Fender Super 60 Red Knob Low power and Hums

[jcollins]

I've got a Fender Red Knob Super 60 on my bench. It hums sounds distorted and has low power.

Narrowed the problem down to the power amp section. Tried swapping all tubes, no change.

Did some signal tracing and it looks like the problem is at the power tubes.

Checked bias. Only 1.5 ma on both tubes.

Measured voltages

3 - 379 V
4 - 370 V
5 - -56 V
Across 2 and 7 6.3 VAC

No voltage drop across the OT center tap and pin 3

Cathode has continuity with ground

So I'm not sure what else to look at here. Obviously the power tubes
aren't conducting but, I can't figure out why.

Any help is greatly apreaciated.

[Enzo]

Reduce the bias voltage. The schematic calls for -50v on the power tube grids. That would be a start.

Those voltage readings, did you take them at ALL the power tube sockets?

Your B+ is like 100v low. COmpare your readings top the schematic. That combined with humming sounds to me like you are missing some filtration in your B+. Go look at the voltage on pin 4. You got 370vDC. Now flip the meter over to AC volts and measure the same point. You SHOULD see about zero AC volts. I suspect you will see your hundred volts there in the AC world. And that would mean your filter caps were either worn out or broken free of the board.

And just to be sure, the schematic calls for 355vAC coming out of the high voltage winding of the power transformer - the red wires. Verify you have that.

[jcollins]

Reduce the bias voltage. The schematic calls for -50v on the power tube grids. That would be a start.

Those voltage readings, did you take them at ALL the power tube sockets?

Your B+ is like 100v low. COmpare your readings top the schematic. That combined with humming sounds to me like you are missing some filtration in your B+. Go look at the voltage on pin 4. You got 370vDC. Now flip the meter over to AC volts and measure the same point. You SHOULD see about zero AC volts. I suspect you will see your hundred volts there in the AC world. And that would mean your filter caps were either worn out or broken free of the board.

And just to be sure, the schematic calls for 355vAC coming out of the high voltage winding of the power transformer - the red wires. Verify you have that.

The first `100 uf cap was a leaker. Replaced it and all is good.

Thanks you very much Enzo.