Hot Wiring Your Guitar
Series Wiring of Pickups
Figure 1 shows the most usual wiring of a standard two pickup, two tone and two volume guitar, reiterated for those who are either unfamiliar with it or who missed earlier articles. The principal features to note in connection with this article are the ways in which the volume controls and pickup selector have to do their jobs. The selector switch needs to be a special type, in which all three contacts are connected together in the switch's mid position (to put both pickups on) rather than the normal "centre off" variety of three position switch.
Second, the volume controls work by shorting the pickups out; for the technical reader, a guitar pickup looks like a voltage source in series with an inductance. When fed into a low impedance, for instance a turned down volume control, the result is a progressive loss of treble as the volume is reduced; a well known feature of guitars with this wiring.
To see why the controls have to work this way, refer to Figure 2 which shows an alternative circuit used on some guitars. At first glance it may seem identical to Figure 1, but the difference is in the volume controls which are wired the other way round. Now, they don't short out the pickups — so no loss of treble — but they do short out each other!
Imagine that both pickups are on at the selector, but volume control 1 is at minimum; you'd expect pickup 2 to come through loud and clear, right? In fact, nothing at all comes through because the signal is shorted through volume 1. Figure 1's wiring avoids this at the expense of loss of tone: each circuit has its pros and cons.
Figure 3 is a completely different method which I've not seen used commercially, but have employed successfully on one of my guitars. The pickups are in series rather than in parallel, and the selector switch works by shorting out the unwanted pickup. This time, the other pickup is unaffected, and the switch can be an ordinary centre off type so that the centre position leaves both pickups alone.
This circuit has pros and cons of its own, of course; apart from the simpler and cheaper pickup switch, there is no shorting problem with the volume controls. Shorting is what's required this time, after all! The treble loss is not as bad as with Figure 1, but there still is some; because the pickups are in series, the signal from each one has to find its way out through the other if it's in circuit. In technical terms, the inductance has been doubled; on the other hand, the output will have been doubled too, and with a decent amplifier and a not too long output lead, a full meaty sound is the result.
When constructing this circuit, the important thing to remember is that the 'top' pickup (no. 1) and its circuitry aren't earthed at all, so the pickup you use must not be of the sort where one side is connected to the screen of the output cable. Similarly, the controls associated with this pickup should have their cases earthed as usual, but none of their terminals should be earthed. In some cases, this means that you won't be able to re-use the old pickup or controls.
As for values, 500k for the volume controls with 250k tone controls and 20nF capacitors should be a good starting point. As always, experiment to obtain the sound you want. The switch is a SPDT centre off type, but should be a type meant for low level signals, preferably with gold contacts. Some mains rated switches, although physically robust, have quite high contact resistances and aren't reliable enough.
Any of the coil switching arrangements described in previous issues can be included, but unfortunately stereo wiring is not really a simple proposition.
Feature by Peter Maydew
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