Ground switch?

Discussion in 'The Workbench' started by Kunnz, Sep 11, 2010.

  1. Kunnz

    Kunnz Active Member

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    Hi, A couple of questions.

    Can any of you give me an explanation of what the ground switch does on an amp and do I need have one?

    My 1969/1970 Marshall doesn’t have one but I see pics of other amps like mine that have three rocker switches on the front panel; is one of those the ground switch?
    Also my Fender Vibrosonic reverb doesn’t have one either but looking at the Fender specs it says it should have a 3 position ground switch. What’s the deal with a three position ground switch?

    I could go and google this but I want to hear it from the workshop tecs first for a ‘sound’ explanation.

    :)
     
  2. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Hey, Kunnz!

    In the olden days before 3 prong, grounded and polarized outlets and power cords, power cables could be inserted either way into the back of the chassis, and conversely the plugs weren't polarized, so they could also be inserted either way into the power outlet. This, in turn, made it so that the ground reference could thus wind up on the hot OR neutral lead - depending on which way either end of the power cord was inserted. Thus, the switch was provided (with a cap to ground on the center pole - the dreaded "Death Cap") to correct for this. Basically, the user could just flip the switch to either position - whichever had the least hum. In the amps I've seen with 3 position switches, the center position is a ground lift - in essence the switch is an ON/OFF/ON switch.

    Ground switches are no longer necessary for the aforementioned reasons...polarized plugs and ground lugs. These switches are dangerous, and should be disconnected from any amp they're in - in tandem with installing a 3 pronged plug (observing proper polarity). The danger lies not only in the unwise accidental grounding of the wrong lug, but also in the "death cap" failing and shorting to ground (the chassis), which is, needless to say, all kinds of BAD.

    As a side note, I once repaired a Fender Twin Reverb Reissue, and the ground switch was installed, but not connected to anything! I was incredulous, and checked the schematic - sure enough, it was drawn by itself to the side of the main circuit...with no connections indicated. It was there simply for vintage layout accuracy. Truth is really stranger than fiction, no? ;)

    -Lane
     
    Last edited: Sep 11, 2010
  3. MajorNut1967

    MajorNut1967 New Member

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    Good response Lane, nothing more need be said.
    +1
     
  4. chuckharmonjr

    chuckharmonjr Well-Known Member

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    In the bad old days, if you're ground switch polarity was different from the PA's...if you made the mistake of touching your lips to the mic while playing you'd hit vocal notes you never thought was possible...lol...that shit HURT!
     
  5. Kunnz

    Kunnz Active Member

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    I see now, so the metal frame of the amp and me! could be live with respect to a true ground.
    Thats why I was always getting a stinging zap from the mic in some of those bars I played in NY. In some of those places the armour of BX doesn’t make a good enough ground either if you could find an outlet receptacle with a ground plug on it. I checked those damn amps a hundred times and swore that it was the bars fault and I was right.

    At least I got free beer for making loud noise, LOL.

    Cheers
    :cheers:
     
  6. Ken

    Ken Well-Known Member

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    Even today I NEVER touch a mic before I do a quick hand slap while holding my guitar. Lots of vinatge equipment is still out there being used, like my Music Man.

    Ken
     

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