ChrsiP's JTL-1 "The Sphinx" - 1st Build

[Bakersdozen]

Can you please elaborate on this:
"630V DC safety cap separates hardware from cable negative"
Awesome build by the way mate!!

[chrisp]

Can you please elaborate on this:
"630V DC safety cap separates hardware from cable negative"!

Sure.

To begin, most guitar wiring confuses two separate circuits - ground and signal negative. The ground path includes things like the strings, bridge, control bodies and cavity shielding, and it's main purpose is to shield the signal from RF interference. The signal negative, on the other hand, is the reference 'zero volts' against which the pickup circuitry's voltage signal is measured. If everything goes right, you can simply connect these two paths together - and every time you see a potentiometer lug bent back and soldered to the pot body, that's an example of connecting the ground and sig neg paths.

In the vast majority of cases, connecting the two together doesn't matter. It all gets connected to ground in the amp via the guitar jack sleeve connector. But what if things go wrong ....

Some older tube amps can fail in a way that puts their HT (typically 300-400V DC) on the signal negative circuit, which in turn can make anything connected to that circuit dangerous. If your wiring connects ground and and signal negative, you've then got 400V on your bridge and strings, which might find its way to ground through you!

This mod is achieved by connecting all the ground circuit elements together, and all the signal neg elements (including the jack sleeve) together, but never having a direct connection between the two. Instead, you solder a 630V high value capacitor between the two paths. Caps will block DC voltage but pass AC voltages, so the RF interference (AC) will get shunted off to the amp ground but any DC volts finding their way onto the sig neg path won't get passed on to the bridge, strings or controls - it's likely that something else will blow first, rather than you getting fried.

The risk is not at all likely in modern amp design, but I do play with some older tube amps and its a really simple mod in any event. It has no effect on tone or volume, its really nothing more than a 20c bit of insurance against something that's not likely to happen. But in my view that's 20c well spent!



You can see the capacitor at the selector switch in this diagram of my build, but it can sit anywhere between the ground and sig neg paths that's convenient.

Awesome build by the way mate!!

Thanks. Your PRS is looking very pretty itself.