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howe0168 3rd October 2012 01:44 PM

Capacitor from 120VAC to ground
Hi, I've noticed a lot of old amps have a .05-600V or so capacitor going from the hot side of the 120V to ground. I was under the impression this was meant to filter out noise but I rarely see builders do this anymore...any reason why? Am I missing something?


DF96 3rd October 2012 01:58 PM

The purpose of this capacitor was to ensure that the switch outlived the guitarist. At first it would reduce arcing so prolonging switch life, but then it would fail and electrocute the player.

The correct way to minimise switch arcing is to put a lowish value (10nF?) X-rated cap across the mains transformer primary (i.e. after the switch). The cap reduces the voltage developed across the opening contacts, and then the transformer resistance dissipates the stored energy as heat. A snubber can also be used.

Any cap from mains to chassis must be Y-rated and even lower in value; in practice it may be better to buy a packaged mains filter.

howe0168 3rd October 2012 02:09 PM

Thank you.

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