#1
Hello.

So, I was just using a bass amp and got mildly electrocuted. With some experience in electronics, I should have known better. Some dildo ripped out the ground prong on the plug- I already knew that's bad news. To keep a long story short, I was just eager to try it and didn't replace the plug first like I should have. I took a break to vape with this box I made hooked up to an ATX power supply (the ground wires have a very low impedance to the ground prong on the mains plug), and I got a little zap on the lip because I was touching the bass strings at the same time. Careless, stupid and potentially lethal mistake on my part- I've read about these kinds of things before.

It got me thinking. Why does this happen? I'm pretty sure the vast majority of amps use transformers so the secondary side of those where the amp gets its power from should be isolated from mains as well as the bass guitar, but something else is going on here. I know sparks don't fly from an oscilloscope's ground clip when you hook it up to the secondary side of a transformer because I've done it before (after researching how NOT to ruin an O'scope before I bought it) and I'm pretty sure I understand how that works; transformers provide isolation from mains so there's no current path to mains ground through the ground clip on an oscilloscope when you're testing things powered by transformers rather than mains. This is the case even if you hook the ground clip on a non-isolated oscilloscope to the high side of a circuit- you're just ground referencing it so no sparks fly. Going on what I know, I figure amps should work the same way- this time the negative/ground wiring of the power supply should function the same as the ground clip on an oscilloscope. Haven't thought about it for awhile though to be honest.

What am I missing? Clearly there's some lack of isolation for current to flow through my bass, through me and to mains ground. I suspect it has to do with reducing noise.

Just measured it with a multimeter and there's 85.5 VAC between my bass strings and the power supply! Could have easily damaged other things as well as killed myself. I'd do a current test, but I'd rather not risk destroying stuff more than I already have.

I'm using an old Roland Super Cube 60 my dad salvaged from a dump. Didn't modify it at all, but the power switch is broken and stuck in the on position. Other than using a power bar as the power switch, crackly input jacks and the ground prong missing it seems to work fine.
Quote by Jesus
Gaza Strip- home. At least it was before I fucked ereythang up...
#2
Put a 3 prong plug on it.  Obviously there is some path to ground.  The non-ground leads include a neutral that is usually tied to ground at the breaker box.
#3
Quote by fly135
Put a 3 prong plug on it.  Obviously there is some path to ground.  The non-ground leads include a neutral that is usually tied to ground at the breaker box.

 
I realize that. Just wondering how that path works and why this particular ground path is necessary for something powered by a transformer.

I've built things with transformers before and this sort of thing isn't an issue; i'll wire things up with a ground connection, but it's just so the breaker flips if there is any kind of short on the primary side of the transformer- like dropping it in water. From what I understand, ground connections are typically used to avoid damage/injury in case of some kind of malfunction. It's like if the outside of a stove becomes energized from a frayed hot wire touching it; you want the breaker to flip instead of something outside it grounding that voltage through the metal exterior and potentially killing someone or causing a fire.

What makes amplifiers different? This seems like normal behavior with them when the ground connection is missing rather than something there in case something goes wrong. I was taught that a constant current flow through ground wires should typically be avoided- that's what neutral is for; ground is basically a backup neutral to prevent damage from malfunctions.

Not trying to justify my mistakes and impatience- I own up to it and just want to learn more about how stuff works. It's hard for me to explain what I'm asking and it's confusing to me!
Quote by Jesus
Gaza Strip- home. At least it was before I fucked ereythang up...
#4
I can understand wanting to know why.  You could try measuring the resistance between the hot lead on the plug and the chassis.  See if there is any leakage.  Although if the leakage is capacitance in nature a DC resistance might not reveal too much.  But if the resistance starts out low and goes high that would indicate a capacitive leak.
#5
Been a few days, but I got around to measuring a few things.

Resistance measurement on the bass amp is just infinity- according to my meter it's an open circuit between both the bass bridge and hot lead as well as the amp's chassis and hot lead.

However, I got pretty much exactly 100 ohms measuring resistance from the bass bridge to the ground prong on the plug (I replaced the plug). Just to compare, I tried plugging my bass to a guitar amp- a Peavey Envoy 110- and got a fairly low 4-4.5 ohms doing the same thing.

I also did a capacitance measurement on the hot lead and the ground prong on both amps. Got 2nF with the bass amp and 1nF with the Peavey. I really can't say how trustworthy the measurement is though- could be some weird stuff going on and the most sensitive setting on my meter measures in 1nF increments but the measurements didn't jump around. I'm pretty sure the capacitance measurement uses some kind of pulse, so it makes sense that it would go across the transformer somehow but I have no idea how it would interact with the ground connection.

To sum it up, I didn't learn much about what's going on. I did figure out that there's not much resistance from the instrument to the mains ground though, so even on the bass amp what my body sees should be at most 1/1000th of the voltage as the resistance of that internal connection on the amp (about 80 mV give or take) and even less on the guitar amp; the resistance between the instrument ground and the amp should work like a voltage divider with a person who happens to touch ground and the instrument, but I'm not sure how the whole circuit works. I did measure significant AC voltage rather than DC with the old plug, so maybe there is some kind of feedback through the transformer which makes me think the isolation transformers provide might not be as perfect as I thought.

I don't have the knowledge to draw a conclusion, but if I learn more at some point I'll post it here in case anyone else is interested.
Quote by Jesus
Gaza Strip- home. At least it was before I fucked ereythang up...
#6
Once you put on the grounded plug, the voltage measure from your Bass strings to ground (chassis) should be close to zero.  That's what counts.  The capacitance looks like nothing that would carry 60hz voltage to a dangerous level.
#7
My guess is the fact you took a break to Vape it was Gods will.   As to that Man Bun?   perhaps a ceiling fan mishap is in your future, Or at least one would hope so,  Oh and by the way Vaping can cause Cervical Cancer for those such as yourself, Electrocution should be the least of your worries