Uni-Vibe - Grounding mains cable to chassis?

[clarisso11]

Hi everyone,

I've been working on a Uni-Vibe clone, and noticed that both the original pedal and other people's clones use a 2-prong AC mains cable, with no grounding to the chassis.

It was my understanding that grounding the mains cable is an important safety consideration, often not done originally on circuits from the 1960s, but a safety standard today.

Am I missing something, and this is not actually an issue in this circuit? And if it is still needed, for safety, would it be as simple as connecting earth to the chassis, or would this introduce hum?

I have attached the original Uni-Vibe schematic and layout, the photos are from recent DIY clones I found online, showing the 2-prong mains cable without grounding.

[x-pro]

If you really want it, install a "Ground-lift" switch, which is applied as in some amplifiers.
In this case, you will have both grounding and the ability to get rid of interference, if any.

[clarisso11]

That's an interesting idea, thanks!

On the chassis I'm using, the inside is covered with non-conductive paint. If all live wire connections are covered by heat-shrink, then maybe that is sufficiently safe as well?

[x-pro]

This solution is an insurance policy, at a certain switch position. In the diagram above, the input connector is connected to the unit's case. If one of the users connects the console to a failed amplifier, you can go to the hospital or straight to heaven.

[clarisso11]

This solution is an insurance policy, at a certain switch position. In the diagram above, the input connector is connected to the unit's case. If one of the users connects the console to a failed amplifier, you can go to the hospital or straight to heaven.

I see, so all the examples I showed in the photos of my post are potentially very dangerous, with the 2-core mains cable?

I'm now thinking I might try a simple grounding first (3-core mains cable, connect earth to the chassis using a locking nut), and if that introduces extra hum, try a more complicated solution.

[x-pro]

It is not the dual core power cable that is the problem. The problem could come on the braided cable connecting the unit to the amplifier. There are a lot of new amplifiers now with switching power supplies. If the mains suddenly has no grounding, these units are dangerous. And there are a lot of old tube ones that have not been properly serviced.

[x-pro]

I've been working on a Uni-Vibe clone,

The device is well made!

[clarisso11]

It is not the dual core power cable that is the problem.

Not sure I completely understand; is the original wiring with 2-core mains cable a safe option? Or should I change it to something safer?

[x-pro]

A two-core network cable is normal.
I can see from your pictures that the first one is your unit and the other two are factory.
Look at how they are connected to the transformer. In a pigtail pattern to reduce interference. The other wires are set back some distance from the transformer.

[The Dude]

IMO, anything that has AC mains connected and you are in constant contact with chassis ground should have a 3 wire AC wire with the ground connected to chassis ground.

[SoulFetish]

Never ever, ever, put a ground lift that disconnects the the chassis to mains earth connection. It’s there as a life saving measure in the event of a catastrophic electrical fault which could cause the chassis to to go ”live” or hot.
the green earth wire is only there to carry fault current, so no ground current should go through that conductor to cause loops.
Obviously it does happen sometimes, so if you need to break a ground loop, there are better and safe ways to do it.
But the dude’s right. If you have mains power coming directly into any device you’re also plugged directly into. It needs a 3-wire power cord, with chassis grounded securely to the Earth wire

[Helmholtz]

the green earth wire is only there to carry fault current, so no ground current should go through that conductor to cause loops.

Interconnecting two pieces of safety grounded equipment always creates a closed ground loop.
This consists of the two earth wires (being connected by the house wiring at their far ends) and the shield of the interconnecting cable.
Nearby magnetic hum fields induce a hum current in the loop.
This current causes a voltage drop (noise voltage) between the ends of the cable shield, which adds to the signal.

The sensitivity of the ground loop depends on the enclosed loop area between the 3 conductors.

[SoulFetish]

Interconnecting two pieces of safety grounded equipment always creates a closed ground loop.
This consists of the two earth wires (being connected by the house wiring at their far ends) and the shield of the interconnecting cable.
Nearby magnetic hum fields induce a hum current in the loop.
This current causes a voltage drop (noise voltage) between the ends of the cable shield, which adds to the signal.

The sensitivity of the ground loop depends on the enclosed loop area between the 3 conductors.

that’s is why I'm so glad you're on this forum.
I had a fundamental misunderstanding of the source of the noise, and cause of the current here.
i wish someone had explained it like that a long time ago. So, it basically becomes a kind of passive l pickup? Oh, dude, that makes a lot of sense now…
the crazy thing is I think I just figured out what is going on in an with an old Gibson amp which is presenting really bizarre noise problems with Reverb connected.
But that’s another story.
Still, there are ways to mitigate the loop noise without disconnecting chassis from earth, unless you wanted to an external and run the unit off that DC.

[Helmholtz]

So, it basically becomes a kind of passive l pickup?

Yes, that's an excellent description.
It works like a (low impedance) single turn PU regarding magnetic AC fields.

[clarisso11]

Never ever, ever, put a ground lift that disconnects the the chassis to mains earth connection. It’s there as a life saving measure in the event of a catastrophic electrical fault which could cause the chassis to to go ”live” or hot.
the green earth wire is only there to carry fault current, so no ground current should go through that conductor to cause loops.
Obviously it does happen sometimes, so if you need to break a ground loop, there are better and safe ways to do it.
But the dude’s right. If you have mains power coming directly into any device you’re also plugged directly into. It needs a 3-wire power cord, with chassis grounded securely to the Earth wire

Thanks, that confirms my worries! Surprising to see all those clones keeping the 2-wire power supply. Maybe to not have to deal with the loop noise in the first place? They do seem to put a lot of care in insulating the mains wire connections, not sure if that would qualify as a double-insulated appliance though.