Bias Adjustment By Randall Smith - Mesa/Boogie Mini Rectifier Twenty-Five Owner's Manual

BIAS ADJUSTMENT
A Feature Article by Randall Smith
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 to this question.
The 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. Noth-
ing 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 accommodate their out-of-MESA tolerance tubes.
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."
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 grids.
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
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