peavey deuce problem (sounds like gated notes)

[mark allen]

hello, My peavey deuce b version. not a vt series, is having a problem on both channels,and all input jacks. Nothing is wrong with speakers,all caps have been replaced ,and all the tubes are good . amp has been working good since the rebuild about 2 years ago. All functions work. the problem is that when you hit a note softly it sounds like there is a gate on it. Notes also sound gated when the note sustains out. distortion fizzes out unnaturally. this amp did not do this before. I also used a different guitar and a different cord. same problem. If you strum hard, amp sounds fine. amp does not cut out and plays at higher volumes. Any ideas on what the problem might be. if you have any thoughts on any specific parts to try , i still have many extra new parts ,caps etc. I do have a multimeter. Power resister reads 426. is this to out of tolerance? Thanks, Mark

[Enzo]

Disconnect the speakers from the amp chassis, and plug the chassis into some other speaker cabinet. Does it sound the same coming from the other speakers? Your description sounds like a voice coil rubbing in one of your speakers.

[mark allen]

peavey deuce gated sounding notes

Enzo, i have tried plugging into another speaker cab already. no change. the cabs work fine with my other amps. I would think this is not a channel problem since both channels are affected. I would think the problem lies in the mixer amp, or the bias section, or the power amp section. Checked and changed speaker cable too. cables are good. Any more thoughts? These amps seam to be one of your specialties. you helped me fix this amp before. is their any thing else that affects both channels i missed. Mark

[Enzo]

Many parts of the amp are common to both channels.

Power supply is, does the +24v rail sit close to that? And is it free of ripple?

When you say all the caps were replaced, does that include all those little 2uf caps in the channels?

The mixer is pretty much just a couple of resistors.

There are four power tubes, is there B+ on pins 3 AND 4 on ALL FOUR sockets? What is the bias voltage on pin 5? Same on all four? Have you measured idle current?

Is the 300v node more or less 300v? And are all the DC voltages in the phase inverter transistors pretty close to the schematic voltages?

[mark allen]

I found the problem. Bias was set to hot . When i lowered it, i immediately heard (the sound of the speakers being ungated) to a normal hiss sound. when adjusted hot the amp would cut out again. amp plays perfect again by a simple screwdriver adjustment. Some tolerance must have drifted from something . Just curious , what do you think may have drifted? THanks again. Mark

[Enzo]

Parts change, mains voltage changes, different tubes will have different optimal bias points. And who knows?