Ampeg B15NA clone with 60Hz buzz HELP

[R.G.]

The signal waveform is a clipped sine wave, and the clipping is consistent with the HV secondary immediately before the rectifiers. The voltage rises, diode turns on and clamps the voltage to the first filter cap, so it rises more slowly. It stays at the capacitor voltage until the diode turns off, then drops suddenly back to the sine wave slope underlying it; that little vertical section.

That's where it comes from; how does it get into the signal chain? All of the transformer secondaries will have the same waveform, just different voltage levels. I don't have a great guess on that so far. That little circle of yellow wire - what is that?

[sluckey]

That little circle of yellow wire - what is that?

It's the unused 5VAC PT secondary.

[R.G.]

I wonder what happens if that is uncurled and folded outside the chassis...

The waveform makes me think that ...something... is getting capacitive pickup on its grid wires from transformer wires. Maybe the 5V winding, maybe the heater winding. I don't see a smoking gun in the pictures, but that may just be me missing it.

I didn't do the usual where's-it-grounded stuff yet, largely because of the waveform.

Hmmm. Here's another question: what happens if you un-lift the heaters from the output tube cathodes and run a grounding wire from the heater CT to the negative of the first filter cap?

[lonote]

I have done the "heaters on a 6V battery" thing a few times at a similar point, chasing an issue.

It can potentially eliminate the heaters as the culprit.

[R.G.]

That would make for an interesting test, all right. Pull the heater wires loose at their first solder joint with the transformer winding and tack-wire in the battery. If it makes any change in the hum/buzz at all, it's at least part of the problem.

[Helmholtz]

As of post #1 the OP aready tried powering the heaters from a DC supply with positive results.

The scope pic looks like a typical AC heater supply voltage itself.
Wondering where this was scoped and if a x1 probe was used.

[Stevem]

But this is what the heater supply looks like when it’s biased up from the output cathode dc?

I ask because I have never scoped out a dc biased up heater feed.

[R.G.]

As of post #1 the OP aready tried powering the heaters from a DC supply with positive results.

Oops. I dis-remembered that.

The scope pic looks like a typical AC heater supply voltage itself.
Wondering where this was scoped and if a x1 probe was used.

Probably not. The scope says it's 0.49Vrms. But all of the transformer windings will have the same wave form, just different voltages. They're magnetically clamped together.

Sure is looking like something is leaking into V2P4 grid circuit somewhere. Perhaps review really hard where those wires and resistors go.

[Helmholtz]

How does the hum look at the speaker output?