My Guitar Amp Tried to Kill Me

[74westy]

I was playing electric guitar in my living room last night when the strings became electrified. Interesting experience--screaming hard rock, wild new hairstyle and all that.

If it matters, I was using an old beat up Peavey Classic 2-12 (2 12 inch speaker tube amp.) I was also wearing headphones connected to my laptop but I'm almost certain that's not relevant. Besides the strings, the metal ends of the guitar cord and the power switch on the amp were also shocking.

I tested the 3 pronged outlet the amp was plugged into and found that the third prong was not connected. I've rarely use the electric guitar at home but when I do it's usually plugged into that outlet and I've never had a problem before. I moved the amp to a different outlet and it worked fine.

So I'm pretty clear about the fact the outlet in the living room has to be fixed. My question is: is there something wrong with the amp that I have to fix? If so, what?

[beowulff]

Sounds like you have a Hot-to-Chassis short in the Amp. If the ground was working, you would have blown a fuse. So, you have a two-point failure, and are lucky you didn't get seriously injured. You need to fix both the outlet and the Amp.

[engineer_comp_geek]

Originally Posted by 74westy

I tested the 3 pronged outlet the amp was plugged into and found that the third prong was not connected.

This is the problem. There is nothing wrong with the amp.

In an electric guitar, the strings and most of the metal bits (the bridge, tuning pegs, etc) are all electrically connected to the guitar's "ground" connection. There's usually a wire under the bridge that makes this connection. This connects to the amplifier ground through the guitar cable. As long as the amp is properly grounded, you're fine.

If the ground on the amplifier is disconnected, the ground "floats", and it will often float up to the supply voltage of the amplifier (120 volts AC). So, with your hands on the strings, you are basically hanging on to a live wire. All you need to do is touch something that is electrically grounded, and ZAP!

Keith Relf, former guitarist for the Yardbirds, was killed by an improperly grounded amplifier.

[Bijou Drains]

A good reason to have a wireless system.

[Mike Fun]

Keith Relf of the Yardbirds agrees.

[Mike Fun]

Too slow!

[Uncertain]

What do you expect? Nobody plays a suspended chord after a dominant seventh.

[muttrox]

But did you guitar try to kill your mama?
http://www.youtube.com/watch?v=nmQYRcmUjfI

[74westy]

Actually, I was playing both at the same time (7-13 chords) so I'm lucky it wasn't lightning. I can't believe the things I do for my Art.

Thanks to all who responded. I'm going to go with engineer_comp_geek's advice but maybe double check with a local repair type about the amp lest I end up jamming with Keith Relf.

[74westy]

Originally Posted by muttrox

But did you guitar try to kill your mama?
http://www.youtube.com/watch?v=nmQYRcmUjfI

Maybe I can rig it to kill fascists.

[Fridgemagnet]

Guitar amps should always always always be earthed, no exceptions. If the earth is floating then, even if the amp isn't faulty, stray currents will place the amp chassis (and your guitar strings) at a high voltage. Usually these stray currents come from mains filtering capacitors connected between live and earth, and neutral and earth. As these caps are invariable the same value the equipment chassis will float at half mains voltage. Other mechanisms might be at work in an older amp, like leakage currents through a dusty board or insulation breaking down.

Mains extension leads with built-in filters use a 3-capacitor delta configuration, and will also float the equipment chassis at half-mains if the earth is floating at the extension lead plug end.

I can't speak for the Peavey, but a lot of old amps of my acquaintance employed fairly dangerous design techniques that wouldn't pass modern safety standards - electrocution was an occupational hazard for musicians not so long ago.

If in doubt, put all your kit through an RCD. It's $10 well spent to avoid a frying.

[74westy]

Originally Posted by Fridgemagnet

If in doubt, put all your kit through an RCD.

Royal Canadian Dragoons? Rally for Congolese Democracy? Ah, it's another name for GFI.

Thanks Fridgemagnet.

[Rucksinator]

Originally Posted by Fridgemagnet

Guitar amps should always always always be earthed, no exceptions. If the earth is floating then.....

[Hijack] I'm curious as to where are you from where people use "earth" the same way I (Southeast US) have always used "ground"? (regarding electricity)[/Hijack]

[Airman Doors, USAF]

Marty McFly sympathizes.

[HongKongFooey]

Originally Posted by Rucksinator

[Hijack] I'm curious as to where are you from where people use "earth" the same way I (Southeast US) have always used "ground"? (regarding electricity)[/Hijack]

I'm guessing England. He also used 'mains' to refer to the supply voltage. Personally, I prefer their terminology to ours for both of those.

[smithsb]

So, do you have a recording of your inspired riffing? The atmosphere must have been electric.

[74westy]

Originally Posted by smithsb

So, do you have a recording of your inspired riffing? The atmosphere must have been electric.

This was after ground problems were resolved but I think I wasn't quite electrically neutral yet.

It ain't Stevie Ray Vaughn.

[Fridgemagnet]

Re: "RCD", "Earth" etc - I do apologise, where are my transatlantic manners? And that should be apologize I guess.

HongKongFooey is quite right, I'm a British analogue designer. We indulge ourselves with three grounds: (Signal) Ground, which is the 0V of an electronic circuit, say; (Chassis) Ground, which is the metal enclosure, and Earth or Safety Ground, which is a good solid connection to the planet. Generally if a bit of mains-powered kit has a metal box then that box will be solidly connected (to a strict legal standard) to mains earth, for safety reasons, so here the chassis ground is earthed. The same for an electric guitar because of the metal strings. The strings need to be connected to the guitar signal ground or the guitarist's body will pick up mains hum (interference) and conduct that through the strings, from where it is radiated to the pickups, causing a 50/60 Hz (delete as applicable) hum several orders of magnitude louder than the music.

Invariably the guitar amp is the only part of the guitarist's mains-powered kit to be earthed, to prevent more mains hum pickup from ground (earth) loops. These arise when the grounding arrangements form a closed circle between the signal grounds in the guitar leads and multiple points of earthing.

As an aside, I hope I haven't made anyone nervous about using filtered mains extension leads. I should say that the filter capacitors to earth are strictly, and internationally, legally bound to meet safety standards, in that they will pass no more than a harmless (but still tingly) amount of current under normal operation, and if they fail then they're guaranteed to fail safe. "Y2" spec is what they're called, and the current is limited by setting a maximum value - 5 nF for domestic equipment on 230 V 50 Hz European mains, probably a little larger for 115 V 60 Hz supplies (maybe larger still, as US UL standards generally aren't as strict as European EN standards). Medical equipment has very low earth leakage limits, as you don't want much current passing through the dentist's drill and down that raw nerve, or making those muscles twitch on the operating table.

I did like 74westy's guitar playing; would that I had that talent and imagination.

[74westy]

Thanks for everything, Fridgemagnet. And thanks for settling on the 230V standard so we wouldn't have to. 110 v passing through me is quite sufficient.

[Shadowfyre]

Color me confused.

The tuners are attached to the wooden headstock of the guitar. The tailpiece is attached to the wooden body (or in the case of a strat with a whammy, the tailpiece goes through the wood). The bridge (non Strat style) is also screwed into the wood body. The nut is usually plastic or ivory. Where do the strings touch anything that could electrify them?

If the amp is shorted, it might cause a voltage along the amp cable back to the guitar itself, but that would go through some pots before winding around the pickups. A varying voltage around the pickups could induce a current on the metal strings, but I doubt that would be enough to feel.

Am I missing something?

[Nunavut Boy]

Am I missing something?

Yes. The cord that plugs into the Strat has a ring and tip. The tip (IIRC) touches a wire on the output jack that goes around in a circuit through all the electronics of the guitar. The ring is attached to another wire that goes through the body of the guitar to the back where the trem springs are. Every Strat has this wire soldered to the anchor that is screwed into that cavity to hold those springs in place. Everything in that assembly is metal, so the springs attach to the block which attaches to the bridge which are in contact with the strings which conduct electricity.

[engineer_comp_geek]

You are missing the fact that inside the guitar, there is a ground wire which goes through the body of the guitar and connects to the guitar's bridge. On most guitars that I have seen, the wire comes up through the body and the end of it is stripped and wrapped around one of the screws that holds the bridge in place.

I couldn't find a good picture of this on the web, but I did find a picture of a disassembled Fender Stratocaster:

http://gregboyd.com/images/instrumen...uitar_Body.jpg

Note that there are two black wires going into the guitar body. The black and white wires together go to the 1/4" jack. The other black wire goes to the bridge.

ETA: I see that Nunavut Boy said the same thing while I was grabbing a coffee. I should learn to preview.