Hummmmm from one side of the PI?

[axpro]

Hey guys, still working on my heavily modified 5e3x2 rebuild. done some boneheaded moves lately, but thats all part of getting back into the hobby

Heres the LATEST issue though. the circuit is essentially a "normal" channel from a blackface DR, with the PI from the DR, grafted onto the cathode biased 4x6v6 output tubes.

After some bug hunting, i had it working, but there was a lot of hum when it was turned down... dissapearing as i turned it up... as i got closer to full volume, it sounded like a normal Deluxe as far as hum.

could it be hum only on the inverse side of the PI that zeros out when i get teh sides balanced?

[Steve Conner]

This usually means that hum is getting in at two places in the circuit, one before the volume control and one after. Sometimes you can find a volume setting where the two hum sources cancel each other, and that's what you've found.

The usual troubleshooting guidelines apply, except you're hunting two sources of hum, not one.

The balance of the PI doesn't depend on the volume control.

[MWJB]

This sounds typical of having the preamp filter cap grounded at the same point as the power amp caps, have you tried regrounding the preamp filter to the same place as V1 pin 3 cathode, or the inpt jack ground?

[axpro]

Hmmmm.

I'll keep looking i guess.

Pulled the board and cleaned up the wiring (I am a lil sloppy when prototyping) and ensured cleaned lead dress overall. I was having an intermittent connection problem somewhere (amp sometimes wouldn't have any output until i did the "fonzie" test) so I desoldered and resoldered all my connections.

End result, i still have the noise issue. but no other issues.

When the volume on the amp is turned to 0, lots of hum, around the halfway mark on the pot, it cleans up, and as i turn it up from there, noisy again.
I switched from "vitamin T" paper+oil caps in parts of the amp, to Mallory 150's. Still have the T's in one spot, i think i'll pull them and see what happens. I had gotten them for a test... could be they are why i never liked the 5e3 circuit! (though i doubt it)

I tried adding another stage of Power supply filtering, and it didn't change. which makes me think that the problem is still somewhere in the preamp.

I am way out of shape on amp troubleshooting. but i guess it's like riding a bike, leave it for a number of years and realize it is a lot more work than you remember!