Turtlerock Forum

Text_Edifice wrote:I assume it stops your body acting as a conductor in case of a short?

Text_Edifice wrote:Do you do this with all your guitars or is it just in cases like this where you use series wiring?

Guitarnuts wrote:I've had a few people ask why I specify a 400V capacitor when guitar circuits usually only have a few millivolts on them. The reason is simple, that capacitor is what stands between you (via the grounded strings) and your amplifier. Some vintage tube amps could fail such that they put a potentially lethal DC voltage on the "ground" side of the jack. This type of failure is extremely rare, but why take the chance? The 400V capacitor won't break down like a 35V capacitor would in the admittedly extremely unusual case where you might plug into a "killer" amp. A 250V capacitor is also acceptable as it will probably stand up to 300V or so for at least several seconds. Note that the OEMs don't seem all that worried, most guitars have the strings grounded directly without any isolation.

On most guitars, the jack is a non-insulating type which means that the jack plate will be directly connected to the negative side of the jack even after this modification. Metal control knobs may also still be directly connected to the negative side of the jack even with the isolation capacitor in place. Finally, even with the capacitor, a high DC potential failure will give you a nasty "bite" while the capacitor charges – but the capacitor serves to limit the duration and severity of the shock.

You can omit the capacitor entirely (replacing it with a piece of wire) if you're confident you'll never plug into a malfunctioning vintage amp and if you check the house wiring religiously everywhere you play.

Note that the .33uf capacitor recommended here is really intended for protection from DC from a malfunctioning vintage amp. It will only provide minimal protection (though more than most stock guitars provide) from an AC shock caused by improper house wiring. See the very important article on electrical shock.

The .33uf designation is not critical. You could easily get away with a 0.47 or 0.22uf capacitor. The larger the value the better, up to a point (except that larger values will provide less protection from AC shock). If your control cavity is tighter than normal, you might want to go with a 0.1uf 400V because it will be physically smaller. I wouldn't go much lower than that though or you will start suffering noise.

You can also isolate only the bridge and strings by tying the shield ground directly to the signal ground, but isolating the bridge ground wire from the shield by an 0.1uf 450V capacitor. That capacitor will be much smaller and furthermore it can be mounted in the bottom of the body cavity. This method provides a little better protection from AC shock (still not foolproof) via the strings. However, if you use metal knobs on the pots they will not be isolated. Also, the metal shaft on the switch and all the pickguard mounting screws will not be isolated either. Still, this method provides good isolation for the bridge and strings, good noise performance, ease of construction, and it's still safer than most factory guitars.