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RE: Dynaco Pas Hum

The ACV you are seeing to pin 6 (plate) is what you are hearing. 0VAC from 4 to 5 shows it isn't coming from the heater. But the earlier DC voltage measurement pin 4:

"Only thing is on pin 4 on 12AX7 PC5 should be lees than 1 volt, I have about 6."

This doesn't make sense, or we are on the path to the problem. I suggest you measure it again. This happens to me a lot. I get a measurement that doesn't make sense, and I go back and find I had measured the wrong pin.

Also, please measure 4 to 9 (9 is heater center tap) and 5 to 9. Please check DCV and ACV. Since it is 0VAC 4 to 5, so the only thing left to check on the heater is the center tap.

Another thing that would be interesting is to see what the ACV is on the grids (pins 2 and 7). Like pin 6, this would be AC measurements made referenced to ground. This would help us to see if the hum is coming in from the grid.

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