From moskowi–(at)–anix.com Fri Jan 6 10:47:35 CST 1995
Article: 31710 of alt.guitar
From: moskowi–(at)–anix.com (Len Moskowitz)
Subject: How To Bias Your Tube Amp
Date: 5 Jan 1995 23:12:06 -0500
Organization: PANIX Public Access Internet and Unix, NYC
Xref: geraldo.cc.utexas.edu alt.guitar:31710 rec.music.makers.guitar:36342
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
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.
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.
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
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)
Subject: Re: How To Bias Your Tube Amp
Date: 6 Jan 1995 11:38:27 -0500
Organization: Public Access Internet & UNIX
Xref: geraldo.cc.utexas.edu alt.guitar:31765 rec.music.makers.guitar:36404