strange champ problem

[sir 6v6]

Hi, I´m having problems with my diy champ (blackface)

when I turn up the volume on my amp there is a strange scratchy noise (like a dodgy pot). It can´t be the pot because it is brand new. replaced the old one and when i tested the amp it was all the same as before.
the noise is about half way up, and when it comes to the noise "area" the amp looses volume. when turned up from there it is just fine apart from the fact that there is no overdrive.
I´ve tried changing the tubes, still the same. tried changing the filter caps and some other caps. measured all the resistors, seems to be fine the lot. whatever I seem to do I can´t get rid of the problem....
(tubes: 12ax7 and 6v6)

Any ideas?

[Bruce / Mission Amps]

Originally Posted by sir 6v6

Hi, I´m having problems with my diy champ (blackface)

when I turn up the volume on my amp there is a strange scratchy noise (like a dodgy pot). It can´t be the pot because it is brand new. replaced the old one and when i tested the amp it was all the same as before.
the noise is about half way up, and when it comes to the noise "area" the amp looses volume. when turned up from there it is just fine apart from the fact that there is no overdrive.
I´ve tried changing the tubes, still the same. tried changing the filter caps and some other caps. measured all the resistors, seems to be fine the lot. whatever I seem to do I can´t get rid of the problem....
(tubes: 12ax7 and 6v6)

Any ideas?

Off the top of my head it sounds like it could be an ultrasonic oscillation.
Try moving wires around the 12AX7 grids (lugs 2 and 7) and to and from the volume pot to different angles and locations to see if that effects this issue.
If so, you should try shielded audio cable if you have not.

[6267]

I'd second Amp Kat on the oscillation, as I'm working on getting a pretty high gain design into a cabinet and I get this from time to time. It sounds like someone is crumbling newspapers up rapidly, in my case. It could also be DC on the pot, so you may want to check it with a meter. Pots make scratchy sounds when they have DC on them. Also, some pots are just terrible. I've had some pots where the conductive surface starts flaking off (they were pretty old) and it cause all kinds of crazy problems.

[Mars Amp Repair]

Could also be some residual DC on the pots from possibly a leaky coupling cap or such. It only takes a few milivolts to make the pot scratchy. If the pot came right off one of the tubes, a leaky tube can put some voltage on the pot too, but I'd guess you've replaced all the tubes by now. glen

[sir 6v6]

Originally Posted by Bruce / Mission Amps

Off the top of my head it sounds like it could be an ultrasonic oscillation.
Try moving wires around the 12AX7 grids (lugs 2 and 7) and to and from the volume pot to different angles and locations to see if that effects this issue.
If so, you should try shielded audio cable if you have not.

haven´t tried shilded cable yet... poked around the cabels as you suggested, something did change... (not much).... so, should I try to rewire the input stage and volume control? also... the same noise occures when bass control is used... the only control that is fine is treble...