Peavey Classic 50 50 Power Amp Bias mod/repair

[Enzo]

PV doesn;t like to ask the power tubes to work very hard in most of their amps.

[darthpoodle]

I know this thread is a little old but still looking for some help on this topic too. My Classic 50/50 runs cold and would like to up the bias a little too. Any recommendations. Maybe a pot in r229 and r206?

[Rambo227]

Hey, thanks for all the replies but I ended up selling the amp and I was lucky enough to find one that is virtually mint like it just came from the factory. Some guy had it in storage for over a decade and never used it.

Sorry to brag, lol. It still has the original peavey labelled tubes from the factory. (Actually the 12ax7 and the 12at7's are labelled Peavey, the power tubes are apparently Sovteks without any markings.

Anyway Darthpool I don't know how to do the bias mod but I'd be interested in hearing how it's done if any of the experts here want to give us some input.

[Enzo]

Oh, it is simple enough, make R108 variable.

It is a 47k now, so conceptually a 50k pot in its place would work. But that would allow dialing bias to zero, so what I would do is get a 25k pot plus a 22k resistor, wire them in series and replace R108 with them. Or for even closer control, maybe a 10k pot and a 33k or 39k resistor.

That adjust the bias for both channels.

If you want individual bias, then you have to work. The bias is fed to the two tube boards by pin 2 of J201 and J211. J211 connects right to R108, then a trace carries it to J201 with a jumper in the path. SO remove the jumper, and in the power supply duplicate R107, R108, C110, and connect them also to C109 to make a parallel bias supply, and wire it to the J201 end of the jumper pads. REmember that R108 in this scene is now a pot plus resistor.

[darthpoodle]

Enzo,

thank you, this mod saved this amp from being replaced.