What are these components between main ground and chassis for?

[Cirrus]

On my AC30, the ground from the HT centre tap and ground side of the main filter caps goes to the chassis via 4 parallel components - two diodes, a 10r resistor and a 220nf cap. What's the purpose of that?

I'm just trying to get a better understanding of why moving the ground of the HT centre tap so I can use a chassis mounted Zener diode to drop some HT has resulted in a ground loop. I think I basically get it, but can't quite understand what these 4 parts do.

[tubeswell]

Hum-loop blocking.

[pdf64]

They help to stop ground loop hum via the mains safety earth, if, eg you linked the inputs of 2 amps.

[HotBluePlates]

... I'm just trying to get a better understanding of why ... a chassis mounted Zener diode to drop some HT has resulted in a ground loop. ...

Since the 4 parts stop ground loop hum, when you added the chassis-mounted zener you defeated the purpose of those parts & caused hum.

To use your zener without creating hum requires the "chassis-side" of the zener go to the point marked "GND_MAIN" in your diagram.  That is Power Transformer Center-tap ---> Zener ---> GND_MAIN (not chassis)

Except you can't really do that because of the design of most chassis-mounted zener & the fact they're using the chassis as a heat-sink to achieve their power rating.  Which means if you're trying to knock down the supply voltage you probably need to use a different method.


So what about the function of each of the 4 parts?

Problem:  The amp's chassis has a Safety Ground that connects to the 3rd wire of the power cord.  That connection will go to the outlet, through the 3rd wire of other equipment, to that other equipment's chassis, and the ground of that other equipment, creating a giant ground loop that picks up hum.

Safety Ground:  The chassis connection goes to the building's circuit breaker via a 3rd wire.  This is bonded to Neutral (0v) at the service panel, so if a voltage appears on the chassis (which the player could contact & be shocked), then a high current is pulled through the 3rd wire to trip the breaker, removing voltage from the amp.  People will break off the ground pin of the power plug, but they're accepting risk for being shocked (and maybe don't realize that).

Resistor:  Vox went out of their way to keep the entire amp's Ground System on the circuit board & off the chassis.  The Ground System is then connected to the chassis at only this one point, and the resistor reduces any ground currents that may happen to negligible levels.

Diodes:  We want to avoid blowing up the resistor above if a fault situation occurs, so the amp doesn't need to be repaired as a result of incorporating a safety device.  If a voltage appears on either the (amp's) Ground System or the chassis that's big enough to burn out the resistor, the diodes take over & pass that current.

Capacitor:  The amp chassis is now safe, and the resistor/diodes have separated the amp's Ground System from Safety/Chassis Ground, but we can still encounter odd noises due to radio frequency interference.  The Capacitor from Ground System to chassis kills that RF noise without undoing the things we did to kill ground-loop hum.

Check out sections 15.9 and 15.10 of Valve Wizard's info on Grounding.

[shooter]

avoid blowing up the resistor above if a fault situation occurs, so the amp doesn't need to be repaired as a result of incorporating a safety device.

hey, I got paid really good to fix those kind of problems 
where fault circuits were, 40% of fails were fault circuits that failed without real circuit faults 

[Cirrus]

Except you can't really do that because of the design of most chassis-mounted zener & the fact they're using the chassis as a heat-sink to achieve their power rating.  Which means if you're trying to knock down the supply voltage you probably need to use a different method.

Cool, thanks for the explanation of each part's function.

As for the above - yeah, I was figuring that'd be the case.

When I was trailing it, I put 3x 6.8v 5w diodes in series on a little vero board and interrupted the wire between the transformer and MAIN GROUND on the board, and that worked perfectly other than the diodes getting pretty hot.

I've got a couple of choices now, which I think will work.

1; Instead of mounting to chassis, mount to metal bracket and electrically isolate that from chassis with Nylon bolts/ standoffs. It'll be a 25w 24v diode dissipating 6 watts at most into just a metal bracket, though I could add some extra metal for more heat sink. I've got some thermal paste to make it more effective.

2; String 5x 4.8v 5 watt diodes together on vero board, but with more space between them. Each diode will be dissipating just over 1 watt. Could again add some extra vero board to increase surface area radiating heat.

option one feels better because there's less components to fail, and a better practical setup for attaching things securely.

[sluckey]

Stud mount rectifiers and zeners used to come packaged with mounting hardware that included a nylon bushing, two mica washers, metal washer, and nut. Everything you need to properly mount the stud to the chassis or heatsink and maintain electrical isolation. Just add heatsink grease. You can buy the mounting kits separately.

[Cirrus]

Stud mount rectifiers and zeners used to come packaged with mounting hardware that included a nylon bushing, two mica washers, metal washer, and nut. Everything you need to properly mount the stud to the chassis or heatsink and maintain electrical isolation. Just add heatsink grease. You can buy the mounting kits separately.

That would have been nice!

[HotBluePlates]

... You can buy the mounting kits separately.

I should have thought of those insulating/heatsink kits!

That would have been nice!

As Sluckey says, you can buy the needed stuff separate from the zeners.