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Hot Wiring Your Guitar

The electric guitar is a simple beast, indeed in terms of today's micro-chip technology, downright primitive. I don't propose to change that, I like it, but what I will do in this series of articles is show you how you can exploit an existing wiring system more fully in order to make your guitar capable of coping with a variety of tonal demands. A couple of humbuckers can give a wide range of sounds, and can be persuaded to do so without going active, although ultimately, dropping output impedance will help the treble you achieve on the guitar to make it to the amplifier.

Firstly, it'll all make a bit more sense if we understand a few pick-up basics.

A single coil pick-up consists of a coil wound around a bobbin, and in the centre of the bobbin are either six magnets, one bar magnet, or six poles that conduct the magnetic field from magnets underneath the unit. The wires from each end of the coil are connected to two stronger wires, or a single conductor and shield.

The vibration of the string (which contains nickel) over the pick-up excites the magnetic field, and generates a minute alternating current, the frequency of alternation of which is exactly the same as the frequency of alternation of the strings. That is, simply if you twang A 110, the string is vibrating 110 times per second, so the current from the pick-up will alternate positive to negative 110 times per second. This current, when suitably magnified by the amplifier, will push the speaker cone back and forth 110 times per second, and a beautiful, loud A will sound as the air between the speaker and your ear is vibrated 110 times per second. Do not believe the occasional expert who tells you a pickup gives D.C. - guitar mythology has to stop here.

The variation in the number of windings, choice of wire, type of magnet and so on give the pick-up its power and tone characteristics, and single coil pick-ups are available that give you the option of earthing out part of the coil to vary said characteristics accordingly.

Many single coil pick-ups can cause problems with intonation, and Fender Strat- or Telecaster owners know them well. Where a pick-up has magnetic pole pieces, the magnetic field is closer to the string and interferes with its free vibration, causing a double note effect particularly on 6th and 5th strings in higher fret positions. The only cure for this is to lower the pick-up and sacrifice a little power. A pick-up which has magnets underneath and conducting poles will not give this effect to all practical purposes, but for the purist this type may not be tonally desirable. This problem crops up less on humbuckers as few manufacturers use magnetic poles in this situation. You may notice a slight pull on some power pick-ups, for example the Dimarzio X2N, which uses a very strong magnetic field. On this pick-up, the extra magnets under the pick-up are cut away under sixth and fifth strings, so installing the unit the right way round is important. In fact, there is such a healthy output from pick-ups like this that dropping them a little further from the strings gives a negligible power loss - I run three X2Ns in one guitar with no problems.

A humbucker consists of two coils wired in series and over a magnetic field that varies from maker to maker. Here, I'm merely concerned with the alteration of the wiring up to get different sounds, so suffice it to say that various arrangements of pole pieces conduct the field to the area of the vibrating string.

The out of phase bit can be confusing, because as experienced players, we tend to associate 'out of phase' with the honky type of sound given by two pick-ups operating together in a mix, but with the output of one taken from the opposite end of the coil(s) to the other - that is, the phase of one is completely reversed in relation to the other. An apparently similar tone quality can be had from a single humbucker (with a corresponding drop in volume) by running the two coils in series, and in phase. This sound most of us would instinctively call 'out of phase', and the smooth fat sound of a normally wired humbucker, coils actually out of phase, we would regard as sounding 'in phase'. Where we get to wiring up for this in this series, I shall therefore refer to normal and reverse coil phase, and hope that that avoids confusion. If you should find that you have a coil phase problem, remember you solve it simply by swapping around the two wires from one of the coils.

The actual hum-cancelling effect is achieved because the coils are in opposite phases, and when one 'hears' electrical interference, the other 'hears' exactly the opposite and cancels it out. The system does not cancel out string vibration, mainly because the strings are doing a much more thorough job of agitating the magnetic field. In fact, a similar hum-cancelling effect can be had on a Fender Stratocaster. I found that on my test-bed Strat, which has phase reverse switches fitted to all three pick-ups for test purposes, reversing the phase of a back and centre pick-up mix cancelled out hum. Unfortunately, it also sounded vile. This brings up another old chestnut, the straight mix between these two pick-ups has been referred to for a long time as an 'out of phase Strat sound'. To get that sound, the pick-ups have to be wired in phase. Wire them actually out of phase, and your Mark Knopfler licks will sound not the slightest bit like Mark Knopfler.

Figure 1. Pickup connections

If you are going to rewire an existing humbucker from single conductor and shield cable to three or four conductor, the phase of the coils will need checking carefully. In some humbuckers, the phase reversal of the coils is done by winding the coils in opposite directions, and in others, simply by running the output from one into the equivalent wire end on a same-way-wound second coil. Diagrams throughout this series will assume the latter, as in Figure 1.

For humbucker wirings, I shall stick to these colour codings as they also apply to most four conductor pick-ups already on the market.

Figure 2 shows the standard arrangement of volume and tone controls for a single pickup (in this case a humbucker) guitar. The volume pot acts as a potential divider with the pickup connected between the most clockwise end of the track and earth, the anti-clockwise end earthed, and the output taken from the wiper. In the full on position the wiper is connected directly to the pickup and the full signal is fed to the jack socket. As the control is rotated anti-clockwise the wiper moves towards the earthed end, tapping off a progressively smaller proportion of the pickup signal, until the live of the jack socket becomes directly connected to earth in the off position.

Figure 2. Standard volume and tone circuit

The tone control is connected as a variable resistance in a simple low-pass filter circuit, giving a variable amount of treble cut. The capacitor bypasses high frequencies from the pickup to ground, so that when the tone pot wiper is at the pickup end of the track, the amount of treble is reduced. At the other end, the full track resistance is in series with the capacitor, and so most of the treble is allowed to reach the volume control: The capacitor is usually of disc ceramic type, with a value of 50 or 22 nanofarads (0.05 or 0.022 microfarads). The treble cut effect will extend to lower frequencies with larger capacitor values, and this will also depend on the pot resistance, so that at maximum treble the highest frequencies will be cut slightly leaving the lower treble unaffected, and with the tone control at minimum the highest frequencies will be cut a lot, with the lower ones cut slightly.

Figure 3. Alternative tone control wirings

Two alternative wirings of the tone pot, which give exactly the same result, are shown in Figure 3. Sometimes the unused end of the tone pot track is connected to the wiper: this reduces the effect of dirt on the track and causes the pot to act as if it was in its clockwise position (max. treble) should the wiper become totally disconnected.

Next month we'll start with a description of how to rewire two pickup guitars for independent operation of the volume controls.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Mar 1981

Feature by Adrian Legg

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