Do People Still Use a Two Prong Cord?

Discussion in 'Amp Central Station' started by Larmo63, May 8, 2019.

  1. Tonetele

    Tonetele Poster Extraordinaire

    Jun 2, 2009
    South Australia
    Haven't had a shock from a plug but WAS an IDIOT working on a refrigerator with power on. shocking experience.
    Good thing I was sitting on the floor feet closest to appliance and kicked myself free or I wouldn't be doing anything right now like typing this.o_O
    telemnemonics likes this.
  2. TeleFunk Man

    TeleFunk Man Tele-Afflicted

    Jul 1, 2015
    Gatineau, Québec, CANADA
    Yes, I do:

  3. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Mar 2, 2003
    Lawndale CA
    Has anyone else noticed that the OP disappeared after his one post??

    I assume if he asked the question he doesn't know how to do it; and he doesn't know how to work on any amp safely.

    The only answer in that case - take it to a qualified tech to have a 3-prong cord installed. It will have no affect on the amp's value unless it's museum quality (meaning it's in absolutely perfect showroom condition).

    The electrolytic caps MUST be replaced at the same time if original or over 20 years old. In fact, they are a bigger priority than the cord - a blown power cap can cook the power transformer.

    To the OP - if you're still here, PLEASE take it to a tech!
    LuthRavin and BobbyZ like this.
  4. Larmo63

    Larmo63 Tele-Afflicted

    Feb 1, 2018
    Timbuktu, Earth
    I didn't disappear. I've been reading the posts. I don't work on amps, I can assemble them, but I know nothing about the electronics of them. I'm fascinated by how smart a lot of you guys are about these things.

    I'm here, but I won't work on it myself.
    mgreene and BobbyZ like this.
  5. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    If you want to simulate what a two prong cord does from an electrical stand point. Your guitar's parts that are "grounded" are just hooked to the shield on the instrument cord, that's plugged into the amp's input jack and the shield is "grounded" to the amps chassis.
    So potentially the strings you're holding have the same voltage as the amp,s chassis. All my guitars do, but some don't.
    Now think about that "death cap" a .05 mfd @ 600 volts. It it's working right it'll pass 120 volts to the chassis, but not a lot of current. It'll shock you pretty good though! If it's shorted it'll pass a lot more current.
    However it depends on how you plug in that two prong to the wall, one way the "death cap" is connected to the common, flip it and it's connected to the hot.
    So if you want to eliminate all the extra stuff, stuff electricity just passes through and get a good idea what's going on, here's how to do it.
    Get you a .05 mfd @ 600 volt cap, to simulate a good "death cap" and straighten out a paper clip, that's your shorted "death cap".
    Now walk over to an outlet with one of those, and stick one end in the outlet and hold the other. (just the flat holes, no need to try the ground because with a two prong that's unused) Be sure and try both those flat ones.
    If your wearing shoes you probably won't get a shock at all, barefoot on concrete, you'll get a shock.
    Now put your lips on a microphone, plugged into a properly grounded PA and repeat the test.

    Does all that sound like a really bad idea?
    Of course it does!
    Even worse is that in most US amps use a single on/off switch on one wire coming from the outlet. The "death cap" doesn't go through the switch. That means if you plug it in "wrong", switch it off you can still have voltage on the chassis and the guitar. (I've never seen a vintage amp that didn't do that) So a kid that doesn't know might touch it while barefoot on concrete. My youngest daughter did exactly that on a Vibro Champ. Just scared the crap outta her fortunately, but since then I don't like two prong amps and won't have a "widow maker" amp in the house.

    It's funny how many people will argue on these threads but won't take a few minutes with a volt meter to get a good idea what's going on. Seeing 120 volts AC on a Telecaster's strings is an eye opener, lighting a light bulb off a Tele's strings is too, I've done that. :)
    King Fan, nojazzhere and peteb like this.
  6. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Nice write up Bobby.

    I used to believe this, but not any more, recently l received a couple big shocks and I don’t understand how it happened because I wasn’t grounded.

    First, I demonstrated to myself that if I wasn’t grounded I could touch a live tube pin with 400 volts on it.

    Then, under very similar circumstances, I was inside the same amp, it was turned off, I carelessly made contact with the same plate pin and I got maybe the biggest shock I ever got in my life. I was like, hold it, I was just the other day touching that pin with the amp turned on no problem, now it’s turned off and it gives me a big shock. I’ve never seen filter caps hold a charge like that. So I had to re do my original test, I turned the amp on and touched the pin. I got another big shock.

    My theory of what happened is either I was not grounded and I got a shock or I was was partially grounded and I got a shock. Either way, I was shocked.

    How close was I to ground? When I did not get shocked and when I did get shocked I think I was in the same place relative to ground. On both days I was wearing socks on top of a shag rug (poly) over a vinyl floor on top of a concrete slab.

    The big difference between the conditions of the two days when I got shocked and when I did not get shocked was the second day when I did get shocked was much warmer, I had been doing some kind of physical exertion outside and I was perspiring. So skin condition makes a big difference.

    I still don’t know what the ground path was. Was there a partial ground path thru the rug and the flooring? Or does a person have an internal ground?

    This experience changed my whole outlook on shocks and grounding. There is some vital part of it I don’t understand.

    So be careful everyone and follow the advice of the experts.
  7. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

    Mar 17, 2003
    Spring City, Pa
    For years, I touched the strings to the mic. If I heard/saw a spark, I flipped the polarity switch.
  8. nojazzhere

    nojazzhere Poster Extraordinaire

    Feb 3, 2017
    Foat Wuth, Texas
    Yeah.....the problem with that is (as BobbyZ pointed out) if you flip the polarity switch, all you're doing is matching the amp's and the PA's polarity......maybe the PA is incorrect. If it is, you can still get shocked if you touch something else that's grounded. You need to make sure the amp's cord is "oriented" (hot and common) right.
    A guy I used to play with always checked outlets BEFORE we all plugged might be surprised how many electrical outlets are incorrectly wired.......
  9. 66tele

    66tele TDPRI Member

    Jul 26, 2014
    The Dalles, Oregon
    OK....some info......on the AC wall outlet, the wide slot is the neutral and the narrow slot is the hot (if the AC outlet is wired correctly to current standards.....good to check this with an AC voltmeter).

    To correctly re-wire an amp with a 3-prong cord......the green ground wire should be fastened to the chassis....often at a mounting bolt of the power transformer. The neutral wire (can be different colors, depending upon the cord.....measure continuity from the wide prong to the bare wire) should be connected to one side of the primary of power transformer (doesn't matter which one).

    The other side of the AC cord (hot lead) should go to the fuse (at the end terminal of the fuse holder) then connect the other fuse terminal to the power switch.

    The other side of the power switch should go to the other primary of the power transformer.

    Clip the "Death Cap" from the polarity switch. The two other terminals on the polarity switch can be used for making connections from the fuse to the power switch, and the neutral wire to the power transformer, if you wish.

    Also.....if you retain the 2-prong convenience outlet, be sure that the wide slot is connected to the neutral and the narrow slot is connected to the hot lead. Another option is to replace the 2-prong convenience outlet to a 3-prong outlet and connect the ground prong to the chassis.

    I've seen lots of 3-prong AC cord conversions done incorrectly. And often, the original 2-prong AC convenience outlets are wired backwards from the factory. FYI......
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