Two Rock Buzz

[John G]

Hi, I have a TR studio signature amo on the bench, came in with hum when MV advanced. Traced that down to bad JJ pre-tubes.I am now left with a buzz being generated in the first pre-tube. With the gain all the way down and MV up no noise(buzz). Advance the gain and a buzz appears. This is all with no input to the amp.
If I short wiper of the gain control all is silent. If I short the top of the pot the buzz remains.
Question, should the pot ground be going to the cathode earth point( or somewhere else)rather than the common earthing buss?
Thanks if you can help
John
PS e-mailed TR a couple of times but no response.

[stratomaster]

On a hunch, could you temporarily tack a 10+μF cap across that gain stage's cathode resistor? I'm finding lots of examples of preamp tubes unable to cope with partially bypassed cathodes these days. If that takes care of it you have a leaky tube. You can try swapping it, but elevating the heater center tap may be a better long term solution.

[John G]

Stratomasterm, Thanks for the suggestions, good call re the lowish cathode bypass cap.
Unfortunately, the customer got his amp off me to do some gigs so this problem will be put on the back burner.
I look forward to experimenting with this amp.
Thanks again

[pdf64]

Buzz usually indicates a heater circuit issue; is it properly balanced and referenced?
eg check for a bad 100R balancing resistor or damaged humdinger pot.

[John G]

Thanks, I will add your suggestion to my list to check.