ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Weird Noise In Old Marshall

:

6/22/2000 3:46 PM

Weird Noise In Old Marshall

Hi Everyone, here's the deal: I have a 75 4 input 50 watt that I'm working on for a freind. The wiring has been messed around with (new preamp caps),new filament wiring (not twisted), stuff moved around. Anyway, I put a lot of it back to stock but the amp makes a loud vibrating/rattling sound when cranked up and sustaining a note or chord. It sounds just like something behind the amp is vibrating like crazy but the sound comes thru the speakers. The filter caps are original, could this be it? It also seems a little like a parasitic oscillation. Any opinions are welcome-

Ethan

6/22/2000 10:35 PM
Kremator	I'm not an expert, but i'd try swapping the filter caps, as they might be too old, dry etc. They lose capacity and this adds hum. If you have a spare set of tubes, i'd try to change them all. If this doesn't help, then i don't know. K.

6/22/2000 10:56 PM

Here's a few more thoughts. When power transformers are wheezing their last, which could mean a few weeks to a few years, they will sometimes vibrate severely. This would produce a mechanical noise and vibration that can be picked up by the tubes and out the speakers.

Also, have you checked your power tubes? They can surely produce this sort of noise. With your amp up and running gently tap them with a stick and see if they're microphonic and if it causes the amp to produce these symptoms.

Actually, I'd consider swapping the tubes first just because this is the easiest thing to check.

Carl Z

6/22/2000 11:48 PM

Hi Carl, thanks for your input. Hopefully it's not the tranny! I thought after I posted that I should have listed the stuff I checked. It had 6550's stock, changed to EL34's (new Svetlana's), still made noise. I swapped out preamps, no luck, removed preamp covers, checked all trannies/caps for tightness, checked tube sockets for tightness, check connections on board as well as I could without removing board. I think I'm going to get an assistant to help with this. I'm having a hard time holding a chord on guitar/thinking straight with this thing cranked/and trying to determine source of noise at the same time.

Ethan

6/23/2000 12:02 AM
Carl Z	Ethan; If it's the tranny, it'll be vibrating even when at idle. Just rest your hand on it and you'll feel it! A slight vibration isn't out of the ordinary but this is much worse. Carl Z

6/23/2000 12:10 AM

Oh, one more thing....

If you're shy on test gear here's something that'll work in a pinch. If you've got a portable CD player or something that has a headphone output, you can send that signal to the input of the amp. Start with very very low headphone volume settings and increase the level till you've got about 50mV at the input. It ain't perfect by a long shot but it will free your hands up.

Carl Z

6/23/2000 12:51 AM

If things have been changed, i'd be looking to see what, or if the right parts and values were what they were supposed to be. If you don't plug a guitar into it.. and just crank the amp .. and tap on the face of it ... does anything happen.??
Let us know a few more details and we may be able to help. Could be a few things.. loose ground from power tubes.. OT leads reversed.. 5.6k resistors there?? Does it do it in either channel??
Richie{~}==:::

Page 1 of 2