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Hot Wiring Your Guitar

Last month we considered the basic electronics of a single pickup guitar, with various equivalent ways of connecting the tone pot and capacitor. Two-pickup guitars follow the same plan, with the circuitry duplicated up to the jack socket, where a pickup selector switch connects one or both of the volume pot. wipers to the single output jack. This arrangement is shown in Figure 1.

When the pickup selector switch is in either the up (neck pickup) or down (bridge pickup) position, the appropriate volume control wiper is connected directly to the live of the jack socket and the other is left isolated. When the switch is in the centre position, the two wipers and the jack socket live are connected together and if neither volume control is at zero, a mix of the two pickup signals is fed to the socket. The 'mixing' of the two signals is very crude and if either control is at zero there will be no output regardless of the setting of the other, since the jack socket live is grounded. This can be an advantage since it allows the output signal to be shut off completely by turning either volume control to zero but is undesirable if you want to be able to turn off one pickup without changing the selector switch, gradually. For example, Figure 2 shows how to rewire the volume controls for independent operation.

Figure 1. Electronics of a two-pickup guitar.

To avoid one control shutting off the signal from the other, the input (left-hand tag viewed from the back with the tags facing you) and wiper (centre tag) connections should be reversed for each pot. The circuit now appears as in Figure 2, with the tone and pickup part of the circuit remaining the same. Now at maximum volume setting the pickup is still connected directly to the selector switch (with the full track resistance to earth), but at the minimum setting the pickup is earthed and the track resistance lies between the earth and the selector switch, so the other signal is not shorted to ground. It is impossible to damage either pickup by shorting its ends together, and this occurs anyway with an unmodified two-pickup circuit when one volume is at 10 and the other at 0.

Figure 2. Modified volume control circuit.

If the modification is done on just one volume pot. the other can be used as a 'master', shutting the signal off when at zero, with the ability to bring in the other pickup signal independently.

At this point, I will just explain about two different types of pot. available, for those beginners who don't know already. These are logarithmic ('audio') and linear, which are manufactured to give differently tapered responses. A linear pot. gives a constant change in signal amplitude (when used as a volume control) as the control is turned. But the ear hears this as a nonlinear change in volume. Audio pots. have a response that gives subjectively linear control of volume, and consequently are more useful as volume controls. Fitting a linear pot. instead of an audio pot. will give a large increase in volume over the first third with little else happening for the rest of the travel. Note that the two ends of an audio pot's track are not equivalent and reversing the track connections will give an even more extreme response, in addition to changing the direction of control.

Another modification I want to deal with is probably the simplest, cheapest, and in my opinion, the most effective of any. Leo Fender used it on the Telecaster and it is a major reason why that guitar sounds the way it does. It was also used by Travis Bean on humbuckers on the Standard and Artist models.

Figure 3. Capacitor fitted for treble boost.

Quite simply, it is a 1nF capacitor across the 'in' and 'out' tags of the volume pot. As the volume is decreased slightly, treble frequencies bypass the resistance and proportionately increase the amount of treble in the sound. The increase is particularly noticeable with single coil pick-ups or tapped humbuckers, less obvious as an increase with straight humbuckers. Most guitars on the market now use audio pots, and the effect of the capacitor is most noticeable on these. A linear pot gives a barely noticeable effect as there is in my experience, less treble loss in the first place. Cheaper audio pots will give an immediate effect at a volume setting around nine, as the treble loss tends to occur very suddenly just under full volume. While linear pots give less treble loss, they can be less effective than audio immediately in front of a pre-amp. I've found that in that situation, I got virtually no change in level until I got down to around two, when the sound packed up altogether. Fitting audio pots cured the problem. So, although the 1nF capacitor bypass evolved primarily as a crude means of combating treble loss, these days it is very useful as a definite effect. Simple fitting should be as in Figure 3 but it is possible that you will find this a help in some circumstances, and a hindrance in others. For example, the softening effect of winding down an audio pot on a front pick-up can help in getting a warmer, jazzier sound without actually cutting treble on the usual passive tone pot, which can sometimes be difficult to set accurately, quickly. In this case, the answer is to make the capacitor switchable. An interesting side effect of using the capacitor is that passive tone controls will sound more effective, as, at lower volume settings, there is more treble left for them to cut than on an unmodified guitar, where the top end has already been rolled off by the volume pot.

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


Electronics & Music Maker - Apr 1981

Feature by Adrian Legg

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