hey team,
Just fixing up a Vox AC30CC2 and for the life of me I can't figure out what the purpose of the ground scheme is. All the grounds come to a common point via the two diodes, 10r resistor and 220nF cap and then on to the chassis.
Feel like I'm being stupid and missing something obvious or that its a common thing which I haven't seen, anything greater that the forward voltage of the diodes will go straight through to chassis why the resitor and cap?
If anyone can enlighten me I'd be grateful!
Just fixing up a Vox AC30CC2 and for the life of me I can't figure out what the purpose of the ground scheme is. All the grounds come to a common point via the two diodes, 10r resistor and 220nF cap and then on to the chassis.
Feel like I'm being stupid and missing something obvious or that its a common thing which I haven't seen, anything greater that the forward voltage of the diodes will go straight through to chassis why the resitor and cap?
If anyone can enlighten me I'd be grateful!
Thanks @baudouin0 , so does it evolve in that order then - put the resistor in first to help hum so then you need the cap around it to reduce interference and then the diodes for safety?
Just wondering about the diodes - If HT was on the chassis wouldn't it just go to ground via the mains earth?
thanks again!
Just wondering about the diodes - If HT was on the chassis wouldn't it just go to ground via the mains earth?
thanks again!
HT is isolated from mains earth, so if shorted to chassis, its return would be through this "ground breaker" circuit. The diodes don't conduct below 0.6V, so a difference in grounds of less than 0.6V would be broken by the 10 Ohm resistor, steering the ground current into the individual grounding paths. The capacitor connects circuit ground and earth for radio frequencies.
I am a little bit surprised by that 1/2 W resistor in that VOX circuit.
Referring to the ValveWizard's page on Grounding he says ...
... The resistor should be a power device, say 5W, so that it can withstand fault currents. A pair of high-current diodes should also be connected in anti-parallel to bypass more serious fault currents, thereby ensuring any fuses will reliably blow. A >6 amp bridge rectifier package is quite convenient for this, as shown in fig. 15.16 (it does not need to be a high-voltage rectifier).
I.e. they need to be fairly meaty devices so that the basic safety of the device is not compromised. If there was a serious issue and the 1/2W resistor burnt up, and the diodes frazzled before the fuse box of the house registered the short, then wouldn't the amplifier be at a life threatening voltage?
Referring to the ValveWizard's page on Grounding he says ...
... The resistor should be a power device, say 5W, so that it can withstand fault currents. A pair of high-current diodes should also be connected in anti-parallel to bypass more serious fault currents, thereby ensuring any fuses will reliably blow. A >6 amp bridge rectifier package is quite convenient for this, as shown in fig. 15.16 (it does not need to be a high-voltage rectifier).
I.e. they need to be fairly meaty devices so that the basic safety of the device is not compromised. If there was a serious issue and the 1/2W resistor burnt up, and the diodes frazzled before the fuse box of the house registered the short, then wouldn't the amplifier be at a life threatening voltage?
Use a simple 30 or 60 Amp bridge and you will be safe… Add the resistor and capacitor if you think you need it.
Regards, Gerrit
Regards, Gerrit
Looked up a typical 6A bridge rectifier - it's rated for 200A for one AC cycle, which should trip a mains breaker. There are four diodes, so you can parallel them for 400A. The V/I curve only goes to 100A, where it's 1.3V - so that 1/2W resistor has to handle perhaps 1.4V worst-case - 0.2W for ~40 ms with a mains-to-chassis short. If you're bothered by my first assumption (6A bridge will trip 20A breaker), then use a 12A, 25A, 35A bridge -still a $2 part.
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Hi @Tom Bavis what was your opinion on the 1N4007 in that application? I have only seen chunky bridge rectifiers in this application. Apart from being easy to bolt to the chassis, I assumed a rugged part was required for a potential catastrophic fault.
The rectifiers have to survive long enough to throw the breaker, no matter how old or cranky it might have gotten. The resistor is used to swamp effects of stray leakage conductions, keeping the voltage difference between signal ground and PE (chassis ground) very close to zero. If allowed to drift very far this voltage difference would begin to let a rectifier begin conducting, defeating the purpose of the ground breaker.
The reason to break ground paths here is that PE / safety ground / chassis is a potential other path for signal return currents. Worst case would be signal return > PE > long wire to panel > another long wire from panel > PE of a connected device > signal return of that device > interconnecting cables > and around, making a giant ground loop. If everything is plugged into the same power strip this ground loop path is kept smaller and local, and in almost all home installations, perfectly fine. But sometimes this isn't possible.
All good fortune,
Chris
The reason to break ground paths here is that PE / safety ground / chassis is a potential other path for signal return currents. Worst case would be signal return > PE > long wire to panel > another long wire from panel > PE of a connected device > signal return of that device > interconnecting cables > and around, making a giant ground loop. If everything is plugged into the same power strip this ground loop path is kept smaller and local, and in almost all home installations, perfectly fine. But sometimes this isn't possible.
All good fortune,
Chris
If there are no components there between 0V and ground then a short of HT to chassis would place 0V at -HT. So if the rest of your system is floating then you are in for a nasty surprise. Whatever you use for safety it must not go O/C if a fault occurs. I agree 1A diodes may be a bit weedy if the HT can deliver more than 1A.