Troubleshoot AB763 - buzz and feedback

[Sonny ReVerb]

I have an AB763 style amp that is giving me fits. It's basically a single channel Deluxe Reverb. The schematic is very close to a Hoffman AB763 - 1 Channel. It started as a 6L6/5AR4 amp, but now it is a 6V6/5Y3 amp. It has a raw control and a NFB switch (normal, light [half], and none) added.

It has a nasty buzz that follows the volume control. Also, with moderate to high treble settings the amp will start feeding back at around 4 on the volume control (no signal input).

A couple things I've tried - chopstick all solder connections, preamp tube swaps. I've also rewired the input jack and moved the bias filter cap ground from the ground bus to the main filter cap ground point. I don't think it's a NFB phase issue because the problems don't go away with the NFB disconnected (none - switch position). That's also usually more of a squeal, not this type of feedback.

Could the B+ wire running parallel to the phase inverter wiring be an issue? Suggestions?

[vampwizzard]

what about output transformer leads? Are they wired backwards per chance? its 50/50 on the phasing on initial install

[Sonny ReVerb]

I don't think it's a NFB phase issue because the problems don't go away with the NFB disconnected (none - switch position). That's also usually more of a squeal, not this type of feedback.

That's what I was trying to describe there. I don't believe that was the problem.

I tried elevating the heaters and the feedback problem is gone. Still have the buzz, but it might be tolerable. I'll keep working on it.

[Sonny ReVerb]

Buzz sample

[vampwizzard]

I dont understand how youre using the electrolytic bypass caps attached to the first preamp tube. They are turned 180 degrees opposite to the others on the board which stands out, but not sure why the negative end would go to another cap/resistor pair.

Also not a fan of running the wire up through the board for a non-terminal connection but if it works I suppose it works.

[st]

 

I dont understand how youre using the electrolytic bypass caps attached to the first preamp tube. They are turned 180 degrees opposite to the others on the board which stands out, but not sure why the negative end would go to another cap/resistor pair.

Also not a fan of running the wire up through the board for a non-terminal connection but if it works I suppose it works.

Look at the pics again. First preamp is in the back near the input jack.

[vampwizzard]

I dont understand how youre using the electrolytic bypass caps attached to the first preamp tube. They are turned 180 degrees opposite to the others on the board which stands out, but not sure why the negative end would go to another cap/resistor pair.

Also not a fan of running the wire up through the board for a non-terminal connection but if it works I suppose it works.

Look at the pics again. First preamp is in the back near the input jack.

Fair enough, but where do those electrolytics get their ground reference?

[sluckey]

Fair enough, but where do those electrolytics get their ground reference?

They get the ground via that blue wire that dissappears under the board.

[Sonny ReVerb]

Correct, they're directly connected to the ground lug near the input.