Typical AC on tube plate

[ampgeek]

Tracking down a hum that has recently developed in a single end, Two Stroke'ish design. Heater center tap is elevated at the power tube cathode, 5Y3 rectifier and a 20uf filtering cap at the first node.

I am hearing a 120Hz hum that isn't affected at all by the volume/tone controls that I swear wasn't there before.

The amp is otherwise, generally, fine. Still needs some tone-tweaking now that it is in the cab with speaker.

I am measuring ~6 volts AC and ~320 volts DC on the EL-34 plate. Similar with 6L6 and 6V6 in the output section.

Is that a typical AC voltage measurement at the first node? I will admit that I have never measured for AC there so I don't have a baseline to compare it with.

~2% of the total DC level sure feels ominous though. Ailing first filter cap possibly? Subsequent nodes are well below 1 volt AC.

TIA,
Dave O.

[Jana]

With only 20uf filtering on the first node, your voltages don't seem out of line. You might want to try this: http://www.duncanamps.com/psud2/index.html

[ampgeek]

Thank you Jana!

Very helpful tool.

Cheers,
Dave O.