Another weird hum issue

So I recently built an amp that is based on a Sunn Model T, though I am only using 2 output tubes, and have done some different things with one of the first gain stages.

Anyway, the hum seems related to the phase inverter. If I pull it, the hum goes away. Pulling tubes before the PI and turning any volume or tone knobs don't change anything(the mv is right before the PI). Turning the presence control can introduce noise, though it sounds different from the basic hum.

All voltages are about what I would expect all over the amp.

I tried wiring up the neg fb/presence/pi like a marshall instead of a model t(subtle differences) and the hum lessened somewhat.

One odd thing is that when I check the voltages on the grids of the PI, the hum seems to decrease when I touch my lead to the grid wire.

Any ideas? It's not a bad PI tube, I have tried a bunch of them.

Also, fwiw the amp sounds fine, though I have only tried it at living room levels. I heven't taklen to a place I can crank it yet.

John Ward

Maybe it is hum from the 6.3V AC heaters. Check that the heater wires to the power tubes are in phase (pin 2 to pin 2, pin 7 to pin 7). Did you reference the DC on the heaters in some way (6.3V center tap, or 100 Ohm resistors to ground?). If you check the hum on a scope you can tell if it is 60 cycle (heater hum).
It sounds more like 120hz to me, which should point to a power supply filter cap, but I haven't found anything wrong there yet.

The heaters are actually dc on the preamp tubes, something I have done on several amps now, so I don't think that is it, unless one of the filter caps in that supply are bad. Hmm.
Also, the power tubes are on a heater winding that is center tapped and corect polarity is observed on their sockets.

I might just go ahead and move the PI to the same heater winding and see if it makes a difference.

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