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Trouble with 75 JMP 50w

TheArchitect

Member
Joined
Jul 6, 2003
Messages
490
Twice now my 75 JMP 50w as just stopped producing sound in the middle of playing. Bypass all pedals and plug directly in and still no sound. Let it cool over night and it works for a bit. Rinse and repeat.

I don't think a bad tube would do this. I will check the speaker cables but the replacement amp I put up was fine.

Excluding transformers and tubes, are there any components that can go intermittent? Any other ideas on a possible cause?
 

Mars Hall

Active member
Joined
Nov 26, 2008
Messages
1,824
9 out of 10 times, a problem with a tube amp is a tube.:salude Another thing to look into is the filter caps. Those are the blue cans you see sticking up out of the chassis. Those are a chemical based components with the same build construction as batteries.

You can find more information over at metroamp.com a site dedicated to Marshalls and clones. Lots of info on those old cool amps.:ganz
 

roadrunner

New member
Joined
Aug 25, 2001
Messages
6,835
9 out of 10 times, a problem with a tube amp is a tube

Bingo, we have a winner.:applaude

It's actually more like 99 times out of 100. Try replacing all the preamp tubes first, one at a time, then the power tubes if that doesn't work, and give the amp another try, chances are very good your amp will be just fine.
 

Siegler

New member
Joined
Jul 21, 2001
Messages
956
what about tube SOCKETS? I had the same problem with a tube amp and used socket cleaner spray - then it got a lot better
 

BC Audio

New member
Joined
Feb 16, 2010
Messages
119
You may need to retension the sockets in addition to cleaning them. The metal parts that grab the tube's pins can loosen and not grab tightly. It could be that the head is causing expansion that results in a disconnection from a pin on one of the tubes. This is usually more of an issue with power tubes than preamp. More heat.
 

Wally

Well-known member
Joined
Feb 27, 2003
Messages
3,433
IMHO, with an amp of this age, the electrolytic capacitors are suspect....even if the amp is 'working'. Old caps just don't perform as do fresh caps. I also have never seen a tube that exhibited what seems to be a heat-related problem like this one. Tubes are easy to replace, so replace them all and see if that cures the problem. IF not, you will have a start on a reserve of tubes, right? I am going to think that this amp is overdue for some tech time. There are all sorts of things that could cause the amp to go 'silent'. The amp in fact may be producing signal, but the signal production could be above the range of human hearing. After the amp's components have cooled, then things go back to 'normal'.....not correct, I am thinking, but 'normal' for the condition the amp is in.
Tech time....imo.
 
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