Marshall JMP making squealing noise

[goldtop5]

I have a Marshall JMP that's making a squealing noise when you plug into the high gain input and turn the pre-amp control almost all the way up. I tried different pre-amp tubes, but it still makes the squealing noise. It doesn't do it on the low gain input. Any hints?

[52 Bill]

It sounds like the amp is oscillating. There are many reasons why it happens, poor lead dress, bad ground connections, bad power supply decoupling, etc.

Now you need to figure out exactly what your problem is. Turn down the presence control, does that stop it? How about the treble control? Move the wires at the input away from the other wires, does that help?

[The Dude]

My first question would be, does it do this without a guitar plugged in? If it doesn't, it's not uncommon for pickups to feed back in such a high gain configuration. It doesn't do it on the low gain input because it's LOW GAIN.

[J M Fahey]

To check whether it's a real amp problem or not, plug a shorted jack in the high gain input and retest.

And tell us which JCM it is, there are many.

In early "master volume" ones, which later became JCM800, the first stage oscillated with high gain/treble/presence settings and it was "normal"

[goldtop5]

It does it with a guitar plugged in or just a cable. All you have to do is open up the jack and turn the preamp control up. Juan, this is a 100w JMP with a master volume control. It's a 1959 according to the serial# that was looked up by US Music Corp.

[g1]

And it still does it with the guitar connected and guitar volume turned off, or with a shorting plug like Juan suggested?

[goldtop5]

It makes the squealing noise even with the guitar volume turned all the way down. I'm starting to wonder if it was designed that way as Juan suggested for earlier models. But as long as you turn the preamp gain control about 12:00, and the volume the same, it won't squeal but still be loud enough to peel the paint off the walls.

[g1]

Sometimes in cases like these, trying other preamp tubes may not do it. You may have to handpick for low microphonics and in some cases 80% of new tubes on hand will not make the cut. It is very difficult to find modern tubes that will allow high gain stages to be turned up full with no squeal.

[TimmyP1955]

This might have your answer: http://music-electronics-forum.com/t38879/

[Chuck H]

This might have your answer: http://music-electronics-forum.com/t38879/

A 1000pf in that roll would surely alter the tone of the amp. I'd experiment and use the lowest value that fixed the problem. Other solutions would be similar enough (top end bleeder circuits) and not every amp has the same ticklish spot. But as Juan noted, sometimes these amps do this or just begin to without warning. On some later models Marshall started the practice of tacking the input triode plate and grid leads together with goo onto the chassis near the socket. This creates a small capacitance that results in a local NFB loop. They must have become aware of the issue too. At any rate it's very common to mod those amps for stability. People that pull it off without bleed circuits are considered especially clever

A cap across a plate resistor is a very popular fix/mod for this era of Marshall amp.

You can also try just tacking a cap to ground anywhere along the signal path. The later in the circuit the better as far as tonal preservation. Again, use the smallest value that gets the job done.

A cap or cap/resistor series circuit between anti phase points in the circuit to create a NFB loop.

Some have even used a shielded input lead with the shield connected to the plate instead of ground. This is similar to tacking the plate and grid leads together as Marshall did, but with a dangerous exception. A short here puts HV on your guitar (NOT RECOMMENDED).

Here's the very first thing "I" would try:

Unsolder the input triode grid or plate lead at the socket and wrap the two together three or four turns. Then resolder whichever lead you removed. Easy to undo if it doesn't give you any joy and no components needed to try it.

[goldtop5]

I just talked to the customer who owns this amp. He said that it never made those squealing noises when you turn the preamp gain all the way up. Originally, he needed a new ohm tap selector switch. Apparently, they don't make those jumper style tap selectors anymore. For the new one, it required that you make a bigger hole in the chassis. The customer did not want that done so he asked if I could just hard wire it for 16 ohms for now until he decides he wants to make the necessary modification later. So this problem with the oscillation must have started when I reconnected the 16 ohm tap and left the 8 ohm and 4 ohm unattached. Could this be the source of the problem?

[Chuck H]

The NFB circuit connects to that switch. Did you reconnect the NFB circuit to the same lead it was on originally?

[g1]

Agree with Chuck, and someone has those jumper plugs available aftermarket, will try to find it for you later.

[goldtop5]

Yes, it was originally connected to the yellow wire or the 8 ohm out. It was a purple wire that goes back into the circuit.

[g1]

So just leave it connected to the yellow wire. Does that help the squeal?
The impedance selector is called a P-H502.

https://www.tubesandmore.com/products/P-H502

Marshall Amp Parts - Marshall Switches