House of Hindenach LLC
donh Safety Circuit
Modern guitars with passive pickups, just like their fathers and grandfathers, have their bridges and strings tied to signal ground.
While this may help reduce noise when you are touching the strings, it also creates a condition where you can find yourself knocked on your butt (or worse) if you come in contact with any other gear that has an electrical fault condition.   When this happens, you are finding yourself a conduit for wall current as it heads for ground.
There have been a few efforts to mitigate this possibility, most notably the circuit I found attributed to Adrian Legg in a Dan Erlewine book about guitars.   Adrian suggests taking a 220,000-ohm resistor, paralleling it with a .001micro-farad capacitor, and inserting it in series with the ground wire coming off the bridge.   This helps, but it's still a bit noisy and does nothing to address the guitars with metal control plates and/or metal knobs.   So I decided to look into it a bit further.
I found by experimentation that increasing the parallel cap to .022uf knocks the noise down to almost nothing, and still keeps the current to 1/10,000 or less of the 15 or 20 amps you could encounter off a typical 120-volt circuit.
I then decided to put every possible metal piece on the guitar on the other side of this circuit.   So I had my pickups wired so any covers or back-plates had their own separate grounding-wire run alongside the signal cable, inserted my safety circuit between the low-side solder lug on the volume control and the body of the potentiometer, tied all the grounds and shielding elements to the body of the pots, and tied the pot bodies all together.   This way, the only way I could get poked would be if i decided to grab the output jack itself while playing.   Then I rewired my SG, my two Tele-clones, my Strat clone, and my Fender P-Bass.
When I played them as a test after the rework (ya gotta make sure ya didn't mess something up), I swore they were not only a lot quieter, but they actually sounded significantly better.   Scratch head, keep moving, do the next one.   (more on this later)
So here are some simple examples of the circuit, as wired for a P-Bass, dual-humbucker, and Tele-style guitar:
P-Bass or single-pickup guitar
Dual-Humbucker Guitar or Bass - note that the pickups are four-wire, with the shield run to the pot bodies
Standard Tele-type -- please note that pickups should be three-wire, with the shield (non-signal) wire tied in with the bridge-string ground to the pot bodies.
Standard Strat-type - strat-type single-coil pickups are native two-wire and lack the back-plate or metal pickup cover that would require a seperate shield wire.
Thanks to Emmett Brown, known as Deaf Eddie on TDPRI, for these drawings.   Check out his website!
Here is a variation from Bob Urban, Columbus, Ohio
This one is for an Esquire
Position 1 has Vol + Tone
Position 2 is Vol/Tone bypass with an optional series resistor
Position 3 is a kill
Just the peace of mind of knowing that you can reduce the current from an electrical fault from 15 or 20 amps to less than 1.5 milli-amps is reason enough to do this simple mod.   It's certainly why *I* did it!
So what's the deal with noticing it actually sounded better?
My thinking is that by moving all the metal bits to the other side of the safety circuit, I was also electrically moving them off the signal chain.   They were all little antennas creating parasitic signal degradation and I reduced that problem, thus achieving a significant tone recovery.   It's unnecessary to take this on faith.   All you have to do is wire up the guitar as suggested and then take a clip lead from the output jack to the bridge, thus bypassing the safety circuit, and have a friend attach and detach it while playing.   It gets obvious.
There is a whole school of thought that feels pickup covers and shielding dampens their guitar tone.   The tone recovery action of this mod lessens or eliminates that effect while actually improving the shielding action by moving the shielding electrically away from the signal path.
Every person I've done this mod for has been extrememly happy with not only how quiet their guitar has suddenly become, but how much the tone has improved.   Not changed, improved.   This mod can't add anything to what the original pickup maker did, it can only allow more of what they did to come through.
The parts are cheap, costing me less than a buck (USA dollar) per guitar.   I strongly suggest using at least a 250-volt rated poly .022uf capacitor and a quarter-watt or larger 220k-ohm resistor (half-watt if you live or plan to play in Europe where they have 220-volt mains).
Please feel free to pass this info around, all I ask is that the thoughts be attributed to me when you do so.
Lawyer bits:   Please note that the article above only states that you are likely to be exposed to less current than other wiring schemes.   House of Hindenach LLC, avjoy.biz, and D W Hindenach personally make no explicit warranties of usability for safety whatsoever.   The parts, their function, and their installation are your responsibility.   Any exposure to electrical current due to your personal activity is entirely at your own risk, and your own responsibility entirely.