ASKAMPMAN
Bias Blues
Hi Jeff,
I just read your article in the February 2007
issue of Premier Guitar about swapping out
6L6s with EL34s in a Crate amp and what
goes into re-biasing. I wonder if you might
be so good as to help answer another bias-
related question.
To distort or not to distort, that is only part
of the question! We also need to know what
kind of distortion and where it should take
place. To start things off, consider this hypothetical question.
I own a Framus Cobra 100-watt head which
has Electro-Harmonix 12AX7 preamp and
EL34 output tubes. I read online that it
ships from the factory with a “cold” bias
setting of around 25mA and should be
taken in to an amp tech to “heat it up” to
around 33mA to minimize “crossover
distortion and clean up some of the
buzziness.”
The information that you read online is:
A) probably correct
B) not a bad thing to do
C) maybe not right for you
Okay, it’s a trick question. The correct answer
is “all of the above.”
fier is considered to be “high-gain,” the term
refers to the amount of gain and distortion
created in the preamp section of the amp.
It really has nothing to do with the inherent
distortion characteristics of the output stage.
So when your tech said that the output stage
would now have less distortion, he was not
referring to, nor did he lower, the front-end
gain or change the front-end characteristics
of the amp. If you could have listened to the
characteristics of the preamp section in the
amp prior to and after the bias adjustment,
they would have been virtually identical.
I then took the amp to a repair shop.
When I got it back, the tech said it was
true that the bias was cold and didn’t
have a “clean” wave on his oscilloscope.
He ended up “warming” the amp up
to around 32mA and said that the amp
should now have a lot less distortion.
That took me by surprise because the
Cobra is supposed to be a high-gain
amp, not low-gain. Ever since, the amp
has had a good hard rock sound on
channel 2, but not a really good metal
sound on channels 2 or 3.
I understand that a clean sine wave on
an oscilloscope indicates little or no dis-
tortion of the input signal, and you say
in your article that you take into account
the power output under load and the
overall sound, in addition to the main bias
current through the output tubes, when
biasing amps to find the best balance. Is it
possible that the guy who adjusted my amp
didn’t take into account that I was look-
ing for high-gain performance and instead
cleaned up the amp as much as he could? If
so, what kinds of things should I ask an amp
tech in order to get better quality high-gain
performance?
The amp may come with a slightly cold bias
and may need to be warmed up for players
that wish to extract better clean, bluesy or
classic rock tones, but that doesn’t seem to
be your particular focus. Since you mentioned
that it’s supposed to be a high-gain amp, it
may have purposely been designed and set
up with what is perceived by some as a cold
bias. I’ll try to explain why, but first, I’d like
to point out the difference between gain and
distortion in very broad strokes.
The effect you are now noticing is a
result of the way the output stage is han-
dling the preamp signal. While the 32mA
measurement for the idle bias current
is a pretty average setting, it shouldn’t
be considered the correct or perfect
setting – there is no such thing! It is a
setting that would achieve less crossover
distortion in the output stage and more
faithfully reproduce the signal that was
being sent to it by the preamp. This, as
you have found, is a good characteris-
tic when it comes to some uses of the
amp but not for others. I’ve found that
sometimes a colder bias on the output
stage is better for players that focus
mostly on very aggressive, high-gain
styles of music. The additional crossover
distortion developed in the output stage
seems to add a little teeth to the sound.
This may be the very reason that your
amp ships with a cold bias. It is has been
optimized for its target market – not all amps
can be all things to all players.
If you want the characteristics of your amp to
shine in the high-gain mode, have your tech
readjust the amp back to its original bias setting. If, however, you can no longer live without the cool hard rock sound that the amp
currently has, you could always add a stompbox for the over-the-top metal tones.
Thanks for any help,
Rene
Hi Rene,
First, let me thank you for reading the column.
It’s fun to write and I hope informative to all
who read it. Now, let’s see if I can shed some
light on your particular situation.
Gain relates to the amount that a signal level
is increased. Most of the time, most of the
gain in an amplifier is developed in the first
preamp stages. Too much gain in a preceding stage can cause a subsequent stage to
distort. Early on in guitar amplifier design this
was considered to be a bad thing – now we
know better. Anyway, when a guitar ampli-
Now, go put the fangs back in your Cobra.
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2007 Jeff Bober
