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

[slidincharlie (Carlo P)]

Nothing yet! Trouble with Enzo's test

I am still where I started. No progress .
Tube swap and socket pins retensioning didn't help.
I checked for possible breaks in the board jumpers, but everything seems ok in that sector.
I have redone a couple suspect solder joints to no avail.
A few of my mods are in the dirt channel (V2) area, and I have checked my job in that area. Continuity is where it is supposed to be.
The trial suggested by Enzo above (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) didn't work: with a bare cord plugged in the FX return jack and a guitar plugged in the main input jack the offending noise is still there. The only difference is that the output volume is greatly reduced. Maybe I didn't perform Enzo's test correctly...?
EDIT: the oddest thing (to me at least) is that the noise (like an out-of-tune radio "whistle" on top of the guitar sound) appears only between 7 and 11 on the PRE pot dial. Before 7 and after 11 the amp works normally.

[tubeswell]

Hi Carlo

If it appears at a certain point on your dial, it is probably some oscillation induced by your mods. (Edit - Duh why did I say that?) What did you take out?

You could possibly try temporarily grounding each plate (in turn) in the pre-amp with a 630V cap to ground when the oscillation occurs to see if you can isolate where its coming from. It sounds like its from V2A or V2B, but it could be that those stages are only where the symptom manifests itself.

(Edit) While I'm just shootin' the breeze, I still wonder now whether it could be related to a dodgy trace pad that, with re-soldering, has got torn a wee bit when some component was removed - in which case you could try scraping a bit more insulation off one of the traces next to the pads that you took stuff out of, and re-soldering a more reliable connection?

[slidincharlie (Carlo P)]

Hi Carlo
If it appears at a certain point on your dial, it is probably some oscillation induced by your mods. What did you take out?

I took out C7, that is a part of the RC network between the PRE (dirt) pot and the grid of V2.

You could possibly try temporarily grounding each plate (in turn) in the pre-amp with a 630V cap to ground when the oscillation occurs to see if you can isolate where its coming from.

The main trouble in this very amp model is that it is very hard, when not impossible to work when the pcb's are mounted in the chassis. So to test each job you have to put the boards back in place... I cannot even figure how to ground the plates in this amp...

Thanks for following me in the troubleshooting process, btw

[tubeswell]

Yep there might been a reason they put C7 in there in the first place. Did you also change R9 to 1M or leave it a 2M2? (I also have C7 out, but I changed R9 to 1M. Edit - Without C7 there, this makes the grid leak ~500k when the pre pot is dimed - (parallel pot). If the 2M2 was still there, the grid leak resistance would be even lower still - although I am not sure whether that would cause the oscillation. At 470pF - C7 was obviously put there to take out something)

I know what you are going through - what with the awkwardness of the layout of the amp to physically alter stuff - I've been there several times with my C30.

[Mars Amp Repair]

But not too much 'force' or you'll break them! ;-] glen

[slidincharlie (Carlo P)]

Fixed!

Sorry for posting this late. I've been working on this amp in scattered sessions across a long time (about three weeks), then I needed a period of rest...
I have fixed the odd problems in this amp. It was a matter of bad solder joints in several different points where I made the mods.
Now the amp works.
This is the last time I do these mods in this amp anyway. Too hard and time consuming for me.
Thanks all for the advice.