My amp gave me an electric shock.........

Discussion in 'Amps and Cabs [BG]' started by Petebass, Nov 24, 2005.

  1. Petebass


    Dec 22, 2002
    QLD Australia
    Any idea of what might make a SS amp send voltage down the instrument cable plugged into the unput jack?

    Background:- Usually I set up for a gig starting at the bass, then working through the signal chain back to the amp. (Bass--->Pedals--->Amp--->Speaker cab).

    Yesterday I decided to do it the other way around for no real reason. I plugged the cab into the amp, plugged the amp into the power point. Plugged the instrument cable into the input jack - so far so good. When I went to plug my instrument cable into the output of the pedals, I got an electric shock. Not a big one thankfully, but enough to make me pull my hand away in a hurry. At first I thought it was the pedal, so I tinkered with it but I wasn't able to reproduce the shock. Then when I again went to plug the instrument cable into the pedal , I got a second electric shock. At that point I realised the shock only happens if I'm holding the instrument cable in one hand, then touch something metal.

    The shock persisted even after disconnecting the speaker cab. I was also able to reproduce the shock using 4 different instrument cables.

    I plan to take my amp to a tech and use my spare in the meantime. Any ideas what the tech is going to tell me?
  2. Does your amp have a ground switch on the back? These are usually 3 position switches, the center position typically grounds the chassis to the 3rd (ground) prong on the amp's power plug (well at least here in the US). The other two positions "grounds" the chassis through a capacitor to either the + or - side of the AC supply. The capacitor may be leaking a little, this is a severe duty for a capacitor.

    Some amps don't have a switch but still have caps between the legs of the power supply, maybe if the amp wasn't grounded and one of these caps is leaking a tad...

    It's also possible YOUR amp is fine--and whatever you were touching wasn't grounded properly. Of course, if this occurred no matter where you were or what you were touching, that rules that theory out...

    Here's a good article, the last paragraph mentions instrument amps
  3. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    Q: Were you powering your pedals by battery or by power adapter?

    One of my guitar players used to get shocked constantly because the power strip that he plugged his power adapters into was missing the ground in its plug i.e some one had converted it from a 3 prong to a 2 prong. Gave him a new strip and everything was fine.

    Q: What head are you using?

    If the biasing transistor has developed a short it may casue some juice to leak to the what ever it is attached to, which may be attached to the chassis. I had that happen to me once with my SM400. It was however also accompanied by intermittent distortion.

    It seems like you are being careful but PLEASE be careful.
  4. Figjam


    Aug 5, 2003
    Boston, MA
    My guitarists old amp used to send an electric current through the guitar sometimes, or whenever you touch the amps knobs. Sometimes when you touch the strings it would zap you with that shock feeling that those palm-buzzers give you. It was a weird old peavey. Needless to say he doesnt use that amp anymore.
  5. Petebass


    Dec 22, 2002
    QLD Australia
    No switch unfortunately.

    In hindsight, it could have been the pedals. Now that I think of it, the shock itself was through the hand that touched the pedals, not the hand that touched the instrument cable. I guess in the hurry to be ready to do the gig, I didn't think of this as a posibility. I'll have to try it again at tonight's gig.

    Thanks. I'll have a read through that article for sure.

    Each pedal has it's own adapter (I find this eliminated hums and buzzes). They're all connected through a power strip which I'm certain has 3 prongs. It is quite old though, so it could be faulty. I'll try replacing it tonight and see what happens.

    An Ebony Quake 400. Australian made and so good, I actually own two of them. The main amp has been a real workhorse, It's done 100's of gigs without a hitch.
  6. ESP-LTD


    Sep 9, 2001
    Check the outlet you were using; maybe it's wired wrong.
  7. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    What was the floor material?
    Always plug the inputs first, then the warts into the strip, be sure the strip and your amp are fed from the same outlet, the last thing you plug in is the amp AC. If there is a fault you want all your peripherals grounded together first before you energize the system.

  8. I think that tends to only be in the US that switch is there on amps
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The ground switch is a relic from the days of 2 wire AC. When the chassis is grounded via the 3rd lug a groundswitch is unnecessary.
  10. Petebass


    Dec 22, 2002
    QLD Australia
    Last nights gig was at a different venue, I used my spare amp, and it happened again. I guess that rules out the venue wiring AND my amp(s).

    I forgot to bring the spare power strip to the gig (smacks head). I'll change it over today before I go to tonight's gig.

    The floor on both nights was carpet. The rest of your post is duly noted.

    Thanks for all your help so far guys.
  11. Ah right, its just you still only really see it in US made amps as over here at least we've been three prong for ages
  12. Weird. I assume its the pedalboard then.

    I'd try this with a voltmeter if you have one, but if not, feeling a shock (or not) is good enough test... :eek: :hyper: =bad, :D =good when you touch it...

    Plug the pedalboard into the wall. Plug a guitar cable into the pedal board, hold that and touch something grounded. See if you get a shock or AC voltage.

    Plug a cable into the amp, hold that and touch something grounded. See if you get a shock, or AC voltage.

    Thats it, nothing else gets plugged into the wall. One of them has to be hot to get you in trouble, and it happened with another amp, so the only thing left is the pedal board. Probably one of the power supplies is bad, putting voltage on the ground of one of the pedals. They're all powered by wall warts, so they're not really grounded, so connecting them together passes the voltage to all the pedals, waiting to shock you when you touch the ungrounded "ground" connector of the cord going to the the pedals and something else that really IS grounded.

    Now disconnect all the pedals from each other, so they're all isolated. Plug them all into the wall. Touch the ground of each one while touching something really grounded until you find the 1 pedal that is causing the problem.

    Unless you're getting a shock by holding guitar and touching a mic, then the PA is the one with the grounding problem...

  13. Petebass


    Dec 22, 2002
    QLD Australia
    Mystery solved!

    I replaced the power strip with a brand new one I had lying around, and ZAP, I'm still getting shocked. That rules out the power strip, so I started replacing individual power supplies with batteries one pedal at a time. Sure enough the power supply in the MXR M-80 was the culprit.

    I don't have a spare 9v adapter and I won't get time to buy one before tonights gig, so I pulled out the trust daisy chain. For tonight at least the MXR and the Boss EQ can share the same power supply. It seems to work OK at home (no electric shocks or audible buzzes) so it should be OK tonight. If worst comes to worst, I'll have a 9V battery as a backup.

    I occasionally fill-in for a band that has roadies to set up my gear for me. Funnily enough, while I was doing all of this, one of the roadies rang me and said "apparently your working with us again next week. Last time, your gear gave me a ZAP. Have you sorted that out yet? I don't want to touch your gear if it's still faulty".

    To which I replied "Yep it's sorted, but is there any reason you didn't tell me about the Zap at the time?" He just giggled and said "yeah perhaps I should have said something". Honestly I don't understand some people :rolleyes:
  14. Fuzzhead


    Sep 26, 2005
    I've got a spare adaptor if you need to borrow one Pete. :)
  15. jokerjkny


    Jan 19, 2002
    NY / NJ / PA
    wow, this thread is scary...
  16. I'm feeling rather psychic... :D

  17. Petebass


    Dec 22, 2002
    QLD Australia
    Thanks mate, but I won't need it. I just realised I have a spare PSA-240 in the office/studio/music room.

    And to think, I could have spent that time today putting up X-mas lights instead. Ah well, that will have to wait............
  18. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    This points out a good reason for having all your effects mounted on a pedalboard with a single high quality DC power supply for all. Not only would this ensure your safety, it would also likely clean up your sound having a well filtered and regulated DC supply.
  19. Petebass


    Dec 22, 2002
    QLD Australia
    I've got them on a pedalboard, but using a single power supply isn't possible. Not all the pedals are the the same voltage. Of the 4 devices, one is 12V, three are 9V, but one of those (the cheap little Zoom Multi effects unit) hums whenever it it asked to share a power supply.

    So I have to use 3 power supplies at the minimum. That's what I did last night and everything worked fine. I had the spare PSA240 handy just in case but didn't end up needing it.
  20. I've been shocked through my guitar strings as a result of using my Electro Harmonix Hot Tubes overdrive. It has a funny 2 prong plug that plugs into the pedal. Gave me a nasty buzz, but I forgave it because it sounds so nice!