70’s Fender Pro Reverb - AC noise on output, stumped!

[Enzo]

should I remove it?

You should only remove parts when you have a good reason to remove them. Looking at a part and not knowing what it is for, is not good reason.

[bw1985]

This is my take as well. It looks like the green wire is going to a terminal that may be grounded (incorrect) and also to something on the master pot/switch which is probably also supposed to be grounded.
The resistor for the lamp looks correct for schematic, check that the hum pot is wired as shown in schematic.

I'll double check the wiring to see if the green filament wire is grounded. Don't worry, the power cord is grounded

[bw1985]

This is my take as well. It looks like the green wire is going to a terminal that may be grounded (incorrect) and also to something on the master pot/switch which is probably also supposed to be grounded.
The resistor for the lamp looks correct for schematic, check that the hum pot is wired as shown in schematic.

Removed the humdinger pot from the circuit, measured and everything seems healthy. Also swapped a set of preamp tubes for other known tested/good tubes and no luck.

Also, removed all preamp tubes and the hum still exists!

As well I noticed the schematic differs from the wiring in this amp here, the BLU/YEL is connected to pin 4 of one output tube, and BRN/YEL is connected to pin 4 of the other output tube. They are not connected to one another via the 470ohm resistors.

Here are photos of the terminal, as you can see no grounding on the heaters.

[nevetslab]

So, it looks like there's a couple variations on this amp in the output stage....one using the ultralinear screen taps directly, as this appears to be wired, one where 470 ohm screen resistors are added in series with the screen taps (usually mounted between Pin 6 & Pin 4), and the schematic that The Dude added, using the more traditional screen resistor approach, not using the screen taps on the output transformer. Though in that version, we'd normally see a power supply choke ahead of the power supply tap to feed the screens.

Going back to Mikc Bailey's question, have you yet tried a new matched pair of power tubes? I'd do that before going much further, just to confirm it's NOT a power tube issues.

We see the Hum Balance pot is working, as confirmed earlier, so it's not a bad 100 ohm hm balance pot.

[bw1985]

Wait a sec... my hum pot was measuring around 0-30k when I was sweeping it.

Could I eliminate it and wire an artificial CT instead?

[Tom Phillips]

Wait a sec... my hum pot was measuring around 0-30k when I was sweeping it.

Could I eliminate it and wire an artificial CT instead?

Yes. But realize that the hum pot IS an artificial CT that has the improved feature of being adjustable. Therefore, consider replacing the hum pot down the line.

[eschertron]

Wait a sec... my hum pot was measuring around 0-30k when I was sweeping it.

Could I eliminate it and wire an artificial CT instead?

15k to ground from either leg of the heater circuit is too much. Jumper in the artificial CT around the humdinger and see what happens to the hum.

[bw1985]

15k to ground from either leg of the heater circuit is too much. Jumper in the artificial CT around the humdinger and see what happens to the hum.

the hum has improved with the artifical CT. what happens is it gets loud when turning standby off and then tapers off.
definitely an improvement but still present.

[eschertron]

the hum has improved with the artifical CT. what happens is it gets loud when turning standby off and then tapers off.
definitely an improvement but still present.

If the hum is worst as the power tubes begin conducting, it still sounds like you have poorly-balanced power tubes or something in the surrounding circuitry (stoppers, grid leaks, etc.).

[bw1985]

If the hum is worst as the power tubes begin conducting, it still sounds like you have poorly-balanced power tubes or something in the surrounding circuitry (stoppers, grid leaks, etc.).

I'm still seeing around +4Vac on pins 4&6 of the output tubes, could that be it?

Also I measured around -46Vdc on the center pin of the Vibrato input?

[nevetslab]

Wait a sec... my hum pot was measuring around 0-30k when I was sweeping it.

Could I eliminate it and wire an artificial CT instead?

0-30k? It's supposed to be a 100 ohm 2W pot. Better check that again. Best remove the two heater leads from it, so you can properly measure it.