1. Riscchip

    Riscchip Member

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    Here is a crazy & bizarre question for the technically inclined. My girlfriend has developped a healthy fear of pretty much all of my guitar amps, but especially those that I've worked on myself. ;) I made the mistake of explaining to her how dangerous the currents in amps are after she saw (and heard) me discharge the caps on one of my amps.

    So the question is this...if anything shorted, failed, popped loose, insulation was melted by a stray soldering iron, etc...is it possible or even reasonably likely that you could be zapped from outside the chassis after it's installed back in the shell / combo cab? Touching the chassis, knobs, tube holders, etc.? I'd like to offer my girlfriend an informed explanation about how this really isn't likely and how the dog isn't going to be fried by brushing up against my '69 Superlead.
     
  2. John Phillips

    John Phillips Member

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    It's not a crazy or irrational fear - there have been lots of cases of musicians electrocuted by gear (a few of them quite famous). The usual cause is a combination of two different pieces of gear though, one of which is faulty and ungrounded and the other of which is well-grounded... eg a mic connected to a PA and a guitar amp. The player then acts as the bridge between the two, and ironically it's the good ground on one piece of equipment which completes the circuit. This is why circuit breakers (which do not lift the ground connection when they trip) can give a false sense of security.

    But if you're talking about just a single amp with nothing else nearby...

    If the amp isn't plugged into the wall, it's impossible. Even if a cap somehow shorted to the chassis, there is no other path to ground to complete the circuit and you wouldn't actually get a shock. (This is the same reason as why birds can safely sit on power lines running at tens of thousands of volts and not get fried.)

    If the amp is plugged in, and the power cord is a 3-wire type properly connected to the chassis, it's also impossible that an amp fault could be dangerous because any leakage to the chassis will go straight down the ground wire and safely bypass anything touching the amp. This is true even when the amp is turned on.

    The dangerous situations are:

    An ungrounded amp that's plugged in. Now if a fault develops and the chassis becomes live, there's a potential loop via anything else that is grounded (other apparently 'safe' objects or even the floor) back to the supply, which itself is referenced to ground at the local transformer.

    Or a wiring fault in the building which causes the ground wiring itself to become live. In this case the amp is dangerous if it's plugged in even if it's turned off. It also means that many other household appliances (anything with a metal outer casing) will be dangerous.

    But if the house wiring is correctly grounded, and the amp is also, you can't get a shock from it whether it's plugged in or not.
     
  3. donnyjaguar

    donnyjaguar Member

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    The only thing I can add is that if in doubt, you can use your voltmeter and measure between any two amplifiers to see if any voltage is present. In my experience this usually happens between the guitar and the microphone. I've zapped my lips a couple of times when the band I was in used some a more vintage PA system (solid state) with no ground pin. Our singer never got zapped, but the backup singers (myself and the other guitar player) did.

    Who was that famous musician that got killed on stage due to electrocution?
     
  4. Junior

    Junior Member

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    Comments on John's two dangerous situations:

    Back when I was building/modding amps, I disconnected the ground wire to the strings on my guitars. I had amps I was pushing so hard the tube life was measured in single-digit hours, and all sorts of things were melting. This was before the pronged plugs, and I'm not suggesting it to everyone. I don't do it anymore, myself, but I play nearly stock amps now. Tubes are too damn expensive to have fun with them.

    Once, when I moved into an old house, I took a meter to the outlets in my room, with a wire out the window to a ground stake. I expected at worst a few volts difference between Ground and the ground. What I read was 117 volts!! I've seen the same thing in two adjacent outlets in a GE defense plant, so don't trust electricians with your life. Carry an outlet tester. You don't want to know how many bar owners pay their electricians from the tap instead of the till.
     
  5. John Phillips

    John Phillips Member

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    Keith Relf of the Yardbirds is probably the most well-known.

    And Les Harvey of Stone The Crows.

    Both of these were on stage. John Rostill, bass player of The Shadows (not the original bass player Jet Harris), was killed in his home studio.

    There may be others. Thankfully electrical safety regulations are much more strict than they used to be, and things are definitely a lot safer nowadays.

    I've personally had a shock on stage, between my guitar and a bass that was plugged into an amp which was on another circuit (fifteen years ago in a real dive of a venue with totally unsafe wiring - many other musicians in this area mentioned similar experiences - it's been fixed now). I don't know the actual voltage difference but it was scary enough. Since then I've fitted safety capacitors in the bridge ground path of every guitar I've played outside my own house, including all the vintage ones. It's not a perfect protection because you can still get a shock from the jack or a grounded metal control plate (eg Tele), but it does at least mean you won't get shocked via the strings or trem arm, which are particularly dangerous because your hand muscles will contract with the current and make it difficult to let go.

    (The safety cap should be rated for at least double the supply voltage and preferably a 'Class Y' - fail-safe - type, or whatever the equivalent in the US is. The value isn't so important, but .01 to .1uF seems to give good enough buzz reduction - which is the real purpose of grounding the strings, not safety - while still not allowing a dangerous current to pass.)
     
  6. Riscchip

    Riscchip Member

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    Thanks for the fab-tacular info, gents, much appreciated. Sounds like the I can reasonably explain the safety level of an amp with a grounded 3 prong power cord (all of my amps have these, I believe). That all makes sense to me.

    Thanks!
     

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