From kee–(at)–den.com Fri Mar 1 18:51:31 CST 1996
Article: 82125 of alt.guitar
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From: kee–(at)–den.com (R.G. Keen)
Newsgroups: rec.music.makers.builders,rec.music.makers.guitar,alt.guitar
Subject: Tube Amp Mini-FAQ Posting
Date: 28 Feb 1996 19:42:45 GMT
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This is the (very irregular) posting of my tube amp mini-FAQ. I usually
leave this out on Jamie’s web page at
http://www.wwu.edu/~n9343176/docs/tube.amp.mini.faq
rather than posting the whole thing, but this is the first update in a
year.
Please send any comments, corrections, or – please – additional material
that should be included.
======================================================================
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, kee–(at)–den.com
Most recent revision level is Version 1.07, appx. 2/28/96
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, balo–(at)–up.hp.com
David Mourning, da–(at)–cs.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, deni–(at)–eismo.css.gov
Nathan Stewart, npstewa–(at)–os.ncsu.edu
George Kaschner, gckaschne–(at)–cdavis.edu
David Kohn, koh–(at)–CTC.COM
Michael Edelman, mj–(at)–ookie.pass.wayne.edu
Len Moskowitz, moskowi–(at)–anix.com
Tremolu–(at)–ol.com
Brian Carling, brian.carlin–(at)–cenet.com
Eric Barbour ebarbou–(at)–etcom8.netcom.com
=========================================================================
INDEX
> 0. *** SAFETY WARNING *** READ THIS FIRST!!!!!
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 amp to be more powerful?
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. How can I tell if my output transformer is live or dead?
> B. Where can I get a good replacement output transformer?
C. I want to make my own power and output transformers. How do I do
this?/ Where can I find information about this?
> D. 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?
> D. Matched output tubes – do you need them?
12. Amplifier Modifications
A. Recommended amp modifications
> B. OK but involved modifications
C. NOT Recommended amp modifications
13. Tube Characteristics and substitutions
>14. Maintenance (Still under construction, pretty thin right now)
Appendix A. Tube Stuff Suppliers
Appendix B. Tube Makers Producing Today (Eric Barbour news posting)
=========================================================================
> 0. *** SAFETY WARNING *** READ THIS FIRST!!!!!
> 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 you 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.
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 (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 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–(at)– 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.
> 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”.
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.
> 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.
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. 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.
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.
> 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.
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 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.
> 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.
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.
> There is a document on exactly this at
> http://www.wwu.edu/~n9343176/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.
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
