This is a fairly old modification
that most people know about, but almost no one has installed in their instrument
that I'm aware of. It does not affect your sound at all, but it is
reputed to be a potential life-saver.
Basically, this mod isolates yourself from
your "string ground." A guitar has two grounds, one through the output
and another ground that, particularly on Stratocasters and other guitars
with tremolos, grounds to the strings. Typically this is through
the tremolo, then the springs, and finally to the spring claw where the
wire is almost always attached. This is why you can ground out a
bad cable by touching the strings, bridge, or tuning keys of most guitars.
To read more about this subject, check
out the All About Grounding page.
I won't bother with an actual
schematic here because this is another very simple (and cheap) modification.
The idea is to isolate the rest of the guitar's electronics from the string
ground. Between these two sections you wire in a small value cap
and a relatively high value resistor in parallel between the string ground
(i.e., connection to the bridge) and the rest of the circuit. A typical
set of values might be a 0.02 µF capacitor with about a 250k Ohm
How it works
If you happened to be holding
a typical guitar in your hands at the time when ground in your amp happens
to fail, you would become the path of least resistance to the ground.
With the modification described above in place, the resistor blocks the
full current from reaching you (well, maybe; personally, I have some doubts)
although the capacitor would allow you to feel the voltage change.
If you feel something tingling (or worse), you would probably think to
get away from your gear before it gets you.
So how could your ground go bad?
Well, I'm sure there are other ways, but water will do the trick.
That will short the circuit under the right (or wrong!) circumstances.
I witnessed this first-hand when I went to watch a friend of mine play
several years ago. He and his band were playing under a bandstand in a
park after it had rained. Even though things were sheltered, the
ground was still very wet and it was very humid as well. Throughout
the night, enough errant charge was able to arc through the circuitry to
repeatedly shock my friend. At first we thought he was just putting
us on and spazzing out for effect, but apparently this really hurt... though
not enough to stop him from playing for more than a couple beats.
Thankfully, I've never experienced this.