Note:
The pickup shown here is NOT, at this time, a production G&L item. It is a test pickup - a prototype. Do not compare this to the wiring you have in your current (7/25/2011 and earlier) G&L bass pickups.Now that we have that out of the way:
See the green lead in the upper-righthand corner of the pickup. That lead connects from one end of the green/yellow coil to the baseplate of the pickup. At the lower-lefthand corner of the baseplate a long green lead is connected, and this is brought out of the pickup with the rest of the leads.
Good so far?
Put on your mind's eye glasses for a minute. In the "K" model pickups, the green lead at the upper-righthand corner is NOT connected to the baseplate, but is brought out of the pickup directly with the rest of the leads. A blue lead is added that would connect to the baseplate at the lower-lefthand corner, and this lead is also brought out with the rest of the leads.
For both pickup types, the green lead is connected to ground in the cavity. The blue lead is also grounded in the "K" models.
The baseplate, while copper coated, is ferrous and is there to apply certain affects to the magnetic fields of the underlying magnets. Because it is grounded it also provides a bit of noise reduction by shielding the backside of the pickup. The problem comes, and it is a very minor problem, when the
non-"K" pickup is re-wired to a 3-way switch to provide single coil. The baseplate has an extremely minor effect on the impedance (there's that word again, Fred) of the green/yellow coil. When in either humbucking mode (series or parallel), this effect is so small that it's not worth dealing with. When in SC mode, it can sometimes be noticed because one pickup will use the green/yellow coil and other will use the black/white coil. The green/yellow coil has the baseplate in circuit and the black/white coil doesn't, hence a tiny imbalance. So.... The "K" models add the blue wire to ground and isolate the baseplate, and the coils used in SC mode are now balanced.
Okay. Let's see how much trouble THIS gets me into.
Ken...