External reverb unit causing hum

[Randall]

I just built a Weber kit 5G15 Fender stand alone reverb amp. It works, and sounds good, but it is causing some 60 Hz hum in the amp it is sending to. In this case, a 5E3 that I built, my personal amp. The 5E3 is quiet on it's on, but when plugged into the Reverb it has a low level hum. It does this even with no tubes in the reverb. I am relatively sure my work is sound. I will disclose that I had the wires in the power cable reversed (who uses brown and blue?), but I have corrected this. Not sure if it made much, if any difference. When probing the output jack of the reverb, I don't see any hum, just the usual noise floor.

This is my first experience with one of these. I wonder if I have a slight ground loop. I also wonder if plugging one amp into another is normal for some 60 Hz hum, I don't know?

I should also state that with the reverb off, and power cord unplugged, but signal cable between them, the 5E3 is silent. But when I touch only the ground pin of the power cable of the reverb to the outlet ground, I get a slight hum in the 5E3. It's not much, but it's there. Then it seems to be amplified a little when the reverb is switched on.

https://www.tedweber.com/5g15-c-kt/

[Greg_L]

I just built a Weber kit 5G15 Fender stand alone reverb amp. It works, and sounds good, but it is causing some 60 Hz hum in the amp it is sending to. In this case, a 5E3 that I built, my personal amp. The 5E3 is quiet on it's on, but when plugged into the Reverb it has a low level hum. It does this even with no tubes in the reverb. I am relatively sure my work is sound. I will disclose that I had the wires in the power cable reversed (who uses brown and blue?), but I have corrected this. Not sure if it made much, if any difference. When probing the output jack of the reverb, I don't see any hum, just the usual noise floor.

This is my first experience with one of these. I wonder if I have a slight ground loop. I also wonder if plugging one amp into another is normal for some 60 Hz hum, I don't know?

I should also state that with the reverb off, and power cord unplugged, but signal cable between them, the 5E3 is silent. But when I touch only the ground pin of the power cable of the reverb to the outlet ground, I get a slight hum in the 5E3. It's not much, but it's there. Then it seems to be amplified a little when the reverb is switched on.

https://www.tedweber.com/5g15-c-kt/

Ground loop.

https://music-electronics-forum.com/...d-alone-reverb

[Chuck H]

Ground loop.

https://music-electronics-forum.com/...d-alone-reverb

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[Mick Bailey]

When Fender reissued the 6G15 they elevated the circuit ground relative to chassis/mains earth with a back-to-back diode and resistor combination to minimize ground loop hum with 3-prong leads. I built my own reverb (though not copied from Fender) using this method and experimentally bridged the circuit and it hummed.

6G15 Reissue.pdf

[stokes]

I've built 5 reverb units for myself and a couple other people.I always lift the ground on the reverb plug, no hum at all.

[Mick Bailey]

Is that the signal or mains plug?

[pdf64]

It probably wouldn’t really be that much rework on that build to eliminate the use of the chassis as a conductor for 0V common. Finding insulated washers for the RCA and 1/4” sockets probably being biggest hassle

[Mick Bailey]

Regular 1/4" Switchcraft sockets can be used with a thin fibre washer each side. Open up the hole and use a single turn of insulation in the panel thickness. I have PTFE sleeving, but some stripped from hookup wire does the job. There are stepped bushes available this purpose, but I've never come across these in the UK. Similar procedure for the RCA sockets.

[stokes]

Is that the signal or mains plug?

The AC mains.You can get one of those adapters to test it.You dont want to isolate the jacks from the chassis, this is how the reverb unit grounds to the AC ground via the amp. The first unit I built years ago I used one of those adapters then I realized there is no reason you would want to use it with a grounded main and just lifted the green wire from the chassis.

[Helmholtz]

The AC mains.You can get one of those adapters to test it.You dont want to isolate the jacks from the chassis, this is how the reverb unit grounds to the AC ground via the amp. The first unit I built years ago I used one of those adapters then I realized there is no reason you would want to use it with a grounded main and just lifted the green wire from the chassis.

In that case your life may depend on wimply jack-plug contacts and the shield of the cable between - and if reverb unit and amp are interconnected at all.

Class I equipment must be individually safety grounded.

[Mick Bailey]

That's why I asked just to be clear. I wouldn't ever recommend doing this - it's against the regulations regarding electrical safety and would fail the equipment test (PAT) over here which tests amongst other things the earth bond current capability and resistance for Class 1 equipment with exposed metal parts.

[Enzo]

There is troubleshooting and repair and there is daily operation. It will be OK to use a ground lift to verify the ground loop.

[Tom Phillips]

I fully agree that lifting the power cord ground with a cheater plug is not advised. Furthermore, I don't trust the modules seen seen for sale on the internet that are fitted between the three wire power plug and the wall socket.

My fix is to rewire the 5G15 circuit to a true ground lift configuration and install a "Hum stop" circuit that connects the lifted ground buss to the chassis at a single point. This configuration allows the line cord earth ground to be soldered directly to the chassis. The result is a perfect fix of the ground loop hum issue with no safety compromise.

The hum stop circuit I use is a network of a parallel 10Ω - 5W resistor, 0.047µF Cap & back to back 6A diodes.

This approach is a non-trivial but very satisfying bit of work.

It's a shame that kits are offered that mostly just copy the classic build configurations when it is possible to keep all that is great about the circuit while still upgrading for safety, lower hum etc. (Sorry for the added rant)

Cheers,
Tom

[Greg_L]

I fully agree that lifting the power cord ground with a cheater plug is not advised. Furthermore, I don't trust the modules seen seen for sale on the internet that are fitted between the three wire power plug and the wall socket.

My fix is to rewire the 5G15 circuit to a true ground lift configuration and install a "Hum stop" circuit that connects the lifted ground buss to the chassis at a single point. This configuration allows the line cord earth ground to be soldered directly to the chassis. The result is a perfect fix of the ground loop hum issue with no safety compromise.

The hum stop circuit I use is a network of a parallel 10Ω - 5W resistor, 0.047µF Cap & back to back 6A diodes.

This approach is a non-trivial but very satisfying bit of work.



Cheers,
Tom

This is what I did on mine and it works perfectly hum-free. Beautiful drippy drip spring reverb with no noise.

[Randall]

A cheater ground lift AC plug does indeed stop the hum. The reverb rca plugs are isolated from chassis, BTW.

Saying to elevate ground from chassis to resolve hum, is this ALL grounds?

I'd like to see the hum stop circuit that is mentioned.