High gain amplifier trouble with positive feedback

[blackwhaleamp]

Hi,
I am building a single channel dual mode amplifier (CLEAN/HIGH GAIN).

An externally hosted image should be here but it was not working when we last tested it.

This circuit is a draft, which I need to finish of course.

My problem is that when I increase the DIRTY POT, after 3 I get a 4kHz noise. So, there is a positive feedback somewhere in the circuit that is making me crazy.

I literally unplug the whole TREBLE POT from the circuit, and when I feed the amplifier with a signal, I can see from the scope that the signal is there.

I grounded the back of the POTs to GROUND SIGNAL but this is not helping

Any idea about how to solve this?

Thanks a lot

 

[JonSnell Electronic]

Looks like far too much gain. What happens when you remove V2a from the circuit?

 

[jan.didden]

The drawing doesn't open for me.

Can you attach it (when creating the post, go to Manage Attachments, select the file and upload).

Jan

 

[blackwhaleamp]

Looks like far too much gain. What happens when you remove V2a from the circuit?

Ok, I removed one tube and now it works... So too much gain as you said

 

[nigelwright7557]

Quick and dirty fix is replace 12ax7 with 12au7.

I designed a USB valve mixer and it was terribly noisy with the 12ax7.
Then worked out I had a gain of 10,000 !!!!
Popped in a couple of 12au7's and the circuit settled down and worked well.

Valves are very touchy about layout.
1/ Keep transformers away from audio signal.
2/ Keep high volts ac away from audio signal.
3/ Keep input wires/tracks of amp as short as possible.
4/ Use grid stoppers and place them right up to grid pin.

 

[blackwhaleamp]

Quick and dirty fix is replace 12ax7 with 12au7.

Actually, I also try it. But I was not happy. I decide to remove one stage and keep the 12AX7

Also thanks a lot for the tips