ultrasonic oscillation, help needed!

[sir 6v6]

Hi guys
I´m having trouble with my diy fender champ (blackface)...
It is said to suffer from ultrasonic oscillation... I´ve tried to change cabels to to the first grid of the preamp tube (12ax7) and from the vol pot to the second grid (of the preamp tube). Not much change... What should I do?
Please help me if you can!

[tubeswell]

What do you mean by 'ultrasonic oscillation'?

Have you actually scoped it, or are you going by ear?

Can you describe the sound a bit more, or post an .mp3 clip, or a picture of your scope trace?

What do you mean by 'changed the cables'?

Are you using shielded cable(s) from the input to the grid of v1, and from the vol wiper to the grid of v2 (- with the cable shield grounded at one end only)?

What grounding set-up are you using?

Can you post pics of your layout?

[sir 6v6]

Originally Posted by tubeswell

What do you mean by 'ultrasonic oscillation'?

Have you actually scoped it, or are you going by ear?

Can you describe the sound a bit more, or post an .mp3 clip, or a picture of your scope trace?

What do you mean by 'changed the cables'?

Are you using shielded cable(s) from the input to the grid of v1, and from the vol wiper to the grid of v2 (- with the cable shield grounded at one end only)?

What grounding set-up are you using?

Can you post pics of your layout?

haven´t scoped it, but I got it as an answear to an earlier post... and it is likely to be that... (other guys with the same issues blame sonic oscillation...)
yes, shilded cable from input to grid on preamp tube and from vol wiper to gird 2 on the preamp tube... (it´s a 2 tube amp, (grid 2 = the grid on the second half of the same tube) tubes are 12ax7 and 6v6)
everything is grounded to the chassis, from every ground point there is a cable to the chassis... not stargrounding at least...
maybe I can post some pics later...
sound is a "dodgy pot sound" when volume is touched (same for bass).... apart from that fine BUT without any overdrive whatsoever... low power...

thanx,

hope that helps you to help me

[6267]

Make sure all grid leads are as short as possible. Use stopping resistors straight off the grid pin, after the grid leak (68k at input, 1-10k at power tubes). Try with feedback connected/disconnected. Wire a low value ceramic cap (500vdc rated) from anode to grid or grid to cathode/ground (local HF feedback). You will want to make sure that all wires are short, neat and that reinforcing signals or AC crosses other lines perpendicularly if possible. Good luck, these things can be no fun. How much filtering is there on the B+?