120V chassis on Fender champ

My friend just gave me his silverface Champ to look at, since he was getting shocks while playing. Any advice on what to do would be appreciated, here's what I found so far:

1. I measured 120V AC on the chassis with the ground capacitor on the AC input connected, and 95VAC with it disconnected.

2. There is a three prong cord, but the ground leg is broken, so the chassis is not grounded. Hot and neutral lines feed the power transformer, neutral does not contact the chassis (only through the previously mentioned capacitor). There is no virtual center tap (with 100R resistors) on this amp.

I looked and probed, but I can't seem to see how the AC is getting into the chassis. Could it be because there is no earth reference to the power transformer secondary, which only references the chassis, which then floats up at 120VAC relative to the rest of the world? I'm afraid if I just put on a new three prong cord, I might be shorting line voltage to ground.

Also, what's the capacitor on the AC input doing, filtering line noise to ground? Should I just snip it out? I remember in a previous electronics safety thread that the general consensus was these could be dangerous. I know these questions tie into that thread, but I still don't completely understand how a chassis gets hot.

Thanks,
Chris

SF Champs usually have one side of the 6.3V PT secondary grounded so you'll only see one side (pale green wire) chaining through the tubes. At each tube one side of the filament is grounded to the chassis. Done this way--no pseudo center tap is needed.

If someone has upgraded it to replace the single sided filament supply to the normal way, but did not do the false center tap thing that's not good. Add the false center tap at the pilot lamp and reference it to the 6V6 cathode.

As for the mains part...snip the death cap out. Run the hot side to the fuse then to the power switch and on to the PT primary. Neutral connects directly to the other side of the primary. Fix the ground wire, obviously. Now it should be safe. If it pops fuses when it starts up it may be that the PT is toast (internal short).

What rectifier tube has he got in it? Should be a 5Y3, but a 5V4 can work and will have slower start up (indirectly heated) which is nice since there's no standby switch. While you're in there, replace the 6V6 cathode resistor with a 1k 2W+ and replace its cathode bypass cap with one that's rated for 50V (increased cathode R increases cathode voltage). And if there are any other white Mallory electrolytics left in there, replace them as well.

Champs are great little amps. Try it through a big cab and be amazed!

A P

[quote author="Emperor-TK"]2. There is a three prong cord, but the ground leg is broken, so the chassis is not grounded. Hot and neutral lines feed the power transformer, neutral does not contact the chassis (only through the previously mentioned capacitor). There is no virtual center tap (with 100R resistors) on this amp.

Also, what's the capacitor on the AC input doing, filtering line noise to ground? Should I just snip it out? [/quote]

The capacitor AC couples either line hot or line neutral to the chassis. It's a coin toss whether you get the chassis hot or neutral. It's a bad idea that was the only noise-mitigation option before grounded mains outlets were standard. If that cap goes bad you can die.

Remove both the ground switch and that capacitor. Replace the plug on the mains cord so it has a proper ground connection. At the amp end, the ground wire goes direct to the chassis (there are several screws and mounting bosses you can use for this), and the hot and neutral go to the power transformer primary ONLY (hot of course going through the fuse).

Your amp should then be both safe and quiet.

-a

Mr. Packrat is right.
gb

Got it, thanks guys. I did the power rewiring, but didn't have the parts to redo the caps and resistors. No more hot chassis. Boy, it does sound very nice too. AP, what's the 1K 2W replacement on the 6L6 do?

The internal speaker seems to be 4 ohms. Can these amps handle an impedance mismatch (8 or 16 ohm external cabinet)?

-Chris

Glad you got it sorted, Chris. 6L6 you say? Should be a 6V6 in there. The 1k cathode R puts the 6V6 at a sane operating point where it dissipates about 12W. With the 470R you're hitting way above the 14W max rating (more like 18W in the two VibroChamps I have). They sound better and the tube will last longer at 12W. B+ will increase a bit though. You could use anything in the 800 ohm to 1k range if you've got something lying around.

They want to see 4 ohms but do fine with 8. 16 would probably be pushing the envelope a bit far. I run mine through an 8 ohm 2x12 a lot. Sounds good.

Cheers,
A P