derailment oscillation question

[9 volts]

Hey there, I've just built a derailment. I seem to have a grounding problem. Even when using a 12au7 as the first preamp tube I'm getting a high pitch oscillation when I remove my fingers from the guitar.......all the eq pots work though the treble controls the frequency of the squeal. Not sure where to start to debug. Gets worse as the volume gets louder. Any suggestions? Thanks

[karczochrulez]

its probably a ground loop - check all ground connections

[wallybob]

Whenever I hear of nasty oscillation problems I think of reversed plate leads and the resultant positive feedback through the presence control. However, I know nothing about this circuit nor do I know if the symptoms entirely fit this issue...

[9 volts]

Thanks, there is no feedback loop in this circuit, so I'll have a look at the grounds. I attempted to star ground the amp so maybe this is the problem. Really whines if the input is left open (eg cable not plugged into guitar)
Thanks again

[9 volts]

Well after checking the grounds (which were ok) I found the solution, not sure why but the when the output speaker socket (common) is attached to the chassis the whine/oscillations disappear. I thought I was supposed to isolate this socket? Can anyone explain this? Thanks.

[Structo]

Do you have a shorting jack on the input?

It should take the input to ground when nothing is plugged in.

Most amps run the black OT wire to the sleeve of the jack which is grounded.

[9 volts]

'Do you have a shorting jack on the input?'
Not as yet but I plan to put one in. Thanks for the reply, grounding the sleeve on the output seems to have been the problem. This has been my first trainwreak build. I wanted to hear how the preamp sounds before building a bigger version. Nice sounding amp

[9 volts]

Well things are coming along fine, finishing off the cabinet. Is there any reason I couldn't add a preamp output socket after the final triode preamp stage? Thanks

[Structo]

Did you get the screeching sorted out.

After reading the earlier replies I thought of something else as well.

If the grid stopper resistor on the first preamp tube isn't connected right to the tube pin or has an excessive amount of bare wire before going to the shielded cable to the input jack, it can cause oscillations as well as pickup RF.

I'm not familiar with that circuit.

Does it have a master volume?

Basically if you had a effects loop for it, that would be a preamp out.
But you would still need to load the output.

[9 volts]

Thanks, yep, got the screech out. I had isolated the output socket and not grounded it anyway. A wire to the chassis from the the socket and the screech vanished. I've added a master volume in between preamp and poweramp stage. I've run a shielded wire from the master vol to a preamp output socket. Added a dummy load when speaker disconnected. Seems be be working !

I had a quick look at some schems an this looks like where a effects loop would occur. I really like this amp. Thanks again for advice