Hoffman Single Channel AB763-1 Annoying 60Hz Buzz

[TexasTone]

First of all, the amps sounds great and plays great.

The board is the Hoffman AB763-1 board.
Heaters center tap is elevated 70V via a voltage divider off the C node.

All the input and speaker jacks are isolated from the chassis.  Inputs are grounded to the preamp ground buss- board mounting screw closes to left-front of the amp, near where the first input jack would be.

The Power Amp ground includes:
   Speaker jacks
   B+ Center Tap
   Heater CT via 70VDC voltage divider from Doghouse "C" power [C4].
   Bias supply circuit - caoc
   Power tube cathodes via 1Ω resistors.
   NFB circuit: 1kΩ NPB resistor to 100Ω NFB tail; tied to LTPI 27k tail resistor and 0.1µF LTPI AC shunt.
   C1/C2 100µF 350V Power Supply reservoir caps, Doghouse "A".   (I moved all filter caps to this ground for troubleshooting purposes, and this had no effect.

The issue is the annoying buzz.

• Not affected by any front panel controls
• Not affected by pulling V1-V4
• Pulling V5, the long tail phase inverter, stops the buzz.

Here's what I've done so far:

• Swapped the LTPI tube.
• Reflowed solder on G1 ground bus.
• Grounded 4.7k Mid resistor to ground lug of volume pot.
• Moved LFO ground to preamp buss on turret board.
• Validated heater phase.  All preamp pins 9 daisy-chain to both power tube pins 7.
• Added external 22µF-500V electrolytic across C1, C2, C3, C4, C5, one at a time.
• Grounded all filter caps to power amp ground - per Hoffman_AB763_1.PDF page 2 grounding.  Previously, B, C, and D nodes were on preamp ground.

Nothing has made any difference.

Asking for help.

Thanks.

[AlNewman]

Since pulling your PI eliminates hum, I would think it could be caused by your speaker and nfb being grounded to your reservoir.

[sluckey]

Does the buzz go away if you disconnect the NFB wire from the speaker jack or the board?