Peavey Classic with low output...

[1WoodHippie]

On the bench is an old Peavey Classic that a guy brought in that's been sitting in the garage for years and was DOA. After a new set of 6l6's and new 2n3439's for the 64278's it jumped back up. However it's seems low on power, and the more you turn the volumes up the natural distortion sounds flabby and farty. All my voltages seem to be good except on the outputs of those 3439's where I'm supposed to be getting 150 each side, I'm getting 250 each side. The voltages behind it, 300 and 460 are pretty close. I replaced the 100k's for good measure but no luck, still getting 250/side.

Any suggestions/ideas?

thanks in advance!

[Enzo]

Well, that 150v on the collector is controlled by the currents through the BE junction. Do you have 24v on the emitters and 25 on the bases? And I wouldn't really suspect the phase inverter for your symptom.

Your drawing has other DC voltage references, how are they? You got about 18v atop the JFET? 2v under it? Is there any DC atop the master volume control?

If this were mine, considering all the decades this thing has lived, I'd replace all those 2uf 35v electrolytics in the signal path. I'd replace the two 25uf at the PI bases as well.

[1WoodHippie]

I've got 24 on the bases and 23.5 on the emmiters. I've got 20v on top of the FET and 1.5 under it. I haven't checked for any DC on top the master vol. All my DC voltages from input all the way through look good, up to that 250 in the PI. I'll replace the caps in the signal path tomorrow. If that doesn't do it, what should I check next?

[Enzo]

Oh in that case I'd guess it is time to do some signal tracing - isolate the problem area. Inject a good signal at the master to test just the power amp. Trace signal out of the preamp to see how it sounds.

It has been my experience that in these old PV transistor circuits and in corresponding situations in things like old Kustoms that those small value e-caps tend to dry up and work poorly.

But let us not get ahead of the game.


Actually I am not too worried about your 250v, as long as the PI doesn;t run out of headroom, the voltage there doesn;t matter so much. I just thoought it was woorth a look at the transistor bias and resulting currents.

My working theory is that your power amp is OK, and the preamp is the moldy bread.