Stromberg-Carlson AU34 has 60Hz hum after modification...

[jim_naisium]

I have two Stromberg-Carlson AU-34 tube amplifiers they both work fine as-is and don't have any problems putting out nice clean sound.

Tubes:
1st pre-amp = 6SF5
2nd pre-amp = 6N7
Output = Two 6L6GC's (push-pull)
Rectifier = 5U4

Several months ago I decided to modify one of them and built a wooden cabinet on wheels to make a portable amp, with two 12 inch speakers and move the controls out of the chassis and onto a strip of stainless steel that is going to go across the top-front of the cabinet.

Everything else is done and it was time to modify the amp and move the Volume and Tone controls out of it and onto the ends of two shielded 3 ft. long RG-58 cables, that all went fine (and was easy) then I powered on the amp as long as I leave the volume turned all the way down it makes no sound but as soon as I turn it up loud enough to hear anything a 60Hz hum is coming out of the speakers and gets louder the louder I turn it up.

If I pull the first pre-amp tube out of the socket the hum goes away, just to rule out it was some kind of weird problem with this amp I did the same modification to the other amp and as soon as I turned it on and the tubes warmed up it too now has a 60Hz hum coming out of the speakers.

Does anyone have any suggestions on how to make this hum go away, I have a lot of $$$ invested in this and would like it to work and not just give up on it?

[M Fowler]

move the Volume and Tone controls out of it and onto the ends of two shielded 3 ft. long RG-58 cables

Did you ground the new control panel back to the amp chassis?

Mark

[jim_naisium]

move the Volume and Tone controls out of it and onto the ends of two shielded 3 ft. long RG-58 cables

Did you ground the new control panel back to the amp chassis?

Mark

Both controls are getting their ground through the cable shielding, they have not been mounted in the new control panel yet, although that is something that I could try since it has the holes drilled in it already.

One test that I did do was test the continuity from where the shield brading was attached to the chassis to the chassis ground connection to the power cord it is getting a good ground.

The sheet metal for the control panel WILL be getting its own ground wire, I have been bitten by old tube amps before that were leaking high voltage out of the input circuitry that ended up in my metal guitar strings.

[M Fowler]

I was wondering if the shielding was being used as ground connection on both ends or used as shielding with only one end grounded. You answered that.

The other thing is the bias for the V1 6SF5 if that is going to the input jack grounding to keep the low level hum down.

Mark

[jim_naisium]

Figured it out finally, I talked to my dad this evening, he has been fixing tube equipment since the 1960s and said that it sounds like I have a grounding problem.

Before when I had been working on this I either had on my boots or shoes and was pretty much isolated from ground, this time I was in a thin pair of socks on a wood floor...

I turned the amp on as soon as it started humming when I picked up the volume cable and touched one of the shield braids part of the hum went away, and then when I touched the back of the Volume pot to one of the ground screws sticking through the chassis the hum went completely away.

So it sounds like I just need to mount the sheet metal and ground it and then mount the controls in it and everything should be fine.

[M Fowler]

We'll see