Marshall JTM60 volume drop

[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

[bdc]

I had a JTM 60 and it seemed to eat PI tubes.....when it happened there was a volume drop but it was accompanied by an out of phases kind of tone....

[Nooner69]

Sounds like the recitifier is out. Common to these amps

[Nooner69]

Rectifier problem

[mrbassman]

Hi, a bit late so I don't know if you fixed it.
I too have a JTM60 2x12 and had exactly the same problem, in fact I bought the combo cheaply because of it.

It took me a while but I found it to be a faulty jack socket in the send/return loop. It is a real pig to take apart and one of Marshall's worst mechanical designs I think. It took me hours to replace a broken volume pot. However the tone is one of the best and why I keep it.

Anyway, I fixed it quite simply by putting in a short effects pedal cable plugged into both the send and return jacks. It has been 100% reliable ever since. I since found out that this is a common fault with these amps and it is documented all over the net.

If that doesn't work, and you have another amp, you can check the Marshall's preamp and power amp simply by plugging the send into the return of your other amp If that works then the preamp is fine. You can also take the other amp's send and plug it into the Marshall's return, this will then check the power amp of the Marshall. I am sure though it will be the send and return jacks so these tests may not show up as faulty. Try the effects cable as a jumper first.

One day I'll fix mine but 3 circuit boards in a U shape and many jumper leads have put me off. That's assuming I can find 2 replacement jacks as well. So it has a send/return cable fitted all the time. Doesn't worry me!

Good luck.

[lowell]

Some interesting replies. I found this thread cause I have one on the bench with low output. After testing all voltages and replacing power tubes because one was toast, I found the output transformer is toast. Confirmed by measuring ohms from CT to each end of primary. One side is half of the other. Subbed a tweed deluxe OT I keep on hand for just this scenario - fixed. Now on to finding the right replacement.

This kind of problem would NOT be a rectifier issue. Rectifiers blow fuses, resulting in NO POWER.

Usually when there's low output and all tests reveal nothing odd (tube pin voltages, fx loop jacks, tubes etc), it almost always comes down to the OT.