mic zap while playing guitar

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jeancoltrane, Feb 1, 2015.

  1. jeancoltrane

    jeancoltrane Member

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    classic.. i know, but its the first time ive experienced it in my practice space. i plug everything into 1 outlet with 2 plugs. 1 plug is a power bar with a new unpowered mackie(2 prong) mixer into a keyboard/pa(3 prong) amp. the other plug has a power bar with my pedals/amp. the amp is an old magnatone m20 that has had a 3 prong cord installed. it has an anti hum (polarity?) switch, but i assume it was disconnected when installing the grounded cord.

    i got zapped a few times in several minutes, but it seemed minor compared to some of the stories ive heard. i havent used this amp much in the last year, but i know i have used it in the past with no issues, as well as my other amp. the only new things in my setup are a strat, a tube pedal and the 2 power bars.

    i have had some crackle + pop with the strat, but assumed it was due to static/winter dryness and not the wiring because i havent noticed any weird hum/buzz issues.

    ive noticed some extra hum with the power bars if the amp plug isnt seated perfectly, as if the plugs themselves are stretched and not making good contact unless wiggled. i havent tested the wall outlet/plugs, but i assume they are ok because ive ran the same type of setup before with no issues.

    could it be a bad mic or patch cable?
    static? its very dry in my house, was playing with socks on the carpet, but it did happen every time my lip got close.
    guitar wiring?
    amp ground?
    power bar plug not making good contact with amp?
    wall outlets?

    any advice is appreciated, especially any that helps me troubleshoot without repeatedly shocking myself :)
     
  2. zenas

    zenas Member

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    Take a voltmeter and put it between where you got shocked. Amp chassis to mic or whatever. With vintage two prong amps seeing 120 volts AC is normal depending on which you flip the plug.
    Also with vintage amps even though you get 120 volts the current depends on weather it's a live chassis "widow maker" or something like a Fender with a good "death cap."

    Real simple test with a volt meter.

    It sounds like the ground on your power strip might be "wallered out". That would explane why wiggling helps.
    Amps will work weather or not the ground prong is connecting. But they might hum, buzz and shock you.

    I carry an outlet checker they're only about ten bucks because you never know if an outlet is right unless you check. You can check them with a volt meter too but I can never rember if the hots on the left or the right. :)

    I highly recommend screwing around with a volt meter to anyone with amps. You don't have to take any thing apart and it only takes a few minutes.
    Just set the range to AC for the wall voltage in your country.

    And rember whatever voltage and current is on the amp chassis is on you guitar. It could be ground or it could be touching the strings is the same as sticking a key in an outlet.
     
  3. jeancoltrane

    jeancoltrane Member

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    thanks for help. hopefully going to pickup an outlet tester and volt meter tomorrow. the amp has a grounded cord, im going to try it straight into the wall along with the pa tomorrow to rule the power bars out. i just tried without the pedals and swapped the mic cables but it still persists. with the amp on standby, i could hear a hum come through the pa when touching the strings + mic, with the amp on the hum seemed to come through both amp and pa. the weirdest thing is when i went to turn the pa off and was standing near it(the speaker) the mic started to feed back, and would stop when i moved away... and then start again as my guitar got close to the pa amp. dont know if that means anything but it makes no sense to me. there was nothing turned up.

    edit: dont know if i explained that properly. with my guitar volume turned down, and both amp and pa on, when i bent down with my guitar to turn the pa/speaker off it started feeding back as my guitar got close to it.
     
  4. Chris Scott

    Chris Scott Silver Supporting Member

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    I know it's been beaten to death, but anytime you've got more than just you and your guitar rig, you NEED to have some form of a circuit tester handy, especially if you're playing out with others and/or using a p.a.

    Aside from the obvious life-or-death issues, I can't even begin to tell you how many weird wiring things a simple tester uncovers. Most common down here is one outlet on a breaker strand's grounded, the other two or three are NOT - that one's great for all kind's of confusion when trying to sort an errant buzz or hum.

    Few gigs back, ritual testing of new room, drummer checks ALL the "new outlets they ran for us" and voila! - they're 220vac!

    You've got spare picks, strings etc, the tester's every bit (if not way more) important.


    Oh, and I'm guessing it started to feedback because you bent over out of the "line-of-sight" between your mic and either the monitor(s) or (less likely) the foh.
     
  5. EFK

    EFK Supporting Member

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    I cannot tell you how many old amps I've had come through that were supposedly converted to a grounded AC line properly and in fact were not. Leaving the neutral fused, leaving the polarity switch connected, leaving the polarity cap ("death cap") connected - I've seen it all. It would be best if you verified the conversion: AC line (black) to fuse, then to switch, then to one side PT primary. AC neutral (white) directly to other side of PT primary. AC ground (green) bolted to chassis. The polarity switch should not be involved in any of this in any way, nor should there be a polarity cap still attached to anything involving the AC line.

    You know the expression, trust but verify.
     
  6. schmidlin

    schmidlin Member

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    Me too:
    [​IMG]
     
  7. EFK

    EFK Supporting Member

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    Jesus, you got one up on me on that one! Holy crap.
     
  8. schmidlin

    schmidlin Member

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    Definitely a WTF moment. :messedup
     
  9. xtian

    xtian Member

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  10. twangbanger

    twangbanger Supporting Member

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    Wow , good read. I've run into the bootleg ground, but not the reverse bootleg ground. That could very well kill someone.
     
  11. xtian

    xtian Member

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    Heck, just throwing out the term, "reverse bootleg ground," with a straight face might get you killed!
     
  12. jeancoltrane

    jeancoltrane Member

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    thanks for the help. i think i figured out the issue - the power cord for the pa amp was a bit stretched at one end with the wires slightly exposed, it looked intact but i switched it with a spare and no more zap/hum when touching the mic. does that seem like a likely cause? i did switch outlets on the power bar so that could still be an issue but i dont feel like messing with it while its working heh. i still plan to pickup an outlet tester when im not snowed in up here in canada. feeling lucky it was a minor shock and that i dont sing and play guitar together often :)
     
  13. drbob1

    drbob1 Silver Supporting Member

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    Yes, a damaged cord could easily do that! Throw it away!
     

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