Need some help fixing a faint hum

[MikeSattler]

Finished up my mainline kit the other day and it sounds excellent except for a very faint hum. It sounds more similar to 120hz than 60 hz and it's not affected by the volume or any of the front panel switches. So far I've tried a few things to no avail. Moved it to a different room, tightened up the transformer, reflowed the solder for as many of the ground points as I could find, and cleaned the tube pins. Voltage and resistance checks are spot on. What should I try next?

[Doc B.]

Is it the same level in both channels?

[MikeSattler]

Yep, right in the center of the image.

Also forgot to mention - I replaced the buss wire with the ground breaker diode mod and tried a cheater plug to lift the ground connection.

[Paul Birkeland]

Can you tell us more about the source you have hooked up to it?

[MikeSattler]

Seems unaffected by having anything plugged into the RCAs. When I moved it to another room I just plugged the amp straight into the wall with nothing else hooked up to it and it’s still there.

[Paul Birkeland]

Perhaps it wouldn't hurt to post some build photos.

[MikeSattler]

Okay thank you for looking! Here's a Google photos album with the build: https://photos.app.goo.gl/oEUx4thV1rVWwXZh6

The underside of the chassis got a bit of overspray from the powdercoating. However, I sanded off the powdercoat in all the spots where it looked like a chassis ground connection was made. Other than the obvious ground lugs and lugs 3 and 7 on the attenuator is there anything else to make sure has a proper ground?

[Paul Birkeland]

What's the DC resistance between the "E" terminal on the IEC power entry module and the bare metal of the plate holding the attenuator switches up front?

[MikeSattler]

Getting 37.4 kohm

[Paul Birkeland]

The paint on the bottom side of the plate is causing issues.   I've circled some points that need to be scraped on the bottom side of the plate to establish proper earthing.  I would scrape a few of the holes around the standoffs for the front attenuator plate just to be extra sure that grabs earth nicely. 

[MikeSattler]

Thanks for that, though I think that shouldn't be an issue anymore. I sanded off those spots as well as the ground lug at the headphone jack and the lug by the center tube socket, so they should all have a good connection to the plate.

[Paul Birkeland]

The E terminal of the IEC power entry module and the plate that the attenuator switches mount to should show continuity.  Since they don't, you'll have to track down where this interruption has happened and correct it. 

[MikeSattler]

Maybe my weird reading from terminal E has something to do with the diodes I installed? The first time I measured from there I got 37 kohm. Now i'm getting 7.6 mohm. If I measure ahead of the diodes, at the primary ground lug, I get 0 ohms.

[MikeSattler]

Well, I removed the diodes so I'm getting 0 ohms from terminal E but the hum still persists.

[Paul Birkeland]

Do the rest of the locations circled show continuity to E?  I had forgotten about the diodes, that will throw off that reading. 

My suspicion is that there's an earth reference that's just missing and resolving that may fix your hum.