HELP FOR OD STAGE PROBLEM

[sergio]

Hi all, I have a problem with my amp.
When I turn the DRIVE POT(250K) of the OD stage to max(11), my amp
starts to whistle, or squeal like it's going feed back. This even
happens with the OD LEVEL pot at zero. More precisely, it starts
happening from about 9/10 to 11.

My OD input stage is: 120k-250K trim.
The OD input trimmer is set at about one half, or almost 1/3. When I
adjust the trimmer to a lower value (toward ground), the whistle goes
away.

Example:
80 KOhm to ground: whistle
40 KOHM to ground: no whistle

NOTE:

V2a 200V / 1.48V
V2b 202V / 1.51V

No 220K/250Pf in FX loop
No 15Pf cap on master pot
I have a reverb stage one tube between master pot and PI.

Thanks

[Bob-I]

Oscillation for sure. Most likely fix will be a tube or lead dress. For obvious reason I'd try a different tube in V2, but if that doesn't fix it you'll need to adjust the lead dress.

[sergio]

I have tried to replace the tube, but nothing.

[dogears]

Sergio,

You should not adjust the trimmer to higher than about 50K from ground. 120 into 250 is similar to the 220K into 100K that Dumble usually used. Those amps are adjusted anywhere from 1/3 to 1/2.

#124 had 220K into 350K. That is 550K total. That was adjusted 100K from ground.

Also, please try the 100k drive pot. You'll dig it!

[ayan]

Sergio,

You should not adjust the trimmer to higher than about 50K from ground. 120 into 250 is similar to the 220K into 100K that Dumble usually used. Those amps are adjusted anywhere from 1/3 to 1/2.

#124 had 220K into 350K. That is 550K total. That was adjusted 100K from ground.

Also, please try the 100k drive pot. You'll dig it!

What Scott says is common -- and good -- practice. However, it has nothing to do with the reason you have a problem. As already mentioned, you have an oscillation, and whether it goes away or not when you turn "something" down, for all you know you're only tuning your oscillator to a frequency you cannot hear, but the problem is still sucking the life out of your amp.

Ten years ago, I wrote this: http://www.ayanmusic.com/leaddress.htm and some people said it helped them, so it might be able to help you too. If you can posts detailed pictures of your build, perhaps we may be able to help you here.

Good luck! The great news is: if you tame the oscillation, the amp may still be on the verge of instability... and it will sound great. I am not the first to make this comment, but amps sound best when they're ready to take off on their own. Dave Funk mentiones in his book that Ken Fisher's amps and his own were deliberately built with instabilities to get some magic out of them.

Cheers,

Gil

[BobW]

Dave Funk mentions in his book that Ken Fisher's amps and his own were deliberately built with instabilities to get some magic out of them.

Built like the old Teles, just on the edge of instability. It really does bring out the magic.

[sergio]

Hi guys , I solved my problem , I had a big error in my wiring.

I had wrong to wiring the NFB on the impedance selector.
I run with my input OD 120K/250K and now is all ok.


Thanks all very much.

Sergio