Princeton reverb clone hum issue

[Chuck H]

Check your reverb cables for a good ground? V3 is part preamp part reverb recovery.
nosaj

Randall grounded the signal on the pot side of the 470k reverb mix series resistor and reported no change in the hum.

[Randall]

Reverb cables are not connected.

I am thinking maybe it is from the small stray voltages I have on the black fiber board?

[Chuck H]

Reverb cables are not connected.

I am thinking maybe it is from the small stray voltages I have on the black fiber board?

Well... The test for that would be to float the suspect connections off the board and see what happens.

[Randall]

Where to start with that?

[Helmholtz]

It is very unlikely that hum is induced via a conductive board as there are no AC connections.
And ripple voltages are small and would have pass a relatively high board resistance of maybe 100M.
If the board had lower resistance, DC voltages would be off.

[Randall]

I have seen other problems with a conductive board, but hum was not one of them. I am just grasping, because I am not understanding what is going on here.

Several opinions that the new can is at fault, is that correct? I don't see the evidence.

I am still stymied about why grounding pin 7 on V3 silences the hum. Wouldn't that suggest the hum is coming from before that grid? If it were power supply noise on the plate of V3, why would it stop when grounding the grid?

[nosaj]

I am still stymied about why grounding pin 7 on V3 silences the hum. Wouldn't that suggest the hum is coming from before that grid?

Yes.
What happens if you ground pin2?
nosaj

[Randall]

What happens if you ground pin2?

Nothing.

[Helmholtz]

...., as does either side of 3.3M/10pF.

What happens if you lift one end of the 3.3M?

[Randall]

"What happens if you lift one end of the 3.3M?"

The hum stops. That explains why grounding pin 7 also stops it. Also, if I put 3.3M back in and lift the .02 before it, that also stops the hum. But where could it be coming from?

edit: I have disconnected the 100K from V1A, and the 500pF reverb filter cap. I can see a small half wave bump on both sides of V1B 100K as well as both sides of V1B .02 coupling cap. I see that same half wave bump on the unmarked terminal of the can. So I guess there is my evidence that this new can is bad, eh?

[Randall]

Wait, here is something I hadn't noticed, there is a 5AR4/GZ34 in there. The schematic I am looking at calls for a 5U4GB. Would this GZ34 be toasting the filter caps?

I threw a vintage big bottle 5U4G in there, but I still have the hum.

edit: I just spoke to CED, and they are going to send me a replacement. Not sure how thrilled I am with that, but it is what they offered.