heavy theory, heavy
technical. Not coffee table reading, but if you want to know, it's
probably in there. This book is perhaps the most highly sought after
tube related book, and commonly goes for $75-$100 in good shape. You
want the 4th edition.
Old Colony Sound just announced a CD ROM version of this book,
apparently indexed, illustrations and all, for $69.95.
* RCA Receiving Tube Manual. Reprints available from several sources,
including Antique Electronic Supply & others (Old Colony?) Mostly tube
spec sheets & some characteristics charts. The intro is a pretty good
technical primer.
* Electron Tubes, R.G. Kloeffler. little application, but a good easy to
digest explanation of characteristics 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 characteristics, transfer 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.
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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:
* 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.
Premium Suppliers
* Fender Musical Instruments - call 800-854-6230 for a list of
* dealers
* Richardson Electronics - 800-348-5580 for dealer list
* RAM Tubes, 805-962-4445
* VTL, 714-627-5944
Here are "more tube supply sources":
* Triode Electronics, 312-871-7459
* Elmiria Electronics 800-847-1695
* Antique Electronic Supply, oriented to radio collectors, 602-820-5411
* Antique Audio, oriented to radio collectors, 512-467-0304
* New Sensor, mostly imported tubes (here's the source of Sovtek), call
Mike Mathews, 212-980-6748. Min. order is $50.00.
* ARS Electronics, 800-422-4250
* Department of Defense surplus auction. 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.
* Surplus electronics dealers - see the Telco yellow pages
* call everybody in the yellow pages under Electronics, TV-Repair,
Communications, and any other promising category.
* Hamfests
* Angela Instruments, 8600 Foundry St. Box 2043, Savage, Md. 20763,
301-725-0451.
* 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.
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Back to the index...
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.
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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Back to the index...
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.
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Back to the index...
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.
Guitar Player magazine ran a construction article on this very topic,
modifying a Fender Bassman to be the "Ultimate Blues Machine". The
article ran in 1995, authored by John McIntyre.
A recently voiced although intuitively applied idea in distortion is
that tube distortion sounds best when each successive distortion stage
is overdriven by less than about 12db. This has the effect of keeping
the tubes inside the area where the signal is more
compression-distorted than clipped. That is what those resistive
divider chains between distortion stages are for inside those
distortion preamp schematics. Mesa's distortion preamps are another
good example.
Overdriving a tube stage too much gives you harsher clipping, not the
singing, sweet distortion we want. To really get sweet, crunchy
distortion, keep each stage that goes into distortion no more than
6-9db into distortion.
* 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.
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Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?
Back to the index...
* 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) ---
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Output transformer questions:
Back to the index...
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.
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CAUTION CAUTION CAUTION
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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.
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'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 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.
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What is the easiest way to get tube sound at a good price?
Back to the index...
* 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.
There is a document on exactly this at
http://www.wwu.edu/~n9343176/docs/old2new.html The document goes into
excellent detail on the in's and out's of building from old tube gear
and the possible and useful variations of which stages with how much
gain go where in the amp.
* 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.
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How can I modify my tube amp to ... ?
Back to the index...
(also see recommended mods, below)
* Get lower hum?
o Replace the defective humming tube
o Replace or improve the power supply filter capacitors
o Fix the defective internal ground wiring, as on a reverb tank, or
previous "improvements" and modifications
o Run the preamp filaments on regulated DC, not AC, starting with
the input tubes
o Rewire the grounding so the amp is star grounded, and does not
use the chassis as a ground bus
o Move the signal wires around, nearer/farther from the chassis or
60 Hz AC carrying wires
o 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 ground 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?
o 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)
o Use the alternate channel for more gain, perhaps jumpering two
channels together
* have a smoother, less buzzy distortion?
o 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-
o Use series grid resistors to cut the high frequencies in and
after distortion stages
o Use a lowpass filter after the amplifier and before the speakers
to cut out some of the higher overtones.
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When should I bias my amp and how do I do this?
Back to the index...
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.
Note that you may not want matched pairs, depending you your taste. See
section D. below.
B. When should I bias my amp?
You should re-bias the amp whenever you change power tubes or modify the
power amp circuits.
Each power tube needs a certain bias current to keep it operating at the
point where the amount and type of distortion under normal conditions is
well controlled. Individual tubes vary widely in the grid bias that sets
the correct idle bias current. If you change tubes or tinker with the
circuit, you need to make sure the tubes are set back into operation in a
way that sounds good and does not cook the tubes.
Amps typically provide only one adjustment point for bias, assuming that
you will have bought matched sets of power tubes.
It is possible to modify your amp to "match" unmatched tubes by setting the
bias voltage and AC drive level of each tube individually. This may require
some serious soldering, though. See section D. below for a discussion on
matching, and the mods section for what you have to change.
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 adapter, 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 appropriate 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.
D. Matched output tubes - do you need them?
Do I always have to buy matched pairs of output tubes? The issue of
"matching" output tubes, either by buying carefully matched pairs or by
tweaking the bias levels and drive signals per output tube is not a settled
one. It used to be common wisdom to simply buy matched tubes. A few people
noticed, however, that they had a favorite pair of output tubes, which made
their amp sound much better than others. The common assumption was that
these tubes were better matched somehow. When these tubes get measured,
though, it usually turns out that they are NOT matched, at least not
matched for AC gain characteristics.
The concept of matched output tubes comes to us musical amp types from the
hifi community, where they are trying to get the lowest possible
distortion. This was true from the start, when Fender was trying to build
low distortion amps and copied hifi circuits. The concept has simply clung
to us, largely through inertia. It is relatively well accepted even in the
hifi circles now that even-order distortion is euphonic, sounds good to our
ears. It is very likely that the even-order distortion produced when
mismatched output tubes are used sounds better than perfectly matched
tubes.
If you have modified your amp so you can independently set the DC bias and
the AC drive signal, you can tune almost any pair of tubes into AC and DC
matching. You can also tune in a selective amount of AC drive mismatch to
experiment with the selective mismatching sound.
There are technical reasons for matching. Getting enough turns of wire on
the primary of an output transformer to get the right primary inductance
and still using as little iron and copper as possible to do the job
properly is an engineering problem that almost always results in Class AB
output transformers being smaller for proportional power outptu than a
Class A output transformer would be. The (relatively) smaller transformer
and wire size makes a class AB (most guitar amps) output transformer
susceptible to burning out if one of the half-primaries carries too much
current.
If one side of the transformer carries significantly more current (like
double) than it would otherwise in "normal" operation, it is possible it
will overheat or open, effectively killing the transformer. Tubes that are
so mismatched that to get the right total current for a pair means that one
is carrying more than 50% over the nominal DC current for a matched pair is
getting into the region where you ought to worry about output transformer
damage.
If you mismatch, try to get the DC current the same in both sides of the
output transformer, and an imbalance in the AC gain of the tubes. The
logical limit of this AC mismatching is to remove all the AC drive from one
output tube, which is a technique used by at least one commercial amp
maker. This effectively keeps the output transformer happy with respect to
DC, and gives you a single ended output stage; this also costs you a large
amount of your available output power, but, hey, we're after tone, right?
Note that the commercial tube suppliers have good reason for wanting to
sell us matched sets at a premium. I would expect their opinion to be that
matched sets are absolutely crucial. As in all musical matters, let your
own personal ears be your guide.
If you have a set of tubes you know are not matched, or if you have
modified your amp to be able to set the bias and drive levels on each
output tube separately so you can either match or not match the tubes at
will, you might want to try un-matching them and see how it sounds to you.
---------------------------------------------------------------------------
Amplifier Modifications
Back to the index...
A. OK/Recommended amp modifications
Read the SAFETY WARNING first, before you put your hands - or other
personal parts - into a tube amp.
* 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. The fuse
current rating should be slightly larger than the max current rating
for your output tubes, generally much less than 1A.
* 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.
* Put 1500Volt, 1A silicon diodes in series with the two sections of
your rectifier tube (if you have a rectifier tube) so that if the
rectifier tube shorts, the silicon will save the output tubes, and
power and output transformer.
* Gerald Weber advocates using a 270K/27K resistor divider from B+ to
raise the filament windings in a DC sense above ground. This keeps
electrons from the filament from hitting the plate, another source of
hum.
* Put 1500Volt, 1A silicon diodes in series with the two sections of
your rectifier tube (if you have a rectifier tube) so that if the
rectifier tube shorts, the silicon will save the output tubes, and
power and output transformer. The B+ will go up about 50V when (if!)
the rectifier tube shorts, so the amp will have a little more power
and run hotter. This can still hurt modern manufactured power tubes if
it goes on too long, so check the rectifier tube frequently.
* Bill Webb's favorite tone mods for Fender amps
o at the Vibrato channel's second gain stage, change the ceramic
0.02uF coupling cap to polypropylene or polystyrene
o replace the coupling cap at the input of the phase inverter with
a better cap (polypropylene -> polystyrene -> mylar in order of
preference); change its value to 0.001 to make the amp
"sparklier" and to 0.01 to make the amp sound bigger and more
midrangy
o The 3.3M resistor which mixes the dry and reverb at the output of
the 3rd gain stage, vibrato preamp, is paralleled by a 10pF
ceramic cap. Change this to silver-mica to make the amp sparklier
o The power amp feedback loop resistor is usually 820 ohms; insert
another 820 ohm resistor. This reduces the feedback, increases
the power amp's gain, and softens the onset of distortion.
* Remove the single bias adjust pot in your amp and put in two,
connecting one to each output tube. You can now set the bias voltage
on each tube to be different, which can match the DC currents for
un-matched tubes, or un-match matched ones for more even harmonic
distortion.
* Tinker the driver circuit to let you adjust the relative amount of AC
drive to each output tube. This lets you match/unmatch output tubes in
an AC sense just like the bias mod lets you change the relative DC
points.
C. NOT Recommended amp modifications
These are likely to be just plain bad, either grossly (it dies soon) or
subtly (it dies slowly, eats tubes, or other sicknesses). Don't do these or
let a tech do them to... er... for you.
* 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 impedance 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.)
* replacing your rectifier tube with a solid-state plug in module
replacement. This effectively just puts in a pair of silicon diodes
which take the place of the tube. But it also lets the B+ come up
about 50V. This won't kill the amp immediately, but it runs the
outputs hotter. Fender often put more than the rated maximum voltage
on the output tubes to get moe power out of them; old US and Euro
manufactured tubes would usually handle it just fine. Some lower cost
modern manufacture tubes CAN'T stand the extra volts as a steady diet,
and can succumb to the Dark Side of the Force - soon.
---------------------------------------------------------------------------
Tube Characteristics and substitutions
Back to the index...
Some 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. or provided from
the net.
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 - legend 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. I have personally seen trays full of
NOS 7591A's for sale in the Akihabra electronics district in Tokyo,
lending some credence to the rumor.
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.
Rumor Update: The rumor mill on the net says that the Russians will
soon be making 7591's soon. Cross your fingers...
* 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.
---------------------------------------------------------------------------
Maintenance Issues
Back to the index...
Cap Jobs - Do I need one? How often? Why?
What's a cap job? A technician may recommend you have a "cap job".
This means that he will replace every single electrolytic capactor in
the amp, from the power supply right down to the cathode bypass caps.
This is because electrolytic (polarized) capacitors have an inherent
wear-out mechanism and will eventually die even if you don't play
death/metal/country/barbershop through them every day - in fact they
may wear out sooner if you leave it sitting in the attic. Here's why.
A capacitor is essentially two conductive plates separated by an
insulator. The bigger the plate area and the thinner the insulator,
the higher the "capacitance" is. Electrolytic capacitors get a very
thin insulator by "growing" an insulating layer of aluminum oxide on
the outside of a rolled up piece of aluminum foil.
The oxide layer is "formed" at manufacture by feeding the aluminum
foil a very small and carefully controlled amount of current. The
current causes a chemical reaction between the foil and the water
solution (electrolyte! ... hey... is that where they got the name??
yep.) which makes an oxide layer grow. As the layer grows, they use
higher and higher voltages to force the same small current through the
layer, which gets thicker and more resistive with time. When they have
to use the full rated voltage to get the forming current through, the
cap is fully "formed" and ready to ship.
If the capacitor is used regularly, has voltage applied to it, and
does not get too hot, the oxide film lasts up to a few decades. If the
capacitor is not used much, or gets too hot, the oxide film slowly
un-forms, the leakage current goes up, and it will eventually short.
Electrolytic caps are designed to last ten years. It is a tribute to
the quality of manufacture that they often last three, sometimes four
times that.
Old amps, particularly if they have not been used regularly need to
have every electrolytic cap replaced. This cap job may be needed every
ten or so years.
Non-electrolytic caps do not have this wear out mechanism, and do not
need replaced for this reason. Modern capacitors can in some
circumstances be much better than old ones, and you can sometimes get
a clearer, more sparkly tone by changing the non-electrolytic caps -
assuming that is something you want to do.
Do new caps need to be formed?
There's a lot of controvery on "reforming" replacement caps. Here are
a few answers.
Manufacturers of caps design their caps for a ten year working life,
and a five year shelf life. That means that the stresses and heat of
working in equipment will leave the vast majority of caps functioning
OK after ten years of normal operation. After that, it's gravy to the
buyer.
They also design them to work OK after sitting on a shelf unused for
five years, meaning that the cap should not fail if it's put into
operation at rated voltage after sitting unused for five years. As
noted above, the caps do slowly un-form without regular use.
If the electrolytic caps you use to fix your amp are over five years
old as determined by the date code on them, you ought to at least
worry about forming them, and if they're over ten years old (like NOS
multisection cans), definitely re-form them. Other than that, put them
in and turn it on.
How do I "re-form" electrolytic caps?
You'll hear folks talk about "bringing an amp up slowly on a variac";
this can work but is not particularly good for your tubes. A better
way is this:
1. Pull out all the tubes.
2. if your amp has a tube rectifier, solder in temporarily some high
voltage silicon diodes across the tube lugs to be a rectifier
that does not depend on the filament voltages. If your amp has
silicon diodes, you can skip this.
3. open up the wire that goes from the rectifier tube (or solid
state diodes) to the first power supply filter stage and solder
in series with the wire a temporary 100K 2- 5W resistor. This
resistor will limit the current that can flow into the caps and
the amount of voltage that is applied to them to safe values that
will cause the insulating layer to re-form.
4. clip your voltmeter across the resistor
5. button it up. Turn it on (no tubes in it, remember). Watch the
voltmeter.
6. when the voltmeter reading drops to less than 20-30VDC, your caps
are formed.
7. open it back up and pull out those diodes and resistor, putting
it back in original shape.
The forming could take hours to days.
Sockets
Sockets get dirty, corroded, broken, and "arced" To recondition them,
get a can of spray contact cleaner, the kind that says "no residue".
Squirt some in each socket hole, then insert that tube in the socket,
wiggle it around, and remove it several times to get the crud off.
Take a thin tool like a jeweler's screwdriver or ice pick and gently
bend the contacts inside each hole so they hold the pins better. If
the socket is cracked, or has blackened lines from pin to pin (where
an electrical arc has actually burned the socket into a carbon
material that conducts electricity), replace the socket.
Dirt
The dusty, hairy, oily layer that collects on the chassis can conduct
electricity as it absorbs humidity from the air. Vacuum it away
periodically.
Other Issues
Lots of good info is contained in Jack Darr's "Electric Guitar Repair
Book", if you can find a copy (it's now out of print) and in Pittman's
"The Tube Amp Book" and Webers "Desktop Reference...". Look for: -
checking for capacitor leakage
From watching a tech work on Fenders, I picked up a nice tidbit. The
eyelet boards in Fenders have most components mounted across the
eyelet board. A very few parts run along the length of the eyelet
board. Because the eyelet board flexes, there is a lot of stress on
the solder joints at the end of these lengthwise components and the
joints often crack. Every time you open up a Fender, take a look and
maybe a soldering iron to these joints. If it's your personal amp, you
might want to get a new part for these positions with long leads and
bend a loop in the leads so that the leads can flex and not put stress
on the solder joints.
---------------------------------------------------------------------------
Appendix A. Tube Stuff Suppliers
Back to the index...
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
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Appendix B. Tube Makers Producing Today (Eric Barbour news posting)
Back to the index...
(The following is the text of a note posted to the alt.guitar news group by
Eric Barbour )
Different makers of tubes use different designs. There are six makers of
common audio tubes right now:
* Shuguang, China--good 12AX7s, so-so power tubes
* Tesla, Czech--ok EL34s, preamp tubes variable
* Reflector, Russia (sold under Sovtek brand)--good 5881, EL84, so-so
12AX7 (they came out with EL34s recently---I am testing them)
* Kaluga, Russia (only a few types--sold under Sovtek, Audio Glassic)
good 5881s, not sure what else they make today
* Svetlana, Russia--has a 6550 now, good---will introduce an EL34 soon
* EI, Yugoslavia (in Serbia)--fair 12AX7s, fair EL34s, future supplies
are questionable because Serbian products are under economic sanction;
thanks to that Bosnia business!
That's ALL there are right now. That's it. Any NEW tube you buy is from one
of the above.
For your guitar amp, I would recommend the "Sovtek" 5881, it's a really
nice, rugged and smooth-sounding tube. It was a military type used in servo
amps in jet aircraft, so it has to be good. If you have a Marshall or other
EL34 amp, the Sovtek 6CA7 imitation (recently released) is probably most
rugged. If you want more distortion and a more bluesy sound, you want the
skinny EL34s. The Svetlana EL34 will be a skinny type, it should be very
good.
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