TOPIC: BIAS ADJUSTMENT
September 25, 1996
Here's a question we often hear:
"Why doesn't Mesa put bias adjustments in their amplifiers?"
Well, there's a short answer and a long answer.
short answer is that during my 12 years of repairing Fenders,
one of the most frequent problems I saw was bias controls
that were either set wrong or that had wandered out of adjustment
due to vibration. As any honest tech will tell you, there's
lot's of easy money to be made by sprinkling "holy water"
on amplifiers ... uh, what I meant to say is "Your amp
needed biasing." See what I mean? What customer is going
to argue with that?
It only takes a moment and a volt meter: The Fender diagram
shows how: "Adjust this trim pot for - 52 volts."
That's it. Nothing more.
Now don't be fooled into thinking that tubes "draw"
more or less bias, they don't. The way a bias supply is connected
to a tube is akin to a dead end road, it just trails off to
nowhere without really completing a circuit. It's a static
voltage and regardless of what tube is in the socket -- or
even if the tubes aren't plugged in at all, it doesn't change
the bias voltage a bit.
So the end of the short answer is this: Since a bias supply
needs to put out the right voltage and never vary, I wanted
to build amplifiers that were individually hard wired to the
correct values and NEVER needed adjustment. And for 25 years,
that's how Mesa/Boogies have been built.
Time to change tubes? Just plug our tubes into any one of
our amps and you're DONE. No tech needed. NO bills and no
BS about biasing. And most important: The bias is RIGHT because
it can't change!
Now, you want the long answer? Here's more information on
how our hard-wired bias avoids trouble. Please read on.
But first, let's make an important distinction. Our business
is designing and building high performance amplifiers. And
for this we need tubes whose variance is within a narrow range.
Our warehouse is full of rejects ... oh, they work -- they
just don't perform within our tolerance range. We have a very
sophisticated computer - based tube testing system (nicknamed
"Robotube") that matches and measures tubes over
seven important parameters. It can even predict which tubes
are likely to have a shortened lifetime -- even though they
work perfectly during the test.
Because our business is building quality amps, we can afford
to reject a lot of wayward tubes. The guys you hear complaining
because Boogies don't have bias adjusters are primarily in
the business of selling tubes - not amps. They don't want
to throw away 30 percent of their inventory, so they promote
the idea that tubes outside our parameters can be used to
"customize" amplifiers and they criticize us because
our amps can't be adjusted to accomodate their out-of-Mesa
Now you might be thinking, "But I thought you just said
that tubes don't "draw" bias, therefore they don't
effect the bias supply and thus it doesn't need to be adjustable."
And that's right. Tubes don't effect the bias setting, but
the bias setting does effect how the tubes work. But HOW it
effects the tubes is difficult to measure.
When you set the bias (whether it's by selecting the right
resistors, as we do, or adjusting a trimmer -- which is quicker)
what you are doing is establishing the correct amount of idle
CURRENT that flows through the power tubes. But you can't
adjust the current directly, you can only change it by adjusting
the amount of bias VOLTAGE that goes onto the tubes' control
Voltage and current are NOT the same. Current is the AMOUNT
of electricity, the "quantity" -- and is measured
in amperes. Voltage is the degree of electric charge -- like
the "pressure" to use the old water analogy. Let
me illustrate how different voltage and current are:
When you scrape your feet across a carpetted floor in dry,
wintery conditions, your body can become charged with 50,000
to 100,000 volts of static electricity. And when you reach
for the door knob, a spark jumps and you feel it! The voltage
is super high but the current (measured in micro-amps) is
tiny - otherwise you would die from electrocution.
Contrast this with your car battery, which puts out a mere
12 volts. You can lay your hands right across the terminals
and not feel a thing. Yet the amount of current available
can run to several hundred amperes .. enough to turn over
a cold engine and get it started.
So current and voltage are two totally separate electrical
parameters -- though when you multiply them together, you
get POWER, which is measured in watts.
When you set the bias of an amplifier, you are adjusting
the static VOLTAGE at the control grid of the tube in order
to produce a desired amount of idle CURRENT flowing to the
tube's plate. A small change in grid voltage, produces a large
change in the amount of current flowing -- and that's basically
how a tube works. Say that again because it's super important:
A small change in voltage at the grid causes a large change
in current flowing to the plate. See, that's the essence of
amplification: A small change causing a large change. And
here it's a small voltage change causing a large current change.
The bias conditions are what determines how much current
flows through the big power tubes when you're not playing.
And what drives your speakers is fluctuations in that current
flow when you are ARE playing. If the amount of current increases
and decreases 440 times per second, then you'll hear an A
note. If the fluctions in current flow are large and still
at 440 per second, you'll hear an A that is LOUD!
But for purposes of biasing, it's the amount of "plate
current" flowing with no signal applied that's important.
Unfortunately current is hard to measure because the circuit
must be interuppted -- as in "cut the wire" -- and
the meter spliced "in series" with the broken circuit.
But measuring VOLTAGE is easy. It is not necessary to interrupt
the circuit because a voltage reading can be taken in PARALLEL
with the circuit intact.
Thus, as a matter of convenience, most bias settings are
given in volts at the grid ... even though current through
the plate is the important factor. In fact plate current is
so inconvenient (and dangerous) to measure that Fender doesn't
even state what the correct value should be. They only give
the grid voltage that will produce that current. (That's the
minus 52.) But that only happens if the tubes being used are
As long as the tubes ARE "in spec", the right bias
voltage will always give the correct plate "CURRENT"
-- but then there's no need for the bias voltage to be adjustable!
If the tubes are NOT in spec, then the only proper way to
re-set the bias is to cut the circuit and measure the current
while adjusting the bias ... but no manufacturer I know even
STATES the desired current value! Be that as it may, when
the original bias voltage is altered far enough, it will compensate
for the tube's abnormal performance and the correct amount
of idle current flow may then be restored. Clearly this is
something most repair techs should not attempt.
Some newer amps have LED indicators connected to the circuit
which will turn on when the right threshold of current flow
has been reached. This is an improvement, and almost worthy
if you're willing to except resistors and lights added into
your amplifier's audio path -- which we aren't. The other
"advantage" of this system is that it allows some
amp manufacturers to avoid matching their power tubes. The
thinking is that adjusting the bias to each tube separately
eradicates the inherent differences between the tubes by insuring
that the same current flows through each one.
Again, this has some merit .. but it's still not as good
as using tubes that are matched in the first place because
compensating for the mis-match causes the push-pull circuit
itself to become unbalanced. Two wrongs don't really make
Some of the other recommended biasing, "methods"
-- such as -".. tubes running red hot, increase the bias
.. sounds harsh and runs too cool, turn it down ..."
are guesswork at best. Luckily, one of the great things about
tube amps is that they can usually stand some abuse without
causing any real harm ... at least not immediately.
But don't these alterations imply that you are second-guessing
the amp designer and that there's a better set of operating
conditions that the designer missed but the tube sellers have
Now some players may like the sound of their amp altered
by tubes with extreme characteristics and with the bias set
to help compensate. But often it is the mere novelty of change
that they're really responding to and when the amp goes back
to the proper original way, we've seen them be far happier
Because every part in every one of our designs has been meticulously
evaluated, compared and stressed over -- no matter how seemingly
insignificant it might be. And with every design we look for
a "sweet spot" where all the parameters -- including
the bias -- come together to give the best sonic performance,
consistently and reliably. Every part and voltage is important
-- yet no one complains that these other parameters aren't
available for tinkering.
Consider our patented Simul-Class circuitry where there are
two different bias voltages used for separate pairs of power
tubes ... and changing one voltage also changes the other.
Great care goes into getting this just right and we think
we'd be asking for trouble to have it adjustable for the world
to play with ... unless you like paying to have your amp messed
up. Sorry, I meant to say, "Uh, ... your amp needed biasing."
If that doesn't appeal to you, then merely plug a matched
set of Mesa tubes into one of our amps and you're ready for
tone. Guaranteed. You'd be amazed at the number of service
calls we field every day that lead to a diagnosis of out-of-tolerance,
non-spec tube problems. To think these would be prevented
by including a bias adjustment is something of an insult to
you and us. If you put the wrong size tires on your car, do
you think changing the pressure will make them right?
Please, don't think this is a blanket indictment of the other
guys selling tubes -- it isn't. And their tubes aren't all
bad either. It just doesn't make sense to pay more of your
hard earned cash for tubes that were probably made in the
same Russian or Chinese factory and which have the possibility
of being outside the performance window we select for your
amp. And it pains us to hear the hype and mystique built up
around biasing when twenty-five years of evidence affirms
our decision to make bias circuits that "never need adjustment".
How much money and trouble that has saved Mesa/Boogie players
you couldn't estimate.
Our rigorously tested and hand selected tubes are available
at your nearest Mesa/Boogie Pro Center or from us directly.
Nobody offers better price, quality or warranty than we do
... so why swerve?
Next time we'll talk about our part in developing the great
Sylvania STR 415 type 6L6 and how we're on the verge of seeing
something fairly close reappear on the market. Remember, we
still have some of these super rugged mondo-bottles available
for older amps -- Boogies only please! Until then, Relax,
Breathe and Nourish your soul!
Designer & President