Grounding a two prong amp

[jrfox92]

I've got an old Univox that sounds pretty good cranked but hums like a mother most of the time.
Whenever I read people talk about adding a three prong power plug they act almost like it's neurosurgery.
Since I haven't really done much repair work or even studying on amp circuits, is it more than just wiring a ground wire to the chassis or should that be about it?

[ShaunNecro]

Nah, that's basically about it.

[waltdogg]

^

easiest amp "mod" ever.

[jrfox92]

Now I'm really curious why people seem to think that their amp will explode if they ground it.

[waltdogg]

if your amp has a polarity switch it probably has a "death" cap that will need to be removed. that's probably what theyre referring to. a bad "death" cap can cause, well, death.

[CyaNitrate]

First and foremost, if you do not know how to discharge the capacitors, stay out of the amp, they will kill you. It isn't hard to drain them, but learn how properly, and double check with a volt meter.

Now that the disclaimer is out of the way, this is the death cap. I think its a .48 if memory serves.. this is a 74 Fender Champ. Stock, the cap goes from the fuse to chassis, and also on some amps the hot wire (black) goes to the switch, then the trans, then the fuse...which is bad news bears.

You want to make the circuit look like the "after" in the diagram (botom right). If it has a courtesy outlet and/or ground switch, you can leave them in or remove them from the circuit. A Champ has neither, but when I did a Vibrolux I had, I left both in place physically in the amp, but neither were wired to anything, totally bypassed.

2to3prongconversion.JPG (25.06 KiB) Viewed 406 times

This is when it was done. ground wire to chassis at the transformer mounting bolt, hot to fuse, fuse to switch, switch to trans, and trans to neutral (white). All said, from start to finish it takes about an hour. Just be careful and take your time.

This is good practice on an amp, but that said, if your amp has never been serviced, it could still hum afterwards and need to have a cap job. But start here first.

[crochambeau]

it could still hum afterwards and need to have a cap job. But start here first.

QFT, caps are usually the cause of hum, but three prong power cable is a critical (and usually easy) modification.

I like to turn the old power cables into speaker cables, they're usually a perfect length and have years of tone goblins built in.