Peavey Artist VT Fades Out

[misterc57]

I have a Peavey Artist 112 on deck for repair. When I first got the amp I went through the entire amp cleaning all connections, pots, tube sockets, etc. It had 4 Mesa 6L6 that all tested good.

The amp sounded great when first turned on and played. It would work great for 5 minutes to 8 minutes and then would start to fade out (distorted sound and low volume). If I try the amp later on or the next day, the same thing happens, plays great for a while and then fades.

I swapped all 4 output tubes with another set of good tubes and the same thing happens.

I will be getting back to this amp in the near future and I am looking for suggestions on what to look for.

As always much thanks to all for your help!

[Jazz P Bass]

Can you monitor the power supply to see if any/all are failing?

[misterc57]

Will do and report back. Thanks

[Enzo]

Just my guts, but I don't see a power supply failing, I see a vision of a voltage creeping up on the signal path. But DO of course verify power supply voltages.

But first, when this happens, plug a cord from preamp out to PA in. ANy help?

Failing that, Isolate the problem. Preamp or power amp? Plug a guitar into the power amp in jack. Sound now clean and strong, or still low and distorted? Likewise, plug the guitar into the regular input, and run a cord from preamp out jack to some other amp and speaker. How does that preamp sound over there? Like it should, or low and distorted? And turn the reverb up midway and rock the amp to crash the reverb springs. Does that come out loud and strong or low and distorted?

Inside, the preamp is a few op amps, look at their output pins to see if any have drifted away from zero DC. Plug a cord into the preamp out jack and measure at its tip for DC voltage, should be zero.

[misterc57]

Had some time tonight to get back to this amp. I did not remember everything you said to try but here is what is happening now.

There may be another problem going on as this new issue is more dominant. I get cracking noises if the amp is moved slightly, if I tap any of the pots or tap the top of the amp. This noise will last for a short duration and go away. It comes right back if the amp is moved at all. I suspect loud playing will cause this also, from vibration to the amp.

This noise is only present with the Reverb pot above zero. If I turn the pot to zero I get no noise.

Jumping pre-amp out to PA in; still had noise.
Pre-amp out to another amp in; still had noise in other amp.
Pre-amp out from other amp to PA of this amp, no noise and no other issues (fade, etc). The PA of this amp seems to sound great.

So it looks like I have a problem in my pre-amp tied to the reverb. The reverb itself is strong and works great. But those crackling noises easily surface and they do not sound like reverb springs banging around.

I will try the other suggestions you made tomorrow night regarding the op amp voltage.

Thanks, Mark

[Enzo]

Wait, if turning the reverb to zero kills the noise, then the noise comes from before that control. I am breaking my own rule here, but anyway, Look on the end of the board, there is a four wire connector where the reverb cables connect. Unplug that connector and see if the noise is now gone. Look at the pins in that connector and their mates, any of the females spread to be loose? And look into the reverb pan itself. Make sure the two plugs on its side are not corroded and that the springs inside are not flopping loose on the bottom.


But my rule... For online help, let us pick one thing and fix it, then move on. If we start tracking down crackling reverb AND fading out distorted sound we will inevitably get confused as to what voltage reading pertains to what and so on. SO I suggest, leave the reverb turned off for now, and concentrate on your fading distortedness.

So the power amp sounds clean and strong? Good.

[misterc57]

Measured preamp out at tip of inserted ¼ phono plug

With Volume at zero or at 5, same readings below.
Reverb set at zero: -.006 VDC
Reverb set at 10: -.002 VDC

Note that just slightly touching the amp while doing these measurements caused loud crackling noises when reverb was above zero. This noise is coming through even with the volume set to zero.

I will pull the chassis out. I do think that the 4 pin connector plug for the reverb may have loose connections.

I will also run the chassis alone (no reverb connected) into a different cab and see if my fade/distortion issues are still there or gone.

Thanks, Mark

[Jazz P Bass]

Ten to one there is a bad solder joint.
15 minutes & you could have the whole board resoldered.

[g1]

Jumping pre-amp out to PA in; still had noise.

If you did not check whether this cured the fade in/out problem, re-check.
This is why Enzo's rule is so important, the jumper was dealing with the fade problem, not a separate noise issue.

[misterc57]

If you did not check whether this cured the fade in/out problem, re-check.
This is why Enzo's rule is so important, the jumper was dealing with the fade problem, not a separate noise issue.

True. The Fade problem did not materialize last night. Perhaps because I was distracted by the noise issue. I will re-do the testing. Thanks

[misterc57]

I put this amp aside over three years ago and just got back to it. This fade issue is resolved. There are 4 input jacks and it turned out to be either poor mechanical connections or dirty input jacks.