Marshall JTM60 volume drops

[generalbeef]

Hey guys,

I have a problem with my first ever tube amp. I bought my Marshall JTM 610 with extra 112 cabinet from the first owner. He never played it loud, but also never changed the tubes.

I have never had problems with it, until about 3 months ago my volume started to drop. A lot of jtm60s seem to be having problems with heat, but when I opened the amp there were absolutely no signs of heat damage. I did notice a blue glow coming from one of the power tubes when i played.
I tried plugging a short lead between the effects loop in and out but with no effect.

So eventually I brought the amp to my local music store. He checked all the wiring, all the solder joints and replaced the El-34 powertubes and one transistor which he thought was bad.

Well all of this didn't fix the problem of the volume drop. but something changed about the volume drop. I noticed that the volume drop is only present when the amp is too cold or too hot. When I point a fan at the amp the volume drops, when I remove the fan the volume rises to normal level until it gets too hot and starts to drop volume again. I point the fan on the amp again and the volume becomes louder until it gets too cold and drops and so on...

I hope someone is familiar with this problem in marshall amps, or knows which part of the amp could cause this problem. So the fluctuating volume as a result of a too low or too high temperature is the problem i'm dealing with. The volume drops are so bad, with volume at 9 o clock you can barely hear anything! Please help!

Thx for the effort, and have a kickass day

[soundguruman]

You can actually freeze the components one at a time with freeze spray, while observing the waveform on an O scope.
This can be helpful in locating a thermally unstable component. What you have is a thermal intermittent.

The blue glow from the power tube is completely normal, the result of vaporized mercury inside the glass envelope.

It may be quite difficult to narrow it down without test equipment. Intermittents are the hardest to locate, and require the most skill and time to correct.

It may turn out to be a bad tube or it may turn out to be high frequency oscillation that causes the audible volume to drop out. OR it could just be a bad solder connection, a bad FET transistor, but then you would not be able to really narrow this down without some test equipment.

[generalbeef]

Oh god, I hoped it would be an easy fix

[soundguruman]

Well does the volume drop in and out when you tap on the circuit boards?

[Enzo]

And heating and cooling among other effects can cause mechanical changes - thermal expansion and contraction - all of which could be flexing the board or connections enough to break a circuit path. I could be wrong, but that sounds more likely than components actually being that sensitive to temperature.


Isolate the problem. WHen this occurs, plug the guitar into the FX return. Now you are listening to just the power amp. Is it still affected or is it clear and strong? Also, you can run a cord from the FX send to some other amp, so you can listen over thre what is coming out the FX send. Then if the sound over there continues strong while the sound from this amp drops, we at least know it is not the preamp. And alternatively, if the sound over there drops with it, then the preamp is the failure.

[soundguruman]

Yes, particularly on the power amp board. The board gets hot as Hades when it's on, then cools down when it's off, cracking the solder connections.
Resoldering the power amp board has cured more than a few of these things....so that might be all it needs.

[generalbeef]

thx for the replies, im going to try this later this evening