Mic Kaczmarczik's Biasing Information

Biasing

Circuits

There have been a few questions lately about tube amps. I just finished
converting a transition era ('68 - '69) silverfaced Fender Twin Reverb
back to blackfaced specs so I'm fresh with the knowledge.

An invaluable resource is The Tube Amp Book, Fourth Edition. A more
invaluable resource is the storehouse of knowledge here on the net and
the very nice folks who make it available to anyone who asks. This post
is a redaction of that information. Feel free to incorporate this in an
FAQ list.

BIASING

You should re-bias the amp whenever you change power tubes or modify a
circuit. Each power tube needs a certain bias current to keep it
operating at the point where there is minimal distortion under normal
conditions. Individual tubes vary widely in the conditions that set the
correct bias current and amps typically provide only one adjustment
point for bias, so it's smart to buy matched sets of power tubes.

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! 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) in between the
cathode and ground. This doesn't have to be a permanent change to the
circuit; you can make a little adapter that fits between the tube and
its socket that runs all the signals straight through except for the
cathode lead -- that path gets the low value resistor in-line. If you
make the adapter, you don't even have to drop the chassis from the amp
to set the bias. Just pull a tube, install the adpater, and adjust.

Hook up a voltmeter across the resistor and measure the voltage. For a
1 Ohm resistor, if you read 30 millivolts Ohm's Law says that you have
30 milliamps running through it. If you have some other value resistor,
make the approriate calculation. Easy! But since the current at the
cathode is the sum of the bias current and some other leakage currents,
you need to compensate the reading a bit, typically 5 to 10 milliamps.

What's nice about the cathode resistor method is that you're not dealing
with high voltages. The cathode sits very close to ground so the chance
of a dangerous mistake is lessened. You're also reading each tube's
bias current individually.

Other Methods

A third 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. 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. These methods can be more
accurate than the first two methods but they require expertise and tools
that most folks don't have.

GENERAL BIAS GUIDELINES (from Tremolu--(at)--ol.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.

Len Moskowitz
Core Sound
moskowi--(at)--anix.com
--
Len Moskowitz
Core Sound
moskowi--(at)--anix.com

From moskowi--(at)--anix.com Fri Jan 6 10:48:01 CST 1995
Article: 31765 of alt.guitar
From: moskowi--(at)--anix.com (Len Moskowitz)
Newsgroups: alt.guitar,rec.music.makers.guitar
Subject: Re: How To Bias Your Tube Amp
Date: 6 Jan 1995 11:38:27 -0500
Organization: Public Access Internet & UNIX
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Message-ID: <3ejrm3$b1--(at)--anix3.panix.com>
References: <3eifum$2c--(at)--anix2.panix.com>
NNTP-Posting-Host: panix3.panix.com
Xref: geraldo.cc.utexas.edu alt.guitar:31765 rec.music.makers.guitar:36404

Shaun Wörtis wrote:

>> 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)....
>
>I was led to believe 2 or 3 watts was more appropriate...

The original 150 Ohm resistors in the Fender Twin Reverb were 7 Watt
units. Lower resistance dissipates less power. If we use a 1 Ohm
resistor that's 1/150th the power. WHat's 1/150th of 7 Watts? I guess
a 1 Watt resistor will do, no?

>I'm a little confused here. I had thought the optimal current for EL-34's
>was closer to 60ma! Tom Mitchell says never go below 45ma!

Let's let the net tube gurus reply.

--
Len Moskowitz
Core Sound
moskowi--(at)--anix.com

From balo--(at)--up.hp.com Fri Jan 6 13:45:47 CST 1995
Article: 31783 of alt.guitar
Newsgroups: alt.guitar,rec.music.makers.guitar
From: balo--(at)--up.hp.com (Tom Balon)
Subject: Re: How To Bias Your Tube Amp
Sender: new--(at)--pax (News Admin)
Message-ID:
Date: Fri, 6 Jan 1995 19:14:00 GMT
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In article ,
Shaun Wörtis wrote:
>In article <3eifum$2c--(at)--anix2.panix.com>, moskowi--(at)--anix.com (Len
>Moskowitz) wrote:
>
>...[snip]...
>
>> 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)....
>
>
>
>I was led to believe 2 or 3 watts was more appropriate...

Lets see. P = IE, P = I*I*R, so, your bias current (at idle)
is 30ma. The resistance is 1 ohm. So that gives 0.0009 W.
So, at idle, you would need a 0.0009W resistor. Unless your
running class A, the amp will draw more current when your playing,
so you need to take that into account. A big beefy power transfomer
might provide say, 250, hell lets go wild and say 500ma of HV.
If we recalculate we get P = (0.5 * 0.5) * 1 = 0.25W So, a 1/4W
resistor here would do just fine.

In fact, if you use a small power resistor, and the tube
shorts out and draws tons of current, the resistor will burn up.
Better to burn out a small cheap resistor than an output
transformer !

Furthurmore, unless you have a good DVM that is fairly accurate
measuring a few millivolts, I would suggest using a 10ohm resistor
instead of a 1 ohm.
>
>
>
>> GENERAL BIAS GUIDELINES (from Tremolu--(at)--ol.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
>
>
>
>I'm a little confused here. I had thought the optimal current for EL-34's
>was closer to 60ma! Tom Mitchell says never go below 45ma!
>

These are just some ballpark figures that are reasonable for most
of your average guitar amps. Your milage may vary. There are other
factors to take into account (Plate voltage, driver stage output
...) when setting the bias.

An EL34 in pentode mode has a max plate dissapation of 25Watts. If
your plate voltage is say 450V you should keep the bias below
25W/450V or 55ma. More than this and you'll be exceeding the max.
plate dissapation of the tube.

Tom

+-----------------------------------------------------------------------+
T.A.S.
Tube Amp Service "On the trailing edge of technology."
San Francisco Ca.
(415) 334-5200 PST
+-----------------------------------------------------------------------+

From mike--(at)--ol.hp.com Fri Jan 6 22:51:52 CST 1995
Article: 31789 of alt.guitar
From: mike--(at)--ol.hp.com (Mike McTigue)
Newsgroups: alt.guitar,rec.music.makers.guitar
Subject: Re: How To Bias Your Tube Amp
Followup-To: alt.guitar,rec.music.makers.guitar
Date: 6 Jan 1995 19:40:27 GMT
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X-Newsreader: TIN [version 1.2 PL2]
Xref: geraldo.cc.utexas.edu alt.guitar:31789 rec.music.makers.guitar:36429

Okay, lets suppose you have a 100 watt amp with two
output tubes that conduct 1/2 the time (AB1 ?). That means
that each tube has to supply 50 watts into the ouput
transformer (assuming transformer is perfect). If typical
plate voltages are 400V and we assume that a tube can
swing 300V on each half sine wave, you have a halfwave
rectified sine wave with the peak voltage of 300V. If for
extra margin we assume that the signal is really closer
to a squarewave, then the peak voltage swing plate is
300V or 212 Vrms [(300**2/2)**.5]. If the tube is suppling
50W and the rms voltage is 212V, then the rms current
must be 50/212 = 236mA.

So, if 236mA is flowing through the one ohm resistor, then
the power dissapation in the resistor is .236**2 * 1 or
55mW. That is ~1/18th of a watt!

Unless you are worried about the tube shorting out and
taking out the cathode resistor, a 1 watt resistor is
more than adequate. As far as I'm concerned, if the
tube shorts out and takes out my 1ohm resistor but doesn't
take out the output transformer, I'll be very happy.
I'm not saying this is a way to protect anything, but
who cares about the resistor (10 or 20 cent part).

As a note, as the voltage that the tube is capable of
swinging goes down, the current in the tube has to go
up to still deliver 50W. So if my assumption that the
tube swings 300V is high, the power in the 1 ohm
resistor increases. If the tube can only swing 200V
peak or 141Vrms, then the power in the 1 ohm resistor
is now 125mW, still way below 1W.

Anyway, no one with a 100W amp really ever has 100W
continuous going. That would be really loud. So this
analysis is worst case if anything. Unless I'm blew
something bigtime, a 1ohm resistor is plenty for
cathode series resistors. Any comments?

MikeM

From fto--(at)--etcom.com Mon Jan 9 13:11:02 CST 1995
Article: 31878 of alt.guitar
Newsgroups: alt.guitar,rec.music.makers.guitar
From: fto--(at)--etcom.com (Tom May)
Subject: Re: How To Bias Your Tube Amp
In-Reply-To: balo--(at)--up.hp.com's message of Fri, 6 Jan 1995 19:14:00 GMT
Message-ID:
Sender: fto--(at)--etcom.com (Tom May)
Organization: The Planet Eden
References: <3eifum$2c--(at)--anix2.panix.com>

Date: Sun, 8 Jan 1995 07:44:08 GMT
Lines: 55
Xref: geraldo.cc.utexas.edu alt.guitar:31878 rec.music.makers.guitar:36537

In article balo--(at)--up.hp.com (Tom Balon) writes:

>>> GENERAL BIAS GUIDELINES (from Tremolu--(at)--ol.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
>>
>>
>>
>>I'm a little confused here. I had thought the optimal current for EL-34's
>>was closer to 60ma! Tom Mitchell says never go below 45ma!
>>
>
> These are just some ballpark figures that are reasonable for most
> of your average guitar amps. Your milage may vary. There are other
> factors to take into account (Plate voltage, driver stage output
> ...) when setting the bias.
>
> An EL34 in pentode mode has a max plate dissapation of 25Watts. If
> your plate voltage is say 450V you should keep the bias below
> 25W/450V or 55ma. More than this and you'll be exceeding the max.
> plate dissapation of the tube.

Isn't it a major feature of class AB1 that plate dissipation is at a
minimum when no input signal is applied? If so, and you set the bias
so the plate is dissipating its maximum rated power with zero signal,
then when you apply a signal and increase the plate dissipation you'll
be in trouble.

Your derivation makes sense for class A, where the maximum plate
dissipation should be occuring at or near the zero signal point -- or
is my mind too fogged with transistor theory? The load-line graphs in
my RCA Receiving Tube Manual seem to agree with what I'm saying,
though.

Also in the manual it says things like zero signal plate current for
6L6 push-pull class AB1 amplifier typical operation at 360V would be
88ma, and at 450V would be 116ma (for two tubes). Presumably that is
44 and 58ma, respectively, for each tube. That is way more than the
30-35ma recommended above. Do guitar amplifiers operate atypically,
or what? I suppose that biasing the tubes closer to cutoff (less zero
signal plate current) would only increase crossover distortion, which
might not be such a big deal for guitar amps except that crossover
distortion is not good distortion.

Just wondering about this stuff before I become a real tube hacker and
have to learn the hard way.
--
Tom, proto tube hacker

From tremolu--(at)--ol.com Mon Jan 9 13:11:28 CST 1995
Article: 31899 of alt.guitar
From: tremolu--(at)--ol.com (Tremolux)
Newsgroups: alt.guitar
Subject: Re: How To Bias Your Tube Amp
Date: 8 Jan 1995 15:49:56 -0500
Organization: America Online, Inc. (1-800-827-6364)
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Sender: roo--(at)--ewsbf02.news.aol.com
Message-ID: <3epj5k$i9--(at)--ewsbf02.news.aol.com>
References:
Reply-To: tremolu--(at)--ol.com (Tremolux)

You have to use the RCA tube manual (I have a couple of them) as a
guideline for circuits that operate conservatively. Guitar amps are not
designed to operate conservatively. Fender typically runs the plate
voltage on 6L6s at about 450 volts. Consequently, you'll need to back down
the idle current to maintain reasonable plate dissipation.

Take it from someone who does this routinely, and if you don't believe me,
call Gerald Weber and ask him. In a Fender, 30 to 35 ma is right for a
6L6. Anything higher and the thing will sound mushy.

Regards.

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