1940s Oahu Amp with 60hz hum

[Helmholtz]

So you still have hum with nothing connected to the amp and both inputs effectively grounded by the jacks?

What happens if you use the HV CT ground as common ground point?

[misterc57]

So you still have hum with nothing connected to the amp and both inputs effectively grounded by the jacks?

Yes.

What happens if you use the HV CT ground as common ground point?

The HV CT is a shared connection point for the HV CT, heater CT and the mains ground wire, and other connections. This is one of the four different connection points I tried, hum is present.

[misterc57]

I rearranged the 3 caps to one common ground.

I lifted the heater center tap and tried clipping each V1 heater (one at a time) to ground. Hum in each case was louder than having the heater center tap connected to ground.

Wonder if I should try wiring in a humdinger.

[Helmholtz]

A humdinger is always worth a try. I think I mentioned that option earlier.
But it typically doesn't help with 120Hz ripple hum.

[misterc57]

A humdinger is always worth a try. I think I mentioned that option earlier.
But it typically doesn't help with 120Hz ripple hum.

Yes you did but I had been chasing other possibilities.

So I clipped in a 500 ohm pot between the two heater wires at the PT with the center pot tap connected to chassis ground. I was able to adjust for minimum 60 Hz hum, however that minimum was no different than with no humdinger installed.

[misterc57]

Finally no hum.

I was playing around with lowering the heater voltage from 6.5 to 6 VAC to reduce hum. Not much difference. I then swapped V1 with another 6N7 I have and hum is basically gone, very faint, even at 6.5 VAC heaters.

I had tried that V1 swap before without success. But that was before moving the filter caps and some other resistor changes.

Wow! This was a long haul...

Thank you all so much for the help, especially Helmholtz !!!