Marshall JTM60 malfunction

[owenhoulston]

Hey

I've had this Marshall JTM60 combo from new, and have never been able to rely on it. Currently the amp is not only producing the fundamental tone from my guitar, but in addition a dissonant tone which is lower than the fundamental. This was originally only when using humbucker equipped LP, but now my Tele is affected as well. The tone is most noticeable on a G string twelth fret (or maybe I just notice it more there). It has the wonky effects loop problem, which I fix with judicious use of a jack cable. All the tubes have been replaced within the last year, and its seen plenty of repair shops over the years. I've also seen some advice on the web which says this is just a bad amp design and not worth bothering with. Any advice would be welcome!

Cheers

Owen

[Enzo]

Hi, welcome. ANy time someone on the internet tells you an amp is a bad design and not worth bothering with, it means he doesn't know how to fix it. Or he doesn't like someone at the local store that sells them. Or he had an argument with an unhappy customer. Or he tried to modify it and screwed it up. Or some other sad story.

If the amp were such a bad designs, then none of them would work, and everyone would be throwing his amp out. Since that is not the case, we should look at this amp as we would ANY amp with similar symptoms.

My first thought is always tubes, but if it does this with differnt tubes in it, then it must be something else. My thoughts then turn to filter caps. A leaky filter in the main B+ can cause distortions, as can a weak filter in the bias circuit. We also want to make sure all th power tubes are functioning. Have you checked for equal currents in both sides of the push pull circuit? Check for B+ on the screen of each power tube. If screen voltage is absent, the tube won't conduct. The amp will still work, but the sound won't be right.

[Gtr_tech]

It *is* a bad design....the B+ filters are located *above* the output tubes. The tubes are operated in a horizontal pos and the main caps are right above....nothin' but trouble can come of that.

[Enzo]

WOuld you tell the owner it was not worth working on for that reason?

I might disagree with all sorts of design decisions I see, but within the context of the situation, I'd say the amp can be made to work.

[Gtr_tech]

Originally Posted by Enzo

WOuld you tell the owner it was not worth working on for that reason?

I might disagree with all sorts of design decisions I see, but within the context of the situation, I'd say the amp can be made to work.

Well sure it can be brought back to life, but I *would* warn the owner that the reliability will be subpar. Cooking filter caps like that is just bad mojo...
Maybe a muffin fan could be added to blow the heat down and keep most of it off those caps....I didn't look into it last time one was here.