Hot-Wiring your Guitar
Series/parallel and pick-up phase reverse
Series/parallel is an alternative feature to coil tap that is included in quite a few production guitars using humbuckers, and a useful item it is too. The difference between the two is simple. Both will give the normal series coil humbucker sound in one position, but where the coil tap cuts out one of the coils, probably by one of the methods I've shown so far, the parallel setting alters the coil wiring so that effectively, instead of a humbucker, you have two single coils operating in parallel. Each coil has its own output and earth, though the pair still use the same magnetic field, and remain out of phase, so are still hum cancelling. This gives a thinner, more toppy sound than series, but is still not absolutely the sound of a single coil pick-up or a tapped humbucker.
The most common set-up requires a three or four conductor and shield humbucker, and an on/on DPDT switch. Where the overall phase of the pick-up is not going to be reversed after the series/parallel switch, a three conductor and shield type will do. But where phase reverse is required, then the in-phase earth end of the coils has to run via the series/parallel DPDT to an on/on DPDT cross-wired so that in reverse phase, this earth-end of the coils will become the output end. Figure 1 shows a four conductor pick-up linked to a series/parallel DPDT, and on from there to a phase reverse DPDT. This set-up can easily be extended to include a coil tap, either partial or complete, by running a wire from terminal 6 of the series/parallel DPDT to any of the earth type taps I've shown so far.
Obviously, the most sensible one to go for in a control situation beginning to get complicated would be the tone control spare leg tap. Where phase reverse is not required, the wiring would be as in Figure 2. I use here for convenience the colour codes as used on Dimarzio and Velvet Hammer four-conductor pickups. The extra tap here would operate when the DPDT was set to series. Parallel would be unaffected. This gives you an interesting A-B comparison facility on the DPDT between parallel and tap, and the opportunity to draw your own conclusions about the relative values of the two.
The interesting point about phase reverse in conjunction with an earth-type tap, either via the series/parallel DPDT, or direct tapped from the coils link, is that phase selection determines which coil is tapped. Figure 3 shows phase reverse in conjunction with a tone control earth-type tap. In normal phase, coil 2 will be turned off, and in reverse phase, coil 1 will be turned off. In my experience, this extra range of tone is of most use on the bridge pick-up. There are taps where this does not apply but they're for another time. Note that in Figure 3, both the black and the white wire run to the tone control, the series link is achieved from 0-9, when there is sufficient resistance to prevent the link between the coils shorting to earth. As before, from 9-10 the tap operates.
Beware when dealing with multiconductor and shield production "custom" pick-ups. Schecter and Lawrence humbuckers for example, are normally wired for phase reverse and tap as in Figure 4, so series/parallel switching is not possible without rewiring the conductors. Others, like the earlier Dimarzio Dual Sound, were wired as in Figure 5 for series/parallel and/or tap, but not for overall phase reverse. In the latter case though, one could always tap out coil 2, and reverse phase on coil 1 only to get an out-of-phase mix with another pick-up. More about coil phase reverse later on. The reason that you cannot reverse overall pick-up phase on the latter type of three conductor and shield is simply that in reverse phase, the shield would become the output, and if that didn't simply earth out somewhere along the line, it would be horribly noisy. These Dual Sounds are wired four conductor nowadays, and the simplest way to get all the options is to rewire a humbucker with less than four conductors up to four, using a single separate shield to earth the pick-up base and hardware.