PPIMV need help - dual pot with long bushing threads?

[pula58]

I installed a master volume into a Dr. Z Prescription ES. Cathode biased, 4 EL84 output stage.

Works great, until I turn it up all the way. Suddenly, at the last slight turn of the pot, there is hum that kicks-in. I thought maybe it was a bad pot or that perhaps some debris (solder, wire clippings) got into the pot (dual Alpha, 250KA pot). So, I removed the pot and installed another one (another Alpha, dual 250KA pot). Wired it all up, and the same darn thing happens.

I measured the resistance from the wiper of each pot to ground and it behaves as expected, reads about 220k when the pot is turned-up all the way, and ~250K when the pot is turned all the way down. I use shielded cable from the wiper of each pot to the grid-stoppers of the output tubes . The shield is connected to the ground lugs of the pots, and that, in turn, is grounded to where the 220K grid leak resistors (now removed) were connected.

Like I said, I thought it was the pot, but with a new pot I get identical behavior.

Any ideas?

Thanks,

P.

[pula58]

So, I pull the pot out of the chassis hole (while the pot is still connected) and it works perfectly. Something going on with ground loops in terms of the threaded shaft of the pot coming into contact with the chassis. So, I have to bore-out the 3/8" hole to 1/2" and use insulting shoulder washers...but...the threaded part of the Alpha pots is only 1/4"..not long enough to reach through the chassis and (rear) faceplate and the shoulder washers. I think I need 3/8" of threaded bushing length.

So, Anyone know where I can get dual 250KA or 500kA pots for 3/8" mounting holes, with the threaded part of the bushing at least 3/8" long?