Strange sounds from master and presence

[Tdale]

I discovered something today that I don't understand.

When I turn the presence pot all the way up, where the sound gets sharper and more edgy, there is a squeeky or squeeling sound. Especially if I put the master pot around 2-4 o'clock.

Also, the master pot gets a bit noisy when I move it in that same position...a bit buzzy, and something happenes to the sound..

Any idea? Could it be the presence cap?

Tommy

[unklmickey]

hi Tommy,

my guess is there is nothing wrong with the presence cap.

the presence pot is actually the bottom (closest to ground) resistor in your long tailed pair PI.

the node at the cw lug of that pot, is where the NFB is returned from your OT.

when the presence is at max, your presence cap is shunting the high frequency content of the NFB to ground.

so, the NFB is no longer able to clean up some instability that exists in your amp.

beyond that i can't help much.


cheers,

unk

[Tdale]

Ok.. I got that explanation from someone else too.. I got to find the source of the ocillation..

Tommy

[Tdale]

I followed Bob's advice, and found the source. It's one of the OT primaries that makes the noise. When I move it around, the squeeling stops. But then, when I turned the master up a bit, it started again....moved the wire more, and it got quiet again.

The OT primaries are very close to the secondaries...is that a bad idea?

Could shielding the primaries help? I'll try to move them more, and maybe find a perfect place for them....

Tommy

[drz400]

Ok.. I got that explanation from someone else too.. I got to find the source of the ocillation..

Tommy

Put your hand around the input jack and see if that stops it.
Take a look at any wires from the output tubes that are too close to the speaker jacks. I've had this problem and it was solved just by routing the primary wires differently to the power tubes like from the bottom instead of from the top chassis side. Yes you want to keep them away from the speaker jacks

[Tdale]

It's the primary on the OT that does it. It runs paralell to the secondaries, and it runs right by the output jack. I'll re-solder it, and place it differently.

Tommy

[Bob-I]

It's the primary on the OT that does it. It runs paralell to the secondaries, and it runs right by the output jack. I'll re-solder it, and place it differently.

Tommy

I've placed the primaries OUTSIDE of the chassis if the run is too long and needs to pass other wires. Just have it enter right by the tubes so that the chassis acts as a shield. I wouldn't use shielded wire for these, too much voltage and current for shielded wire.

I'd also look at what else is near the primaries and see if relocating some of these wires might help.

[drz400]

It's the primary on the OT that does it. It runs paralell to the secondaries, and it runs right by the output jack. I'll re-solder it, and place it differently.

Tommy

Make sure they are all twisted as well
Also the signal feed (grids) to the power tubes

[VacuumVoodoo]

Make sure you haven't swapped the OT primary connections to plates.It's one of more common wiring errors.

[drz400]

Make sure you haven't swapped the OT primary connections to plates.It's one of more common wiring errors.

That mistake is usually VERY obvious!