Shocks from Fender Amps

[martin manning]

So, I was at the local guitar store looking over the vintage amps and happened to put a hand on a chassis mounting strap of an early 60's Vibrolux and one on some small silverface thingie at the same time, and I got a pretty good shock. The Vibrolux, at least still had a 2-wire cord. Someone there said that this would happen with any two Fenders, even new ones.

I find it hard to believe that modern amps would do this if they have three-wire cords and no ground switch. I tried flipping the ground switch on the Vibrolux, but I still got a shock. What gives here?

MPM

[tubeswell]

Must be that the grounds on both amps are at different voltages for some reason (Duh - okay that's a real bright answer ), and the two-wire job has got something to do with it.

[Rob Mercure]

MM

I find it hard to believe that modern amps would do this if they have three-wire cords and no ground switch That's your trouble - both amps didn't have a three wire grounding cord - with the grounding wire still attached to the chassis at one end and the large round plug prong at the other - while connected to a three wire outlet that's hooked up to a circuit with a common ground that's not so worn out that the outlet makes good connection with the grounding prong. Before national electrical codes called for grounding outlets and grounded chasses shocks - especially for stage musicians - were really, really common. And if you peruse the current forum posts - check out the one about transformerless amplifiers by Carlo under Vintage amps - you'll notice how much attention we pay to correcting practices that were legal and common until the mid-1960s, much less where folks pulled out grounding prongs, used "ground adapters," wired three wire outlets to two wire circuits and put pennies in burned out fuses until they remembered to buy a new fuse.

"Shocking" - no? Why do you think us old farts forgo night lights <grin>?

Rob

[martin manning]

Rob, I couldn't find the cord to the smaller amp easily, but I think you're right. I guess I could have tried flipping the 2-wire plug to see if that would fix it.

The part I didn't believe is that modern Fenders would do this too. I have to believe that is an urban legend. This guy was old enough to have been burned (well, poked) a time or two. Once bitten, twice shy...

MPM

[loudthud]

In theory you shouldn't be able to get a shock if both amps were properly grounded. In practice a bad outlet strip, wall outlet or grounding could be at fault. I've even seen those computer like IEC cords not making a good ground. I like those little gadgets with three lights that tell you if an outlet is properly grounded. They come in handy playing dingy poorly lit old bars.

[Enzo]

Turning the plug over should have cured the problem - for the moment. But there should have been a ground polarity switch on the back of the amp.

Modern grounded amps SHOULD not give shocks. Two wire amps OFTEN did, that was the first thing to do after the stage was set up - go around and get all the grounds on the same page.

Old amps really should get new three wire cords.

[martin manning]

I tried flipping the ground switch, and still got shocked as I said in the OP. There was something else wrong, too, since the power switch was on (stand-by off) but the tubes were not warm... perhaps the fuse was blown. Turning the power switch off didn't change anything either, so maybe something was shorted to the chassis on the primary side. Not exactly ready for sale, eh?

I guess the theory of the ground polarity switch was that the chassis would be connected to earth ground (via the neutral side of the AC power) through the 0.047-0.05 cap (about 56K ohm reactance at 60 Hz). I can see where you would definitely not want the chassis to be directly connected to the mains if the polarity of the power plug couldn't be assumed.

I noticed (by checking the schematics) that the silverface version of this amp has a three wire plug, but keeps the ground switch. This was probably a nod to the fact that the amp would often be plugged into a two-wire circuit.

Would the incorrect ground switch position be immediately detectable by 60 Hz hum, especially when touching the guitar strings or anything connected to the chassis?

MPM

[Rob Mercure]

MM,

The answer to your question is yes. As to your original query - the "polarity reversal" cap is referred to as the "death cap" for a reason. While I'm not actually aware of deaths I have removed at least three shorted caps in Fender amps over the years. Two of the caps had for some reason" exploded - the side of one with the foil/dielectric unravelling and the other with a little "volcano" crater. The third switch looked perfectly good and had a DCR of less than 100 ohms. Who knows, I suspect that some of the others I've removed have been funky but I usually don't measure 'em - I just clip 'em out. Hmmm, got a SF SR I'm recapping and adding individual bias controls sitting on the bench right now - I think I'll measure the cap "for the hell of it."

Rob

[PRNDL]

Would the incorrect ground switch position be immediately detectable by 60 Hz hum, especially when touching the guitar strings or anything connected to the chassis?

Sometimes its silent and hard to detect, except for weird things going on. Until you get a shock like you experienced.

The Vibrolux, at least still had a 2-wire cord.

One side of that cord connects to the chassis. Unfortunately for you, that side was connected to AC hot. Flipping the cord over would correct that. To avoid shock, the best step is to install a proper 3-prong cord and a fuse.

[ArtB]

Shock Defense?

In theory you shouldn't be able to get a shock if both amps were properly grounded. In practice a bad outlet strip, wall outlet or grounding could be at fault. I've even seen those computer like IEC cords not making a good ground. I like those little gadgets with three lights that tell you if an outlet is properly grounded. They come in handy playing dingy poorly lit old bars.

Suppose you do find out that the outlets at the dive are not properly grounded -- is there anything you can do about it? We ran into that last year, and couldn't think of a quick fix other than make sure not to kiss the mic while singing.

[Rob Mercure]

Art,

Tempory measures: A set up with an isolation tranny in a box if this is only one singer - isolate that person's amp (tranny for one amp pretty cheap - several getting up there - but sometimes handy to have for recording - I used to carry one); wind screens ("pop" screens) on the mics to keep your lips off - works well but don't hold the guitar and the mic stand at the same time.

Longer term: Offer to check out the outlets at the club with a circuit tester and if you've got the skills to replace the outlet if that's the only trouble. But if the club owner's a jerk - happens more often than not - threaten to call the electrical inspector on them (or find some neutral person who isn't associated with the band to do so) - even little rural Appalachian counties like the one I'm in has an inspector.

Or, if you can afford it, make yourself a mobile power set-up that provides power for the band. A circuit breaker box with outlets that can be tapped right off the power panel. But this is going to require someone associated with the band to have the expertise - perhaps an electrical license - to do so. Generally you can find a dual 30 amp breaker that's not being used at that time (heater in the summer, A/C in the winter - for example) that can be used to feed the panel.

Lot of this depends on how much pull you have with the owner but at this day and time no one wants the liability of even being blamed for an piece of equipment being fried by bad wiring much less a minor/major electrocution.

Rob

PS: I guess you could wear those Halloween wax lips while you play <grin>

[Jag]

Or, if you can afford it, make yourself a mobile power set-up that provides power for the band. A circuit breaker box with outlets that can be tapped right off the power panel.

Back when I was still playing for a living, most of the clubs up here had a dedicated 220v dryer plug at the stage. We, and most other bands around here, carried a distro panel and always used it. Most of the time, if the club didnt have a dedicated circuit on stage, we could tie in to the main service panel. Don't know if that's still done but it was a really good idea.