Vintage amp with two prong, non-grounded plug - how dangerous?

Discussion in 'Amps and Cabs' started by backdrifter, Jan 31, 2012.

  1. backdrifter

    backdrifter Supporting Member

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    Hello guys and gals,

    Electrical is my weak spot and I'm hoping for some help from the experts (or at least those better informed than myself :)). I've got a late 40's/early 50's Gibson amp that I acquired from my grandfather. It has the original two prong power cord. I've read that this can be dangerous and it is recommended that you swap them out for a modern grounded plug to avoid the risk of electrical shock.

    I currently keep the amp in the basement and have never had any issues. I like the idea that the amp is 95%+ original, so I haven't been motivated to swap out the plug.

    But now, I find myself playing it quite a bit more with a newer guitar and have decided I would like to bring it up into the living room. My biggest concern is that once in the living room, it will be in proximity of my 3 year old daughter. How dangerous is the two prong plug? If there is any risk at all to my daughter, it's a no-brainer and a done deal, I just wanted to get some input first.

    Thanks in advance!
     
  2. Jef Bardsley

    Jef Bardsley Member

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    There's a simple explanation here:

    http://electronics.howstuffworks.com/everyday-tech/question110.htm

    So, IMHO, the danger to your daughter is minor. The thing you need to worry about is her getting in the back and touching hot tubes.

    The danger to you is slightly greater, as, if the amp melts down and drops 120V on the chassis, that's connected to the ground in your cord, which is connected to your strings..... and the amp is unlikely to melt down unless you're playing it.

    All of this assumes your living room carpet is flooded with water, or you have a death grip on a cold water pipe... or some stupid appliance that has a grounded case. If you're not grounded, you can touch hot wires all day and no current will flow. That is, in most cases (PTP!), 3-prong plugs increase the opportunity to get electrocuted by providing more available surfaces at ground potential.

    To be really clear, it would be silly to add a grounded steel case to an amp in a wooden cab and you'd need to cover the front panel with, say, Plexiglass. Otherwise, a 3-prong plug would have no advantage. As the article states, in a three wire circuit, two of the wires are connected at the fuse box end. If they go to the same place at the other end, it's pointless.
     
  3. Peteyvee

    Peteyvee Premium Platinum Member Silver Supporting Member

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    I've been zapped by a two prong plug equipped Gibson amp from the 60s (in a wooden case) on a carpeted floor, while wearing shoes. Roll the dice if you want. I'd get in changed over if you're going to use it.
     
  4. mbeards

    mbeards Member

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    It is always a good practice to add a 3 plug cord to any and all vintage amps. If something goes wrong with the amp and high voltage gets sent to the metal chassis (and through the guitar cable to your strings), any dangerous voltage will go to ground directly because it has less resistance than your body, effectively a short in comparison. Then the fuse will pop and the amp will shut off.

    I was once debugging some noise on a DIY tube stereo preamp for my ipod while it was on. I accidentally shorted out the wall voltage to the chassis with a screwdriver while holding the metal chassis. Lucky when I designed the thing, I put a fuse in the AC wall line and had a grounded plug. There was a huge spark but the voltage went through the chassis into the ground plug and blew the fuse inside of amp due to the increased current draw instead going through my hand and body to ground.

    So yeah a 3 prong plug is always good practice.
     
  5. charveldan

    charveldan Senior Member

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    Get a 3 prong for sure. :agree
     
  6. '58Bassman

    '58Bassman Member

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    If the chassis is hot and you're touching the strings or anything else that's connected to the guitar's ground, you'll be the load if you touch anything in the building that's grounded. This means, if you touch the strings with one hand and ground with the other, the current can pass through your heart and you'll be electrocuted.

    A little tingle is one thing but dying can be avoided.
     
  7. GrungeMan

    GrungeMan Supporting Member

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    :huh

    Not sure what you are talking about?
    Did I read this guys post right???

    Most amps have a metal chassis and the PT's ground lead is usually attached to a PT bolt(the bolt(s) is used to attach the PT to the chassis), the ground wire from the 3-prong cord is also attached to the chassis most likely at the same spot the PT bolt so there is no ground loop which could add hum to the circuit.

    Live or Neutral(common) do not touch the amps metal chassis or metal receptacle/light switch box in your house wiring at all.

    Hopefully ones home is wired with a proper ground therefore it's less likely to have any deaths from electrocution.

    I touch the metal chassis's of my amps all the time and suffer no ill effects because my amp and home are properly grounded.

    Be safe and install a 3-prong cord and make sure your house/home electrical circuitry is wired and grounded properly as well.

    3-prong plug/receptacle
    White wire-neutral/common-wide blade
    Black wire-hot-narrow blade
    Ground-bare wire/green-round blade

    Grungy :D
     
  8. charveldan

    charveldan Senior Member

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    Early 82' i was playing a gig in a lower level through a 50 watt Marshall with a 3 prong adapter that wasn't grounded, i was barefoot on concrete and stepped in water/liqiud......all i can say is i'm fortunate i wasn't killed. :omg:omg:omg:omg
     
  9. modernp

    modernp Member

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    Why would you choose to make yourself the ground. Been bit bit hard in the old days by ungrounded amps. Especially when your lips touch a microphone.
     
  10. Jef Bardsley

    Jef Bardsley Member

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    So, would any of you like to explain just where to connect the extra ground wire to make the amp safer?

    Backdrifter, I applaud your concern for your daughter's safety, I just don't want you lulled into a false sense of security.
     
  11. ROKY

    ROKY Member

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    Ask Keith Relf .
     
  12. Ramblin Hymns

    Ramblin Hymns Member

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    The third prong will help to make sure the polarity is right if hooked up right. I could go into the whole explanation but try walking across a HVAC vent holding your guitar by the neck touching the strings. Make sure the polarity on the amp is wrong and you are barefooted. You will see very quickly why the third prong will help. I really don't advice this because I did it not thinking. You can connect the gnd to the chassis. If anything shorts to chassis it will go to gnd. The neutral and gnd are two different things. At the power companies xformer,if you don't tie the neutral to gnd the neutral will eat you up and it has voltage on it. If you get between the neutral and gnd at the xformer it can kill. That is where the neutral and gnd needs to be tied together. and so on
     
  13. modernp

    modernp Member

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  14. kiki_90291

    kiki_90291 Supporting Member

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    Unless you can levitate, when you are standing on any surface, you're grounded.
     
  15. '58Bassman

    '58Bassman Member

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    The neutral is tied to the ground at the electrical panel, not at the transformer. Service entrance is supposed to have appropriate ground stakes, as deep as required by local code or the NEC and the ground conductor is supposed to be unbroken from the panel to the far ground rod. A residential service feed has three conductors- two hot (these provide your 230VAC for electric stoves, furnaces & water heaters) and one that serves as the neutral/ground reference. The service is grounded from the panel.
     
  16. gatordoc

    gatordoc Member

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    Put it this way, every time my heart rhythm feels a little abnormal I just plug a Strat into my trusty old 2-prong '66 Vibro Champ and hit the switch.........best little defibrillator around and it even has that retro 60's charm.
     
  17. backdrifter

    backdrifter Supporting Member

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    Thanks for the responses, everyone. Sounds like a quick and easy fix, and one that's not worth putting off.

    Sadly, I'm an engineer by education (chemical) and took a few electrical engineering classes, but electrical has always been a rough subject for me.

    Three prong plug it ts!
     
  18. Ramblin Hymns

    Ramblin Hymns Member

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    Hey 58 Bassman,Not to get really involved but the Xformer is grounded. I've been an electrician for over 40 years. I worked high voltage for last 15. 25000 volts. The xformer is grounded as is the panel. In the panel the grd is seperated from the neutral. You are tied at the xformer in the new systems. The grd bar at the panel goes to the grd field and usally the nearest structural ground.That separates the neutral and ground at the panel. NEC does not allow you to wire the neutral and ground together in the panel or the outside disconnect for said structure. I'm not saying it wasn't done in the past but not now. It has to do with computers and noise in the lines and safety.Now if you want to get into some weird problems,play with an open neutral on the power company side. It can fry the electronics in your house. By the way,at the university I worked at we used 7200 or 15000v for the primary. It doesn't jump like 25000 does and is safer. Oh yea NEC supercedes local and state codes. Also panels are four wire. That is the reason you see four wire stove hookups now. I do understand the way people get around the 3 and four wire hookups. I've done it myself.
     
    Last edited: Jan 31, 2012
  19. GT100

    GT100 Member

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    Yes, the only difference is that if you are simply standing on the floor v.s. say holding on to a pipe is that the electrical shock will be less.
    But is it safe? Not likely and I wouldn't want to find out the hard way...

    Lloyd
     
  20. gatordoc

    gatordoc Member

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    I have never been shocked through the guitar itself but have taken a couple of good jolts directly from the amp over the years when I switched it on and just happened to touch the metal face plate at the same time - yes, it's a lot of amps, wearing sneakers did nothing, it does really hurt, and you won't want to do it again.
     

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