Building an amp

Reverb Hum, Measuring

Scope

Because the hum a noticed with the reverb, I had to do some measuring.

Bought myself a simple scope. This is the voltage direct after the rectifier tube.

Expexted more a sine-wave, but nothing special here.

Voltage "D"

Voltage "D" of the pre-amp and the reverb. No rimple at all. So it's definitly not the power supply.

Plate Voltage V4A

The plate voltage of V4A, the reverb tube. As you can see, there's a lot of hum visible. Reverb tank not connected.

Grid Voltage V4B

The grid voltage of V4B. A very loud hum enters when measuring. Visible as well.

Plate Voltage V4B

The plate voltage of V4B. Reverb pot all the way down. No hum visible (or hearable).

Plate Voltage V4B

When turing the reverb pot up, I get a lot of hum, here visible.

Conclusion is that the hum comes from V4A. I think a ground loop, the reverb unit is not connected. The RCA jacks of the reverb are grounded to the chassis. I'm gonna change this by mounting isolated jacks and ground them too the bus bar. I hope and think this will help.