‘67 Dual Showman Buzzzz

[Helmholtz]

No change when disconnecting NFB. Amp was inoperable prior due to condition of electrolytics. Screenshot attached. Sometimes reads as 1.5khz.

That's a rather dense spectrum containing frequencies from 11Hz to above 20kHz.
Can you exclude an external EMI source?

[fdesalvo]

That's a rather dense spectrum containing frequencies from 11Hz to above 20kHz.
Can you exclude an external EMI source?

I agree. I’ve tried it on the opposite end of the house and have the same issues. I’m desperate enough to suspect bad pots lol.

[Helmholtz]

If this is a sound spectrum (as opposed to electrical signal), what is the noise floor, i.e.with amp switched off?

[fdesalvo]

If this is a sound spectrum (as opposed to electrical signal), what is the noise floor, i.e.with amp switched off?

it just listens for audio. There’s about 30hz in the background of the house.

[Helmholtz]

There’s about 30hz in the background of the house.

Means that the whole spectrum is significantly down except for a line around 30Hz?

[fdesalvo]

Means that the whole spectrum is significantly down except for a line around 30Hz?

yes sir

[g1]

Have you found a proper schematic for it? Just to clarify, it's Dual Showman, not just Showman, and has no reverb? Blackface?

[fdesalvo]

Have you found a proper schematic for it? Just to clarify, it's Dual Showman, not just Showman, and has no reverb? Blackface?

Yeah it’s the ab763 dual showman. I think the diff between the two is just a resistor in the feedback circuit. Top of my head. No reverb just trem.

[fdesalvo]

Here’s the schematic. Noise persists with PI input cap and NFB disconnected.

[Helmholtz]

Here’s the schematic. Noise persists with PI input cap and NFB disconnected.

As pulling the PI stops the buzz, this leaves the PI tube.

[fdesalvo]

I have tried 3 new tubes in the pi slot. No luck. Amp is cursed.

[Helmholtz]

No dc on grids. All voltages are with in spec.

There should be DCV on power tube and PI grids.

[fdesalvo]

Correct. Apologies for leaving that out.

[Helmholtz]

You should really get a scope. I don't have much confidence in phone apps.
Seeing the waveshape can be quite revealing.

[Mars Amp Repair]

Hello FDeSalvo,
Any chance of lead dress Issues. Ultrasonic oscillation you can't really hear or detect w/o scope can also have harmonic elements that you can hear.
Any possibility of mis-reading old faded or overheated color code when testing for out of tolerance resistors leading to replacing with incorrect value?
Just a few thoughts.
glen