Mesa Boogie mkiii - Adjustable fixed bias mod

[DrGonz78]

So I am attempting to make the bias supply adjustable on this mkiii boogie amp but so far no luck. Looking at the schematic you will see a resistor that measure 826ohms coming right off the the rectifier diode for the bias supply. Then the bias voltage passes through a 3.3k resistor, looks like 33k on the scheme, and there is that "factory select" resistor in parallel with a 68k. Now on this amp the 68k resistor is really a 221k which has drifted to 246k, R102. Where the factory select resistor is, R117, there is not a resistor installed. So for a test I wired a 200k and a 50k pot in series and replaced R102. I set the 50k full on (250k total resistance) I get about -63v at the grid of V6 or V7. Sweep it all the way down (200k total resistance) and I get like -62.4v. I thought getting that resistor back to 221k again might help and then I could sweep down to 200k to bleed a little more of the bias voltage less negative.

Just gotta make sure I am heading in the right direction here. The lower the value I put into R102 should yield a less negative bias voltage supply right? I am thinking that I will need to take a 100k resistor in series with a 100k trim pot to get more adjustment. Does that sound correct? I am just a little put off the by differences in the Boogie schematic and what is actually in the amp. I know that I should not be surprised with these schematics as they are more like suggestions as some other members here have stated many times before. I just like when what is in the schematic is at least very close to the what's in the amp.

[alexradium]

yes,lowering that r102 will give you less negative voltage

[Jazz P Bass]

Here is a crop from an MKII with a bias pot.

boogie_mkii Adj Bias.pdf

[DrGonz78]

I have looked at all the various bias circuits for all Mark amps. I guess I was a bit surprised that there was little change from 250k down to 200k, like a volt difference. There is an old thread online somewhere that I read where the guy says he replace a 221k resistor with a 200k pot. He had it set to about 150k at the sweet point of adjustment. I guess it has to do with the voltage divider range from the 3.3k in series resistor and that once I start getting between 150k-200k then there will be substantial change to the bias voltage. Where I left off at 200k made me double check if I was getting the board layout correct, which I was.