Plexi problems

[stangwarrior]

Hello!
I have a Marshall 1959 Plexi clone, modded for bass.The problem is every time i switch it on and play for like 5-10 minutes, the amp suddenly drops the volume and after 2-3 sec there is no sound at all.When i turn the volume up there is very, very low level, distorted sound.This happens on both channels.But when i pluck the strings really hard, i.e. increase the input level, the volume is back to normal, but then again, after several seconds it goes dead.Hard hitting the amp does not help, so it's not a bad solder point IMO.The fact that input level brings the sound back leads me to bad cap?Or bad recto tube?HELP!
p.s. sorry for the bad english...

[Stevem]

Hook up a voltmeter set for ac volts across the speaker, play the amp until it acts up and then stop playing, does the meter still read a high voltage output?
If so the amp has a circuit that is oscillating And we can then help you with that.
If the amp craps out and shows no ac voltage at the output then I would have to take a guess that a circuit is losing B+ voltage somewhere! heck it could be a intermittent standby switch even.

[Firestorm]

What happens when you turn the treble pot while the amp is malfunctioning? The whole signal goes through that pot so dirt and debris can interrupt the signal.

[stangwarrior]

After trying to measure the output voltage when there was a volume drop i accidentaly touched the send-return jacks and the sound was back.Several taps on them cleared the problem - dirty jacks.
Thanks for the time guys!

[xtian]

Ah, the old FX loop bug. I think that's a sticky over at TGP.

[Reeltarded]

haha! Oh yeah they put loops in everything now. I had some success tinning the positive jack pins. Once those things do that, they do that, and they do that... and on and on.