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OLD-TIME GROUND SWITCH

 

A lot of older guitar amps, like early Fender® models, have a 2-position ground switch that creates a shock hazard when you're holding a connected guitar and you touch something grounded like a vocal mic.

The following schematic shows how these old ground switches are wired.



2-position ground switch schematic



The ground switch puts a capacitor in between the amp's metal chassis and one side of the AC power cord.  The capacitor is there to help filter out buzz caused by noise on the AC line.

But in one position of the ground switch, the capacitor sits between the hot side of your house wiring and the amp's chassis!  (Follow the red path in the above diagram).

As a result, the chassis floats at a voltage that's neither neutral nor grounded.  And since the guitar and its strings connect to the chassis via the guitar cord, the player takes on that voltage too and gets a shock when touching something electrically neutral.

 

GROUND SWITCH REMOVAL

 

To eliminate the shock hazard, you can remove the ground switch as shown below.  For added safety, change the 2-wire power cord and AC plug to a 3-wire cord and plug, connecting the ground pin to the chassis as shown.  This insures that the amp's chassis (and therefore your body) are always grounded.



No Ground Switch = No Danger

 

If there's a 2-pin AC extension outlet on your amp's rear panel, you might want to leave it there but disconnect it.  If you must use it, consider replacing it with either a 3-pin socket or a polarized, 2-pin socket.

 

COMPROMISE GROUND SWITCH

 

If you want to keep the ground switch because of buzzing issues, you should consider installing a 3-position ground switch instead, as shown below



A Center-Off Ground Switch Gives You A Choice

 

Set the 3-position switch to the safe, center-off position most of the time and only activate it when you absolutely need it.  You can use a "center-locking" toggle switch to make sure it doesn't get flipped accidentally.




WIRING NOTES:
  1. The black lead of an AC cord is the hot lead, the white lead is the neutral lead, and the green lead is the ground lead.  (The wider blade of the AC plug is neutral and the narrower blade is hot.)

  2. The hot lead usually connects to the fuse holder (see above).  For added safety when changing a blown fuse, connect the hot lead to the holder's terminal farthest from the fuse cap.

  3. If the primary coil of the power transformer has one black wire and one white wire, connect the hot line to the black wire. Otherwise use either wire.

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