60Hz Hum Sanity Check

[Shottky]

I need a sanity check on the possible causes of 60Hz hum. I hooked a scope up to the output of the phase-inverter, and I can see (misshapen) 60Hz hum. It goes up as I crank up the gain.

I am using a 2-channel amp. The TMB side is quiet as a mouse. The normal channel (2-stage) has a bunch of hum.

Correct me if I am wrong, but these are the main (only?) sources of 60Hz (not 120Hz) hum:

1) Heaters

2) Ambient Sources

3) Ground loop

Can a socket be defective and introduce hum into a channel via the heaters?

[GibsonGM]

Have you tried several different tubes in V1? A tube which has had its cathode over-voltaged can hum (ok, I'm reaching, but it CAN happen).

I've had that experience, normal channel only - now it's just 'hissier' than the TMB. Could be a damaged tube, ground loop, poor solder joint, wire dress....? Are your jacks isolated from the chassis? I still get more hiss on my normal channel than TMB, so I hear your frustration!

[zaphod_phil]

In my own experience a ground loop is the #1 cause. Radiated hum can also be picked up from heater supply 6.3V AC wiring getting picked up in the amplification chain, but that's usually quite easy to fix.

[Shottky]

Have you tried several different tubes in V1? A tube
which has had its cathode over-voltaged can hum (ok, I'm reaching, but
it CAN happen).

I've had that experience, normal channel only - now it's just
'hissier' than the TMB. Could be a damaged tube, ground loop, poor
solder joint, wire dress....? Are your jacks isolated from the
chassis? I still get more hiss on my normal channel than TMB, so I
hear your frustration!

Yeah, I've tried the tube that's quiet in the TMB in the Normal. No
change. The Normal tube is quiet in the TMB.

Jacks are isolated from the chassis. Removing the jacks and soldering
the 1st grid directly to ground has no effect.

In my own experience a ground loop is the #1
cause. Radiated hum can also be picked up from heater supply 6.3V AC
wiring getting picked up in the amplification chain, but that's
usually quite easy to fix.

I think you're right that it's a ground loop.

I made my own PCB with a large ground plane on the back. Both channels
are grounded very close to each other in the plane. The power section
is very far away, on the other side of the board, near the PT and OT
ground. It'll be a bit tricky to find a ground loop in a ground plane
(especially since I can't modify that) so I'll have to assume that the
ground loop is somewhere else in the gain stage.

[Shottky]

After tearing out the whole channel and pegging it onto a breadboard, I came to the conclusion that it isn't the circuit on the PCB that is noisy - it's actually the socket!

I switched the noisy channel and the clear channel's tube connections with one another - the noise followed the socket, not the tube or the circuit. I have yet to replace the socket, but I'm guessing this will fix it.

I have never seen or heard of this before. Strange.

[zaphod_phil]

Now that's pretty weird