Why does my "ON" switch go both ways?

[J Hill]

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I hope there's somebody here that appreciates an untypical question of curiousity once in a while, maybe if only for entertainment...

My question is this: I've got a sort of large Evans Amp with a metal switch on the back. The "off" position is neutral (straight out) while pushing it to the Left... or the Right turns it on. Why is it made this way?

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[Jack Hanson]

Center position is "off." Left position and right position are both "on," reversing polarity. Basically eliminates the need for two separate "on/off" and "ground" switches.

[David Nugent]

If your amp is a hybrid (solid state pre amp, tube power section) this may apply. Normally on an all tube unit, the functions of the three way switch will be indicated as: 'Off', 'Standby, and 'On'. Solid state units do not normally require a warmup period which may account for the three way switch being wired with two 'On' positions.

[George Biner]

It sounds like this is the power polarity switch found in old amps. With it, you could select which of the two power wires was acting as the hot and which the neutral (return) (before 3-wire plugs), the equivalent of turning the power plug 180 in the wall socket. One of these settings would give less hum than the other. The middle position functioned as "off".

The introduction of the 3-prong plug eliminated the need for it and any older amp with only two power wires should be upgraded to three to get a safety ground onto the chassis to prevent electrocution.

[J Hill]

Was I supposed to understand all that?

[Jim Pitman]

J Hill, yes.

[George Biner]

My main message is that the switch was designed like that to have a way to reduce hum. If you haven't done it already, you should have a 3-prong power cord put on it for safety, and then it won't matter what side you throw that switch to.