Univox amp Buzzz

[hylaphone]

Hi gang, I have a Univox 1236 here with a wild amount of 120Hz buzz happening in the 1st stage. It disappears when the 2nd grid is grounded. I've exhausted all of my usual troubleshooting steps and can't seem to solve it.

Filter caps are new, I replaced the preamp cap twice to be sure. Voltages look good. I've gone over all the grounding, everything earlier than the PI node sees ground at the input jacks, I did try isolated them and running a wire to the PSU ground, that didn't help.

Some values in the PI tail and feedback loop have been modified by a previous creature, but I don't see how that could cause buzz that goes away grounding V1b..

Any wild ideas? Thank you!
Univox1236.pdf

[Jon Snell]

120HZ im your country is main smoothing issues. Just because they are nice and shiny, doesn't make them good!
V1B is the first ECC82?

[Helmholtz]

Hi gang, I have a Univox 1236 here with a wild amount of 120Hz buzz happening in the 1st stage.

Are you really sure it's 120Hz?
Can you post a scope pic of the buzz at the speaker output?

[hylaphone]

Are you really sure it's 120Hz?
Can you post a scope pic of the buzz at the speaker output?

I was mistaken, it's mostly 60Hz --
I did swap V1 to no avail.

[Jon Snell]

Grounding the 1st grid doesn't stop it?
Remove the 1st anode 220k, does it stop?
Check the sharp deep wiring for bad connections.

[hylaphone]

Grounding the 1st grid doesn't stop it?
Remove the 1st anode 220k, does it stop?
Check the sharp deep wiring for bad connections.

Correct, grounding 1st grid just makes louder hum.
I pulled the 1st anode resistor and the hum disappeared

[Jazz P Bass]

V1 .01 coupling capacitor.
Is it possibly faulty and passing Vdc to the second grid?

[hylaphone]

V1 .01 coupling capacitor.
Is it possibly faulty and passing Vdc to the second grid?

Just threw a new one in, no change --

Also, the Sharp/Deep circuitry seems ok, they function correctly and I checked continuity between all the components.
The switches pop a bit when engaged but seems unrelated

[Helmholtz]

I was mistaken, it's mostly 60Hz --
I did swap V1 to no avail.

So I assume heater buzz generated in the first triode.
Is the PT original and does it have a heater CT?

[hylaphone]

So I assume heater buzz generated in the first triode.
Is the PT original and does it have a heater CT?

Yes original PT, and I actually tested that by trying a humdinger pot instead of the CT. No change in the buzz

[Jazz P Bass]

Ok.
You replaced the .01 coupling capacitor.
Did you try to measure it first?
There should not be any Vdc on V1B side.
With that in mind I would measure the next cap, the .02.

[hylaphone]

Measured 0VDC on both V1A and V1B grids.

[hylaphone]

Sorry to be so frantic here!
Finally figured out that it was a nasty ground loop.
On this amp the PCB is about half visible through removable panels in the chassis... in one of those windows the PCB grounds appeared to be split/separate between Preamp and PI/Power amp, with the former ending at the input jacks and the latter ending at the 1st filter node, CT, etc. I had tried isolating the input jacks and running a wire down to the latter, which made an even worse hum. Today I pulled the input jacks from the chassis again, and happened to notice they still had ground continuity -- but where?? Well hidden from view, the PCB ground trace wanders up and around the entire perimeter of the board - connecting both sides and making the ground loop when the input jacks are in. I isolated them without the additional ground wire, and the amp is dead quiet.

Thank you all for the ideas!