Amp squeling when i rotate the gain knob

[Rod]

Hi i need some help here to find what is the problem:

Well, after i have done some mods to my Marshall dsl 15h the amp now starts squealing when i rotate the gain knob to add gain. If i rotate the gain knob up to 1 the amp doesnt squeal, but after this position if i rotate the knob to add more gain the squeal changes pitch and gets louder.

The problema only happens in the overdrive channel.

What could be the problem here? if the squeal changes pitch when rotating the gain knob, could the problem be a capacitor?

The amp has this problem even with anything plugged into the amp. i also have checked the preamp tubes ans eveything is allright.

[Rod]

Ok the squeal started after i have done the following mods:

In the preamp:
R34 changed to 220K
C58 changed to a 500pf 500V silver mica cap

in the poweramp:
R91 to 10K
C69 to 4,7 nf , 500 volts cap

The Marshall dsl15h schematic is in the link bellow:
https://www.dropbox.com/s/0ekws3lamt...2-v04.pdf?dl=0

[mozz]

Why would you change r34 to 220k from 18k? I stopped looking after that, can't find r91, who draws these schematics and numbers them?

[Rod]

Why would you change r34 to 220k from 18k? I stopped looking after that, can't find r91, who draws these schematics and numbers them?

I changed to 220K for more gain, more deep bottom end , the schematic is from Marshall , the R92 is in the negative feedback loop

[Enzo]

Um...

So you increased the gain of one stage by 10 times, and also reduced the NFB, and now the amp is unstable. You can't increase gain forever, there are consequences.

[Rod]

Um...

So you increased the gain of one stage by 10 times, and also reduced the NFB, and now the amp is unstable. You can't increase gain forever, there are consequences.

The problem remains after put again everything like it was , i changed tubes check for microphonics, i have no idea what has happen, does someone knows what to check?

[Chuck H]

The problem remains after put again everything like it was , i changed tubes check for microphonics, i have no idea what has happen, does someone knows what to check?

If the amp didn't squeal before you worked on it, and now you've put it back to the way it was before the problem, but it still squeals, you have probably made a wiring error. These are VERY difficult to diagnose from a remote perspective because it amounts to an incorrect circuit. Symptoms are easier to diagnose for correctly wired circuits because there is at least some point of reference.

The good news (?) is that your gain increase mod might actually work and it's something other than the extra gain that's causing the problem. So once you solve for the issue at hand you might be able to try it again with better success

[g1]

Any chance the output transformer was disconnected or replaced?
How about connector 2A/2B shown below the power tubes, can it be put on reversed?

[Rod]

Any chance the output transformer was disconnected or replaced?
How about connector 2A/2B shown below the power tubes, can it be put on reversed?

If the ouput transformer was disconnected what could happened?

I had connected some wires in the wrong side for a little time.

[Rod]

The connectors that i have connected in the wrong position are the AN1A e and AN1B.

[ATTACH=CONFIG]41720[/ATTACH

[Chuck H]

The image doesn't show where the wire on AN1B goes and the schematic shown no connection to AN1B shown on the schematic. So it's impossible to say.

[Rod]

The image doesn't show where the wire on AN1B goes and the schematic shown no connection to AN1B shown on the schematic. So it's impossible to say.

It is a conection in the power tubes side , you can see it in the schematic.

[g1]

I don't think it matters if AN1A and AN1B are reversed. But I'm curious, one of those brown wires should go to the output transformer. Where does the other one go?
And what about the CON2 I mentioned, can it be put together reversed?

[Chuck H]

It is a conection in the power tubes side , you can see it in the schematic.

Uh, yeah. I saw THAT. AN1A in the schematic is connected to a power tube plate, the OT and the triode/pentode switch. AN1B is just hangin' there connected to NOTHING and there's no other AN1B reference in the schem to allocate it. This isn't my first rodeo. I got that. Yet there's a lead connected to it in the actual amplifier. As you've shown in the image. But there's no way to tell from the schematic OR your picture where the lead connected to A1NB goes.

Just to be clear.