HF or ultrasonic oscillation, what to check?

[Satamax]

Hi everybody!

Well, old ampage is dead for good as it seems! Or was last time i checked!

Just a few questions, i'm rebuilding that SC 50 PA into a SC L100 circuit as per this schemo http://home.earthlink.net/~iktoblikto/B100_schem.jpg exept that it's 50 watts. I have an oscillation problem, the tubes dissipate something like 64ma above five when the iddle is about 45 with no sound comming out at five, so i guess it's a case of ultrasonic oscillation. I've got it pretty narowed down to before the normal volume pot, or the pot itself, since iddle is fine at 0 on the vol. Till five or 6 i get no sound but it dissipate more and above that i get the old valve radio tuning sound, dependent on the pot rotation, you know that screechouichchuchouchiiii. I was checking input jacks, putint the blade of a screwdriver on the tip lug and moving it makes the kind of sound you'd get by moving the aerial i think. What else can i check? Can this come from teh pot itself, or by the dual shielded cable, with normal and brilliant runing together in the same envelope (the real amp has two shielded cables, one per chanel)

Thanks a lot and best regards.

Max.

The link to the page with the pics of teh amp, if ever that tickles your fancy to see it.

http://vintageamps.com/plexiboard/viewtopic.php?t=50494

[R ski]

My first suspect would be the feedback resistor, the 10K connected to
the 16 ohm tap. Lift the resistor. If the high frequecy dies the output
tranny leads are wrong on each tube.

If you cant hear the squeal, a trusty voltmeter across the output
will indicate HF when preseet.

[Satamax]

Hi R Ski!

Well, thanks a lot for the reply. I found what it was two weeks back. It was a very microphonic V2 valve.

Best regards.

Max.