Bizarre ground loop/60 Hz hum issue. 5F6-ish clone

[byoungblood]

I'm trying to wrap up work on a 5F6A(ish) clone that I built out of a old Hammond (AO-29) organ amp. Only real difference is that I subbed in a 12AT7 for V1 (biased appropriately) and only have a single input with a bright switch in lieu of a dedicated bright channel. Otherwise, it follows the stock Fender design. 

Sparing the troubleshooting steps, I tracked down the hum to the NFB circuit, specifically the presence control. If I lift the ground from the presence control, and run a jumper from the tail resistor to ground, the hum largely disappears, down to a negligible amount. Jumper the presence control back in, hum is back, about 2.5v p-p. Thinking it was a lead dress issue,  when I completely removed the pot from the chassis, and jumpered it in, the amp goes into oscillation with the presence control at max, with it at its minimum, I just get the 60 Hz hum but the amp is stable.

I have all of the preamp grounds on one bus, the PI and bias supply on another, and then they tie to the same chassis ground point where the main filter cap (I'm using a 2 section 50/50uF) and center tap for the PT all tie together.

Right now, I can use the amp, but now lack a presence control, so I'd like to get this figured out.

[sluckey]

A couple things to try... The presence pot, OT secondary (ie, speaker jack), and filter cap that supplies the PI should all be grounded together. If still have problems, swap the OT primary plate leads If amp gets worse or howls, pot the OT primary plate leads back.

Also, the preamp ground buss should connect to chassis near the input jack, not tied to the same chassis point with the PT center tap and power filter caps. Some people just use the input jack as the preamp chassis ground point. I always use a dedicated chassis lug near the input jack.

Did any of this help?

[byoungblood]

Thanks, I'll give those a try when I get some time to work on it tomorrow. Can't wait to see how it sounds minus the hum!

[byoungblood]

Sluckey, that did the trick. Thanks for the help! Need to hook it up to a speaker again, but on the bench across a dummy load the 60 Hz noise is down to about 90 mV p-p now.

[Willabe]

Sluckey, that did the trick.

What/which did the trick?

[byoungblood]

Sluckey, that did the trick.

What/which did the trick?

Grounding the secondary of the OT to the PI ground had the biggest effect and got rid of the oscillations.