grounding practices in 2 channel amp to eliminate hum. Ground loop?

[tony321owen]

I am building a 5f2 inspired amp with an extra 6sj7 preamp channel that joins the first channel before the tonestack and volume control. When i have both preamp tubes in, there is a nasty 60 cycle hum that disappears when I remove the 6sj7.

What are the rules for grounding two preamp channels to eliminate hum?

isolate one jack??

[pdf64]

HT ‘grounding’ issues would normally cause 120Hz hum.
Why do you think it’s a grounding issue.
Are you sure it’s 60Hz?
Have you eliminated the possibility that it’s something else, eg a bad tube?

[tony321owen]

maybe its 120hz actually. I switched the tube out with another. same problem. It could be another issue, I just thought I read something a while back about issues with grounding two channel amps and ground loops being created.. I was wondering if anyone had knowledge or experience with this.

[66Strat]

Some 6SJ7s have a tendency to hum. Maybe you have a bad one. Do you have anything attached to pin one? Pin one is the shield on metal 6SJ7 tubes. Pin one should be connected directly to ground.

[tony321owen]

I used pin 1 to mount the cathode resistor and t goes straight to ground. I have tried 2  different 6sj7 tubes. Both channels hum when the 6sj7 tube is added..

[sluckey]

schematic, voltage readings, hi-rez pics

[tony321owen]

will do.

[shooter]

grounding two channel amps and ground loops

fwiw
 can "see" a loop if I jumped the channels together, but I'm on my 3rd build that has 1 in and 2 "channels" jumped after and 2 of three were studio close quiet. the 3rd is under construction 
I ground EVERY thing left of PI (or driver) to the in, the rest go to HT ground and I let the chassis work out the kinks 

[tony321owen]

Heres my schematic with measured voltages. I also included a preamp ground bus layout the way it's wired right now and some chassis photos. Not my neatest work but theres an order to things... Let me know if you notice any blaring layout errors or even If you have constructive criticism of my schematic . Like I said above, Its dead quiet when I play through the first channel without the 6sj7 installed..

There are the remains of a failed tremolo that I lifted from the dearmond R5T but I disconnected from the circuit
to try and trouble shoot my hum issue. I think I will add a vibrochamp tremolo circuit after I get this hum issue worked out!

[tony321owen]

more pics

[tony321owen]

pics

[tony321owen]

Just realized that i drew my schematic wrong at the input. the grid leak resistors for each input stage are currently wired from tip to sleeve as they should be.

[sluckey]

Pin 3 is not connected to anything so connect pin 3 to pin 5. And connect 1M screen resistor to node D rather than node B. Any better?

[pdf64]

Also a good idea to bypass early stage cathodes, as unbypassed they’re liable to pick up hum from their heater.

[silverfox]

There are numerous poor solder joints. Reflow any of the connections that are not completely filled with solder.


silverfox.

[shooter]

I circled a few "tails", they don't appear to be an issue, but they will
wrap n solder, or snip and remove, blood from a nicked finger, or carbon from an arc are a  to clean up 

[tony321owen]

Thanks for the input fellas! I connected pin 3 to pin 5 as sluckey recommended and the noise disappeared!

Next I will fine tune voltages in the power rail, clean up solder joints, bias the power tube and build the tremolo circuit!

The only up side to a pandemic is that there’s nothing to do but build more amps!