High-pitch noise in modded Peavey Classic30 (oscillation?)

[slidincharlie (Carlo P)]

I have installed a master volume in a Peavey Classic 30 amp, as per Steve Ahola's instructions. The master replaces the R48 grid resistor on the phase inverter tube (schematic attached).
I did this mod in another Classic 30 before, and in that case the amp developed an oscillation issue that I fixed with a different layout of the master volume wires - see picture attached below: the green arrow shows the correct final layout of the two master wires (indicated in their former wrong layout by the upper red arrow).
Now I have a similar problem: when the Drive channel is engaged, the PRE pot develops a loud fizzy scratchy high-pitched noise (similar to an out-of-tune radio) between 6 and 11 on the dial, while it works fine below 6 and then at 12 (end of the dial setting). The noise is on top of the note, does not replace it entirely. It is not a squeal by all means, just a loud fizzy scratchy noise. The starting point (#6 on the knob dial) does not change with different master and POST settings.
There is no problem of any kind on the Normal volume channel (i.e., Drive switched off).
I have checked the PRE pot with my DMM, it seems fine.
I have replaced the phase inverter to no avail.
I have also modified the master wires layout of course, with no results.
What else should I check?

[tubeswell]

Hi Carlo

When I did this mod I used twin shielded cable and never had any probs at any volume. I wired the cable in from the 'inside'/reverse side of the pc boards.

But a 1M pot wired as a voltage divider in a little aluminium box with two phono jacks inserted into the FX send and return will do the same job

[slidincharlie (Carlo P)]

I used twin shielded cable, too. But I soldered the wires on the outer side of the pcb (components side).
I'll try to change side.

[slidincharlie (Carlo P)]

Things are not quite as decribed

I have disconnected the master volume and gone back to the pristine configuration of this section. The noise is still there, so it is not directly linked to the master volume mod.
I have not specified that the noise happens when I play a guitar with the Drive channel engaged. Without a guitar plugged in I can only hear a very slight noise when I set the Pre pot in the disturb zone (7-11 on the dial), more like a weak scratch with an almost inaudible high-pitch whistle on top.
I am really puzzled. I just did a few other mods in this amp. Could a bad solder joint or a less than optimal connection cause such noise and/or oscillation? Will the use of a jumper wire help me in the troubleshooting process?

[Enzo]

Wait. Try reversing the OT primary connector. I'd be thinking when you took it apart to do your mod, you might have put it all back together but reversed that connector. That would be the connector with the blue, red, and brown wires.

[slidincharlie (Carlo P)]

Wait. Try reversing the OT primary connector. I'd be thinking when you took it apart to do your mod, you might have put it all back together but reversed that connector. That would be the connector with the blue, red, and brown wires.

No, I have marked all connectors' ends to avoid similar errors prior to disassembling...

[Enzo]

Does it help if you do it anyway?

[slidincharlie (Carlo P)]

Does it help if you do it anyway?

No, it causes a loud high-pitch squeal even with the volume pots set at zero.

[Gtr_tech]

If there's a NFB loop in the output section and you have the primary leads connected in reverse, it usually sounds somewhat like a theremin operating at a lower midrange freq.....and *loud*. High pitched squealing or whistling usually points to a lead dress issue or some abnormality in the front end. Got a scope? You may need one to find the oscillation.

[slidincharlie (Carlo P)]

No scope.
The only thing that can be related to bad lead dress in my mods could be a cap that I had to bend right over a nearby resistor to get it flush to the pcb. For the rest I really can't realize where a bad lead dress may have occurred...

[Enzo]

OK, so we have a problem. Let us forget what we did and then undid, let's just say we have the problem and solve it.

Isolate the problem. PLug something into the FX return jack. Does that kill the noise? I expect it will, since your problem only occurs on the dirt channel, so it seems preamp related.

But try this: plug a cord into the FX send, and send the signal to some other amp for a listen. The sound still comes out the speaker in the C30, but we can also now hear the preamp from the other amp. Does the noise still happen? Does the noise come out the other amp too now? If it does, leave all that set up, but plug something into the FX return jack to silence the power amp of the C30. Now play. Does the noise still happen, heard through the other amp? If so, the problem is localized to your preamp. If silencing the power amp makes it stop, then that points to possible B+ decoupling issues, or maybe feedback from the powr amp.

You changed the phase inverter tube, but I don;t recall you saying you changed V2, did you? Try that. V2 is only used for the dirt channel.

Inside. I don;t recall, did you check every little bare wire jumper between boards? A broken one could cause this.

[slidincharlie (Carlo P)]

Isolate the problem. PLug something into the FX return jack. Does that kill the noise? I expect it will, since your problem only occurs on the dirt channel, so it seems preamp related. (....)

Enzo,
Plugging a bare cord into the FX return jack only affects the output volume. It does not affect the noise, it only reduces dramatically the amp's overall volume.
Is this a clue?
In case it was not clear, I would stress that the noise (sort of out-of-tune radio rumbling hiss with a weak trebly whistle on top) happens when I play the guitar between 6 and 11 on the PRE dial, AND also without anything plugged in, only weaker.
Replacing V2 did not change things.

[tubeswell]

It could be from a dodgy connection. Either a strained jumper tugging at a trace pad, or a bad soldering joint, or maybe - have you tried re-tensioning the tube sockets?

[slidincharlie (Carlo P)]

This amp is driving me crazy... Fool me to accept such jobs...
I'm going to bed now, tomorrow I'll check jumpers, bad connections, etc. (I already did it actually... but I'll do it again).

[tubeswell]

Anger will lead you to the dark side. Use the force - you are on the verge on important self-discovery. And check the tube socket pin tension.