Ground Loop(s)

[trem]

Maybe I have asked this before, I searched my history...could not find it.
Building an amp that is in a very narrow chassis...stuff is really crammed in there...real estate is hard to come by.

So.....it seems B+ for Tremolo Circuits are frequently pulled from the power tube screens...and that is how this amp is set up as well.
How important (ground loop wise) is it for the Trem Foot-Switch to be grounded at or near the Power Tube Ground location.?

Space is real tight...it would be easier to install the Foot-Switch/Cathode Ground of the Trem tube more in the area of the Preamp...and not in the Power Amp where it shares its current with the B+. of the power tube screens.

I would have the Filter Cap for the Power Tube Screens grounded near the PT (along with the phase inverter, power tube cathodes and power tube B+), and then the Foot-Switch/Cathode ground for the trem circuit much closer to the Preamp and Input Jack grounds
When do you need to worry about this situation.?
Thank You

[soundguruman]

The further apart two redundant grounds are, the more ground loop occurs.
This is why RCA jacks are set up on plastic insulators in newer amps, or the grounds
are closer together.
If the distance is short, it usually won't be an issue.
Now if we combine front panel ground with rear panel ground (like an effects loop),
we often start to get a hum from a ground loop.
But depending on distance, chassis resistance, the result varies.
And in a lower gain amp, like a Fender twin, the hum may not be noticeable.
In a higher gain amp, like a boogie, Marshall, etc...it becomes louder and more noticeable.

You might build it and find out what you end up with...

[Chuck H]

It may not matter at all since the trem isn't in the signal path.

I have a build where the trem and the reverb share a stereo footswitch jack. The jack isn't isolated so the trem does indeed have two grounds to the same node of the oscillator when the footswitch is engaged (trem off). Same with the reverb recovery output. I decided to chance it since many amps are built this way and after testing I found no reason to change it. No hum problem. It's harder to hunt down a loop hum than it is to plan for one you know might happen. I say just chance it and be prepared to change to an isolated footswitch jack if there's a problem.

[trem]

Appreciate the answers. So let me ask the next logical question.
My FS Jack is isolated.

What is better, or maybe worse:
1. Ground the FS Jack where it is installed, in the preamp area.
2. Install FS Jack as above, but run a ground wire back to the screen grid ground.

Would you guys worry about running the FS Ground back to the power amp area, if the jack is located in the preamp section.?

Thank You

[Chuck H]

If the jack is already isolated locate the ground for the FS exactly where the 0V reference for the signal it's grounding is. This assumes the FS is grounding a signal and not operating a relay or SS switching device.

[trem]

Hey Chuck -
Swicthcraft jack, but it has those shoulder washers so it does not touch the chassis.
The jack is just for a Mono Foot Switch that will ground (or lift ground) of the Cathode of the Trem Tube.
So I was thinking to locate the jack where it is convenient (somewhere close to the preamp section) and then run a ground wire for the Foot Switch Jack over to the Power Amp section where the filter cap for the screen grids is located.
It is THAT wire that I am wondering about. Will there be "enough" current (for the Trem Cathode) running in that wire to create a problem.?..... if i have to run it for a ways...kind of from one end of the chassis to the other.
Thank You

[Chuck H]

What you propose sounds good to me. Don't sweat the long lead. On it's own going to the proper ground that lead doesn't pose a threat to anything.

[trem]

OK Chuck.....Thanks for clearing that up.
I agree with your "Try It And See" comment from earlier...and normally I would.
But like I said...elbow room is so tight in this chassis, it can be a real challenge to install something, and then change it if it does not work out too well.
I can now proceed with an easy mind
Thank You So Much