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.