Mesa/Boogie II - Manual (Page 31)

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BIAS ADJUSTMENT: (Part of a continuing series)
NOTE: NOTE: NOTE: An article written by Randall Smith that
we thought you might fi nd interesting.
Here’s a question we often hear:
“Why doesn’t MESA put bias adjustments
in their amplifi ers?”
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 amplifi ers ... 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 amplifi ers
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 fi rst, let’s make an important distinction. Our business is designing and building high performance amplifi ers. 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
Boogies Boogies
of their inventory, so they promote the idea that tubes outside our parameters can be used to “customize” amplifi ers 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 fl ows 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.
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PLATE
Structure of a 6L6 / 5881
Beam Power Pentode.
BEAM-
CONFINING
ELECTRODE
SCREEN
GRID
GRID
CATHODE