Jcm900 4500 hum

[lowell]

Hey folks, this came in with sound cutting in/out. All the pots were super dirty. Gave it a good cleaning amp sounds great all the pots work. Went to reassemble it into the head cabinet, with 2 chassis bolts, to test the reverb since it's mounted in the cabinet. There is a hum, which I believe is 120 cycle. I was moving reverb cables around and also tapping on the chassis, the hum would come and go. I thought okay maybe bad reverb cable something loose in the amp. Took it out of the cabinet to poke around with my sharpie. Nothing responded to pushing around or tapping... nothing. This amp has a star ground that is lifted off of Earth by a 22ohm resistor with 2 diodes and a cap in parallel. The star ground measures 22ohms to chassis with my meter.

Here are some tests I tried, and while they haven't pointed me in the right direction maybe they'll point you guys toward a clue...

The hum is there with ALL knobs at 0 except for the Master Volume, yes even the reverb at 0 (on BOTH channels)
If I touch the red reverb return plug shield/ground to the chassis, the hum goes away (this doesn't happen to star ground)
If I turn the Presence up to 12 the hum goes away
If I pull V2 the hum goes away
If I short V2A grid to the chassis the hum goes away, (doesn't happen when use star ground)
The hum goes away if I connect star ground to Earth

I've tried new preamp tubes, didn't help
The low voltage bipolar supplies are good

Whatcha think? I'm stumped... I even put the chassis back into the cabinet thinking maybe the cabinet shielding plate played a part. I couldn't make the hum go away anymore after tapping around on the chassis with it mounted tightly in the cab.

Here's the schematic - https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

Aside from the hum, does the reverb work?

[Helmholtz]

If I turn the Presence up to 12 the hum goes away

That's typically a sign of power amp oscillation which also causes increased 120Hz ripple.
Do you have a scope?

[lowell]

Reverb works yes.

Scope is elsewhere at the moment. What would I check with the scope?

[lowell]

Any reason I shouldn't just short star ground to earth/chassis ground? Problem solved?

[Helmholtz]

What would I check with the scope?

Especially see if you have ultrasonic HF oscillation.

[lowell]

Especially see if you have ultrasonic HF oscillation.

Where would I connect the probes to for this? Out of curiosity.

Also, I feel it's extra odd that cranking the presence kills the hum.

[Helmholtz]

Where would I connect the probes to for this?

If you can't see it at the output, try the PI plates. Scoping oscillation can be tricky as it sometimes stops when probing a circuit point.

Also, I feel it's extra odd that cranking the presence kills the hum.

Exactly that makes me think of power amp HF oscillation. Turning presence up reduces HF feedback.

[lowell]

If you can't see it at the output, try the PI plates. Scoping oscillation can be tricky as it sometimes stops when probing a circuit point.

Thank you sir... How would HF oscillation cause 120 cycle hum?

[Helmholtz]

Thank you sir... How would HF oscillation cause 120 cycle hum?

Power amp oscillation can produce kV amplitudes at the plates associated with increased power supply current. This in turn increases audible 120Hz ripple.

[pdf64]

Maybe remove the HT fuse FS2 and fit a current meter across the fuseholder terminals. Then compare the power tube current levels, with the controls adjusted for 'hum', then again with them readjusted for 'no hum'. Ultrasonic oscillation would typically cause an increase of power tube current draw.

[Enzo]

You can just monitor mains draw in the same way.

[lowell]

You can just monitor mains draw in the same way.

Thanks that's what I did, too lazy to remove chassis again.

I had 500ma with amp ON. No change in mains current at all with hum vs no hum.

I was just reading about the ground circuit on Rob Elliot's site. This is supposed to REDUCE hum is it not?? So why in the heck does the hum disappear when I connect star ground to chassis/earth? I also read that the diodes will cause hum if there is a ground loop. Could it be, that if ONE diode is short circuit or open, that may be the cause of the hum? I'm baffled because I just don't understand the concept of the 2 diodes, 22ohm, and small cap.

Finally, can I just connect the dang star ground to earth and call it a day if I can't figure this out?

[Helmholtz]

Is C16 good?

[lowell]

Is C16 good?

I'll pull it and check