Mesa/Boogie Rectifier Stereo owner manual Bias Adjustment part of a continuing series

BIAS ADJUSTMENT: part of a continuing series

An Article written by Randall Smith

HereÕs a question we often hear:

ÒWhy doesnÕt MESA put bias adjustments in their amplifiers?Ó

CATHODE (	)
GRID ( , )	..
SCREEN	.....
	.
GRID ( )	.
	....
	.
	.
	....
	.
	..
	..

Structure of a 6L6 / 5881

Beam Power Pentode.

BEAM-

CONFINING ELECTRODE ( )

PLATE

( , )

Well, thereÕs 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, thereÕs lotÕs of easy money to be made by sprinkling Òholy waterÓ on amplifiers ... uh, what I meant to say is ÒYour amp needed biasing.Ó 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: ÒAdjust this trim pot for - 52 volts.Ó ThatÕs it. Nothing more.

Now donÕt be fooled into thinking that tubes ÒdrawÓ more or less bias, they donÕt. 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. ItÕs a static voltage and regardless of what tube is in the socket Ñ or even if the tubes arenÕt plugged in at all, it doesnÕt 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, thatÕs how MESABoogies have been built.

Time to change tubes? Just plug our tubes into any one of our amps and youÕre DONE. No tech needed. NO bills and no BS about biasing. And most important: The bias is RIGHT because it canÕt change!

Now, you want the long answer? HereÕs more information on how our hard-wired bias avoids trouble. Please read on.

But first, letÕs 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 Ñ they just donÕt perform within our tolerance range. We have a very sophisticated computer - based tube testing system (nicknamed ÒRobotubeÓ) that matches and measures tubes over seven important parameters. It can even predict which tubes are likely to have a shortened lifetime Ñ 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 donÕt have bias adjusters are primarily in the business of selling tubes - not amps. They donÕt want to throw away 30 percent of their inventory, so they promote the idea that tubes outside our parameters can be used to ÒcustomizeÓ amplifiers and they criticize us because our amps canÕt be adjusted to accommodate their out-of-MESAtolerance tubes.

Now you might be thinking, ÒBut I thought you just said that tubes donÕt ÒdrawÓ bias, therefore they donÕt effect the bias supply and thus it doesnÕt need to be adjustable.Ó

When you set the bias (whether itÕs by selecting the right resistors, as we do, or adjusting a trimmer Ñ which is quicker) what you are doing is establishing the correct amount of idle CURRENT that flows through the power tubes. But you canÕt adjust the current directly, you can only change it by adjusting the amount of bias VOLTAGE that goes onto the tubesÕ control grids.

PAGE 10