Choosing right bridge rectifier

[Guitarist]

Hi, looking for a bridge rectifier to use in Valve wizard's hum-blocking network. Mouser has options but I'm sure the best to select with the plethora of specs. https://www.mouser.ca/c/semiconducto...ge%20rectifier

"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). A silicon diode will not turn on until the voltage across it reaches about 0.6V, so the ground loop still appears to be blocked as far as small hum voltages are concerned." Merlin

[Mick Bailey]

Mouser 905-KBPC2504T is what I would use - I usually fit 25A 400V here in the UK (upwards of 240v mains). The rectifier needs to pass the maximum possible ground current in any fault situation at full mains voltage. In the US you could use a 200V rectifier. Whilst a rectifier will fail short under excess voltage or current, the short will only hold up if the shorted junction can still pass current without blowing like a fuse. With a bridge though there's extra security of multiple junctions.

[R.G.]

The current rating is quite important. While 6A might be adequate, it might run afoul of actual real-world requirements. I don't know where you live (appears to be Canada by the mouser.ca url) but in the USA, safety grounding has to be able to pass a 25A current long enough to blow the fuse or the wall-socket breaker. 6A rectifiers ... might... do this, but 25A diodes would be more likely to do so. If I were doing this, I would get a 25A bridge. The funny world of rectifiers is such that a 1000V rectifier is not much more expensive than a 100V rectifier, so the actual voltage rating is cheap to go high on.

The voltage does not matter. By their connection, none of the diodes can have more than a volt of reverse voltage. They're all "protected" from reverse voltage by the other diodes.

The "if I were doing this" is not a throw away line. If you're in a situation where you think you need to lift the AC safety ground wire with a rig like this, then you by definition have AC leakage issues. Introducing this in the safety ground line is removing some of the electrocution protection that safety ground is intended for. A broken wire to/from the rectifiers removes the protection entirely. And adding this entices the installer to skip the special connection items for safety grounding wires, if the installer knows those. I might set this up in one of my amps (and I have done so) for temporary use, but I'd be very aware of the safety issues and disconnect it after the use that required it (and I did). If I were in a situation where I had to get rid of AC leakage based hum all the time, I would (and have done) dig in enough to fix the underlying leakage, not rely on what is basically a band-aid for the long term.

Is this setup safe? Well, maybe. It depends on how you personally define "safe". It does not meet USA safety standards, and probably not CSA or CE. It will probably be OK nearly all the time. I have worked with live AC mains professionally for a very long time, and I really, really want to keep that particular genie inside its bottle. So I personally might take a short cut for a short time, but I would not rely on it all the time.

[Helmholtz]

I understand the chassis is always correctly safety grounded and only the circuit ground is "lifted".
Is the 25A test really performed between chassis and circuit ground?

[R.G.]

Yes, the intent of this mod is to get a small resistance (10 ohms or so - I have not read Blencowe's books, but the other places I've seen this from years ago often quote that value) between chassis/safety ground and circuit/signal ground. The purpose of the 10 or so ohms is to get a static reference between the two while allowing any safety ground leakage to not pull the signal ground around when signal ground is attached to other stuff. The diodes actually play no part in the process unless the leakage can force currents bigger than 1.4V divided by the 10 ohms through the network.

Edit: forgot to mention that the AC leakages in question can come from anything else the unit is plugged into. This is especially obvious when two amps are paralleled with a Y cable.

The standards say any "accessible metal", meaning anything metallic that the user can touch. If the user can't touch signal/circuit ground, including with a finger-emulating metal probe, Safety inspection engineers can and will hunt for any metal surface that doesn't show near zero ohms to safety ground, then test those weak points. Or at least that was true the last time I had something safety inspected.

[Chuck H]

It's nice to have you posting R.G. I know you have a recent schematic inquiry so you "happen to be here" at the moment as it were. But your experience and wisdom are always appreciated here.

[Guitarist]

Thank you guys. Your expertise eclipses mine but it is encouraging that you don't offer that point.
R.G., I just said a prayer for you; hope you feel better!

[loudthud]

The standards say any "accessible metal", meaning anything metallic that the user can touch. If the user can't touch signal/circuit ground, including with a finger-emulating metal probe, Safety inspection engineers can and will hunt for any metal surface that doesn't show near zero ohms to safety ground, then test those weak points. Or at least that was true the last time I had something safety inspected.

I've never seen those chrome corner covers being grounded, but there are plastic ones available.

What about the connections to the speaker in an open back combo amp ? If you ever touched those when a tube amp is being overdriven, you'll wish you hadn't.

Then there are those pedal board power supplies. Don't some or all of the outputs have isolated grounds ?

[R.G.]

I've never seen those chrome corner covers being grounded, but there are plastic ones available.

Good point. I have no experience with something like that - a completely disconnected piece of metal.

What about the connections to the speaker in an open back combo amp ? If you ever touched those when a tube amp is being overdriven, you'll wish you hadn't.

The guys who looked at the speaker outputs for our amps did in fact look for continuity to safety ground on the speaker jack output. It had it, and was OK.

Then there are those pedal board power supplies. Don't some or all of the outputs have isolated grounds ?

They do. A lot of those meet double insulated for the isolated output grounds. And a lot of the cheaper ones say they have "isolated outputs" when they really mean "separately regulated" and share a ground or are "isolated" by a resistor to one another. Or simply have their contract manufacturer print the certification symbols on the case without bothering to actually do the tests.