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?

[TexasTone]

Latest.

I moved some grounds.  The issue now is that the entire amp chassis is microphonic.
Pull V1 preamp.  No change.
Pull V2 reverb driver.  No change.
Pull V3, reverb recovery / PI driver.  Totally silent.
I tried three different tubes in V3, a new JJ, and NOS GE, and a NOS RCA.  The entire amp is microphonic no matter which one.
I'm going over the V3 circuit with a fine-tooth comb, but so far, nothing.

[passaloutre]

There is a component in the signal path with a loose connection.

[tubeswell]

Latest.I moved some grounds.  The issue now is that the entire amp chassis is microphonic.Pull V1 preamp.  No change.Pull V2 reverb driver.  No change.Pull V3, reverb recovery / PI driver.  Totally silent.I tried three different tubes in V3, a new JJ, and NOS GE, and a NOS RCA.  The entire amp is microphonic no matter which one.I'm going over the V3 circuit with a fine-tooth comb, but so far, nothing.

There is a component in the signal path with a loose connection.

Check that all input jack tip-switch contacts are making firm connection with the jack tips when there is no plug inserted. Even a slightly open connection will rattle microphonically. You can verify this by pressing the tip switches against the tips to see if the hum stops. The fix is to re-tension the tip switches slightly to make firm contact.

[AlNewman]

Can you isolate the problem?  I happens with the reverb level at 0?  What happens if you unplug the reverb tank?  Ground the grid of V3a pin 2?   Ground the grid of V3b pin 7?