Capacitor on ground switch in Fender Tweeds

[wcrankshaw]

I'm about wrap up my first 5E3 build, and I'm doing it by the book. So...what is the purpose of the 0.05uF cap from the ground switch to ground?

[JHow]

It was to allow you to select chassis ground to either hot or neutral to help cancel hum and avoid shocks. If you are using modern three-wire cord, you don't need this cap. There is lots of contention on the forum about this. Do a search for "death cap".

[wcrankshaw]

Sounds scary. I'll check it out.

[JHow]

That's just how people refer to it. Modern wiring practice is to use three wire plug, in which case, you don't install this cap. It is not part of the musical circuit, in any case, and since you are not restoring a vintage antique, you can leave it out. One less component to buy. I fall in the "three-wire" camp.

[Wilder Amplification]

Sounds scary. I'll check it out.

The purpose of that cap was to give the chassis an AC coupled reference to neutral. The switch allowed you to switch between the black and the white wire on the mains cord because plugs back then weren't polarized (i.e. one prong bigger than the other like they are today) and as such could be flipped and plugged into a socket either way. The switch was a convenience option...without the switch you would have to manually flip the cord around to ensure that the cap was in fact on the neutral line.

Where it got the name the "death cap" was due to the fact that if that cap were to short and the switch were to have the cap switched over to the hot wire instead of the neutral (or the plug were plugged into the socket in such a way as to allow the cap to be connected to the hot side of the mains), you'd end up with a live chassis on your hands and as such could suffer electric shock.

The 3 wire cords give the chassis a solid DC connection to the mains ground, which is then referenced to neutral via a bonding strap in the mains panel, so with a 3 wire mains cord this cap is not needed.