Supro Sahara Reissue HUM

[samgraysound]

Seems like I'm always here asking about hum. Here we go again.

Supro Sahara tube Amp.  It's a Zinky design based on a 1958 Supro design (so I have read on random forums).  I don't have a schematic. I  contacted Supro and Zinky Electronics for one and waiting to hear back. Two 12ax7 and two EL84s.

It has a significant 60hz hum with a source plugged into it (in this case my oscillator). No hum without a source. This hum is entirely controlled by the volume pot.

I have:
Swapped all tubes
Checked for ripple on the B+. Negligible ripple, all filter caps are good.
Checked the ground reference for the filament supply. It is referenced through a CT. Continuity and voltage checks are good.
Checked the ground connection on the input jack.
Checked the ground connection from the AC power cord, and transformer chassis connection.
Checked all ground connections on the preamp PCB

The internal signal routing is all in shielded cable and I've checked the ground connections for all the shields.

Suggestions?

 

[samgraysound]

Hmmm. It hums with my oscillator plugged in, and from the output of my mixer, but not with a microphone plugged in.

Sam

 

[pucho812]

does it hum with a guitar?

 

[pucho812]

"does thew hum change with high tide or low tide" - CJ

 

[radardoug]

Gosh Sam, the clue is right there in front of you!

 

[Bo Deadly]

It has a significant 60hz hum with a source plugged into it (in this case my oscillator). No hum without a source. This hum is entirely controlled by the volume pot.

If you terminate the input with a 15k resistor (simulating a guitar source impedance) and the hum goes away, then it pretty much has to be upstream. It can't be tubes or heaters or anything like that. It's just the ground loop created by the external gear. We are talking about a guitar amp so the input is unbalanced and the amp is relatively high gain. So it's really designed for a floating source (meaning not connected to ground elsewhere) like a guitar. If you want to plug in an externally grounded source, you'll need a re-amping device which could just be a transformer like the sort of thing Radial would make or active would be perfectly fine too. Or just rig up a 10k:10k transformer (maybe with a split secondary so that you can also switch to 10k:2.5k). Or you could use one of those XLR to 1/4" transformer adapters use with old unbalanced Reel-to-Reels. Hosa makes one.

 

[CJ]

schematic>