External reverb unit causing hum

[Enzo]

Instead of running various circuit grounds to chassis, you have a COMMON wire on the board and all "grounds" go to that. Then there is one point where that common is connected to chassis through the reverse diode pair with a cap and resistor, like on this Marshall amp, lower left corner.

[Mick Bailey]

Take a look at the service manual in post #4 to see how Fender does it.

[dai h.]

hi, here's (what looks like) a good article on the circuit:

https://sound-au.com/earthing.htm#s9

also, on merlin's site (go down to p.275 to the section titled "15.10: Ground Lift")

http://www.valvewizard.co.uk/Grounding.pdf

also (from my novice undertanding) might be good to use 10nF-ish ceramics with very short lead connections on (assumed to be insulated in this scenario)
inputs and outputs (maybe footswitch lead as well) from shield to chassis to shunt(divert) any possible incoming RF that might be picked by the cable
shields (acting as an antenna). (This needs to be done RIGHT AT inputs and outputs, ("at the border" in a manner of speaking), and not after the shield/ground
lead has already made its way inside the chassis (where it can spray RF around). This might be a bit tricky to do, and Cliff makes a "FCR14422 EMI／RFI SCREEN FOR 6.35mm JACK SOCKETS" to make it easier (I don't know about Switchcraft or other makers--I guess you'd need a toothed washer plus tab to connect the cap to shield)).

(My non-expert understanding: ) this is to 1) decrease chance of any possible obvious nuisance noises, 2) keep RF from intermingling
with the audio and degrading it (even frequencies you can't hear can create frequencies you can and affect the audible range)
, and 3) thus help to increase consistency of sound if the RF environment changes (strength of source, weather, gear
location, etc.).

[Greg_L]

Instead of running various circuit grounds to chassis, you have a COMMON wire on the board and all "grounds" go to that. Then there is one point where that common is connected to chassis through the reverse diode pair with a cap and resistor, like on this Marshall amp, lower left corner.

Exactly.

[Helmholtz]

hi, here's (what looks like) a good article on the circuit:

https://sound-au.com/earthing.htm#s9

Excellent article, thanks for sharing!

[Randall]

If I had known, I'd have approached this from the beginning, but I am not going to re-engineer it after the fact. I was hired to build this Weber kit, not improve upon it. So I guess like many originals, it gonna hum. It is what it is.

[Helmholtz]

Ground loop hum is proportional to the area enclosed by the ground loop. Zero area = zero hum voltage.

To minimize the ground loop hum, the interconnect cable between the reverb and the amp should be as short as possible, the two mains cables should be tied together and plugged into adjacent outlets.
This will minimize the ground loop area and thus hum.