3 Prong Power Cord Install???

[lakehaus]

Yep. I now know why it's good to have a grounded power cord on my old Vibro Champ. Shocking experience! Well, more of a buzz...

Can anyone recommend a good site with pics that will guide me to do the install? I've searched, and there's a lot of blabbing about it, but to me, a picture is worth a thousand words - but I understand the blabbing part is needed, too.

[ampdoc1]

I took this off the net a while back.

[Structo]

http://www.theguitarfiles.com/guitarfile644.html

http://www.rru.com/~meo/Guitar/Amps/Kal ... /safe.html

[mhuss]

It wasn't really brought out in the posts above, but *always* remove the ground cap (aka "death cap") on old amps which have these. You can leave the ground/polarity switch in for cosmetic reasons, but lose the cap.

--mark

[selloutrr]

maybe this will help. Make sure to remove the death cap you may be able to reuse it, if you wire it back up like this.

[lakehaus]

Thanks for the advice on the death cap... but what's the deal with it? Why the ominous name? And, correct me if I'm wrong, but is this the death cap? What function does it serve, and if it's not needed, why was it put there in the first place?

Sorry, I'm new to this stuff.

[img:498:441]http://i256.photobucket.com/albums/hh17 ... ight-1.jpg[/img]

[rfgordon]

yes, that's the death cap. Clip it out and throw it away.

Onto replacing your cord. If you are careful, you may be able to squeeze the compression grommet and pull the old power cord out--being able to reuse the grommet. Otherwise, they're cheap.

Before you stuff the new cord into the amp, solder a ring terminal onto the end of the ground wire (it'll be the green one in the cord.). Attach it to that tranny bolt close to the power cord hole.

solder the Black (hot) lead to the fuse, and the white (neutral) to the other PT primary.

[Structo]

In the US, conventional power outlets have one side of the line at ground potential and the other side connected to the "hot" lead. The ground switch works by creating a high-impedance connection, through the grounding or "death" capacitor, between the chassis and one side of the AC line. The "correct" connection attaches the capacitor to the grounded side of the AC line, thus putting the chassis at ground potential for AC frequencies.

The cap really was necessary; without it, leakage currents (mostly from the power transformer) would put some small amount of AC voltage on the chassis. With the entire circuit floating up and down on the AC voltage, a certain amount of imbalanced voltage would get injected into the circuit and cause hum. The cap simply shunted the chassis voltage to ground, greatly reducing the magnitude of the voltage and reducing hum.

If the death cap shorts out, it renders any protection from a hot chassis null and void.
These were outlawed sometime in the 60's.

A three prong cord helps in keeping the hot wire off of the chassis because it polarizes the plug so it can only be plugged in one way.

But, if the outlet you are plugging into is not wired correctly, you can still have the hot side of the AC go to the chassis.

It's a good idea to carry a simple outlet tester with you if you gig away from home.
You can test the outlet at the venue and make sure it is wired correctly.

A lot of older buildings only have two wire systems. If at all possible, use an adapter that you can wire a ground to.
If the building has metal conduit this is usually grounded.
If not, you will need to run a ground wire from the adapter to a cold water pipe or other earth grounded object.

[Wayne]

You have to wonder - just how many older amps are roaming the earth sporting the safer 3 prong plug but still having the death cap intact? I "took care of" another just last week.

W

[selloutrr]

i've removed more then a dozen in the last year from amps bought on ebay. But since they out lawed firecrackers and made it law to wear a helmet, maybe we need some sort of idiot control. it's to bad it usually never stays with the guy who wired it.

[mhuss]

A typical old-fashioned two-wire ground cap will connect to one side or the other of the incoming mains. If you have it switched to the neutral side, it works as described above, shunting noise to 'virtual' ground.

If you have it switched to the hot side, it will actually inject a small amount of AC onto the chassis, and you could get a slight shock if the plug is put in the right way, or a pretty good shock if it's put in backwards.

All original AC cords old enough to have one of these caps is also old enough to be not polarized, and are a potential shock hazard.

Normally (unless you're standing in a bath tub full of water or have a bad heart) these are not fatal, but are unpleasant -- particularly when the guitar amp was wrong and the microphone screen was properly grounded -- your lips would complete the connection! In the Bad Old Days I used to touch my guitar strings to the mike screen and look for sparks.

--mark

[drhulsey]

... I used to touch my guitar strings to the mike screen and look for sparks...

Don't worry, ladies and gentlemen It's all part of the show

[Normster]

So what do you guys think of Dumble's method of using a center-off switch with the cap "available" in case of bad house wiring?

[mhuss]

No disrespect to HAD, but that's actually the (silver face/CBS) Fender method. My '74 Twin had one.

Proper modern amp wiring (with the two mains wires going ONLY to fuse, switch, PT primary and perhaps indicator) protects you automatically, even if the hot and neutral wires are reversed in the socket. The earth ground is the only mains wire that should be connected (any which way) to the chassis.

If the socket ground is missing or not connected, you're in trouble anyway!

For new work, I suspect that any of these "cap between hot or neutral and chassis" methods are forbidden by the electrical code in most if not all jurisdictions.

--mark

[C Moore]

OK -
Green ground wire to filter cap ground.
Hot black wire to center of fuse. Black side of fuse to switch, black from switch to PT.
Neutral white to PT.
Hopefully you can see it all from my pictures.