Marshall Jmp squeals like a pig.......

[stompjunkie]

Hello all..... I've got a 72 50w Marshall Jmp that I absolutely love. It's been modified a bit (I honestly have no idea what was done to it, nor who actually did it, but she sounds great! ).... anyway, a couple of days ago it developed some wicked oscillation. It only shows up after about 3 on the master volume, and it's only on the two inputs of the modified (higher gain) channel.... at first I thought it might be a tube, but everything works wonderfully on the other set of inputs.....so, I know where to start, but I'm a bit lost as to what tests to try....I'm comfortable working on amps, I've even scratch built a couple, but I'm not so great at trouble shooting..... So any help would be appreciated.... I need her workin again....I miss that giant wall of guitar.....

[bnwitt]

What model is the circuit? Just because it's only on one channel doesn't mean it's not a tube. Do you have some you could substitute? Have you tried swapping tubes around?

[stompjunkie]

to be honest.... I'm not really sure what circuit it is. How many models of the JMP are there? I have swapped out in all positions and nothings changed.....

[Darkbluemurder]

to be honest.... I'm not really sure what circuit it is. How many models of the JMP are there? I have swapped out in all positions and nothings changed.....

There are actually three JMP models - the lead amp, the bass amp and the master volume amp. The latter has only two inputs. From your description your amp started its life as a lead or bass amp since it has 4 inputs (almost the same, only four parts different). Keep in mind however that many 4 input models were converted to master volume amps with the two gain stages cascaded. It seems that this has been done to your amp, too, especially since Marshall didn't make master volume amps in 1972.

The first thing I would do is to "reverse engineer" the circuit, i.e. tracking all values and drawing the schematic. At least it would be interesting to know whether it has three gain stages or four gain stages or any tubes added. A standard three gain stage amp should be stable at a master setting of 4-5.

You could check the following lead dress issues:

1. No wires going to pins 2 or 7 of any preamp tube should run in parallel or close to wires going to pins 1 or 6. This can make a big difference.

2. The wires going to pins 5 of the output tubes should not run close to the purple feedback wire or to any of the output transformer primary wire (the ones coming from pins 3 of the output tubes). They should also be as short as possible.

3. There should be a 5k6 resistor soldered directly to pin 5 of the output tube.

Does the noise go away when you turn the treble and/or presence down?

[bnwitt]

quite unusual for a perfectly good amp suddenly to develop squeal and have it be lead dress unless he's been poking around inside. More like bad tube or other component failure.

[Darkbluemurder]

quite unusual for a perfectly good amp suddenly to develop squeal and have it be lead dress unless he's been poking around inside. More like bad tube or other component failure.

You are probably correct but there's no harm in checking.

Stompjunkie said he swapped tubes but of course the other sample may be microphonic as well.

Stompjunkie, I assume that you played the amp at the same volume and gain levels before without a problem. Does the amp also hum more than before? Do any of the gain controls suddenly scratch when turned? Did you use a different speaker cab?

Sorry for all these questions but we try to isolate the problem.