Musical Instrument Repairs and Electronics in Oswestry and the Marches


Guitar Wiring and Pickup Modifications

Using a little ingenuity and a few inexpensive parts, there is a great deal of scope for customising or "hot-rodding" your guitar to dramatically increase the range of tones it can produce.

Before I get into the nitty-gritty of how to chop up your wiring of your guitar, I shall take a brief look at some of the principles involved in the function of typical guitar electronics, including pickups and the various components inside your guitar, and how each of them will affect the instrument's tone.

How Guitar Pickups Work

A guitar pickup is a very simple electro-magnetic device, which would be instantly recognisable to Michael Faraday, the great pioneer of electromagnetism.

Construction of a typical single coil pickup
Construction of a typical single coil guitar pickup

In its simplest form, the standard "single coil" guitar pickup, as fitted to Tele or Strat style instruments, the pickup consists of (usually) six permanent magnets, with several thousand turns of fine copper wire wound around them. To those of you who still have a distant memory of physics lessons in school, this should bring to mind the definition of a basic generator or dynamo - "The production of an electrical potential difference (or voltage) across a conductor situated in a changing magnetic flux". The vibrating steel string cutting the lines of magnetic flux causes the change in flux in the "conductor", ie the coil of copper wire.

As guitarists who use instruments equipped with single coil pickups will testify, they are equally effective at picking up stray 50 or 60Hz fields radiated by AC mains wing and equipment as they are at picking up the vibration of the strings! I am sure we have all ended up playing our guitars in weird and wonderful positions, just to minimise the infernal buzz! This led to the development of the "humbucking" pickup. This is again a simple device, imagine two single coil pickups placed side by side and both connected in series - if these pickups were identical, then they would both pick up the same amount of string vibration and background hum: the voltages would be added together but the ration of signal to noise would be the same.

Construction of a typical humbucking pickup
Construction of a typical humbucking pickup

The clever twist that makes the humbucker so effective is that in, one pickup, the permanent magnets are effectively mounted upside down (usually there is only one bar magnet with the North pole touching the base of one set of polepieces, and the South pole touching the other set), and the coil is reversed. This effectively means that the signal produced by the vibrating string is unchanged, whereas the hum signal produced by pickup of stray magnetic fields is reversed (remember that the string signal is produced by the interaction of the magnet and the coil, but the hum is purely picked up by the coil). When the signals from the two coils are mixed together, the signal from the vibrating strings is effectively twice that of a single coil, but the hum signal is reduced to almost zero, as the hum signal from the two coils have opposite polarities. In practice, there will still be a small amount of hum, as the two coils are very rarely identical.

Factors Effecting Tone

For such a simple device, the tone of a guitar pickup can be affected in both subtle and not-so-subtle ways by a large number of factors.

1 - Magnet Material

The magnets are at the heart of the pickup and the material used will have a great bearing on the sound of the pickup. There are three main types of magnetic materials used:

2 - Inductance, DC resistance and Distributed Capacitance

A pickup will have an inductance, measured in Henries - this figure can be anything from 1 Henry for a low output single coil pickup up to around 9 Henries for high output humbuckers. This inductance depends on the number of turns of copper wire and also on the physical shape of the coils.

Similarly every pickup coil has a characteristic DC resistance, measured in Ohms - this depends on the number of turns and the diameter of the copper wire used. Typical values can range from about 1k (1000 ohms) up to about 15k (15000 ohms).

Pickups also exhibit a "distributed" capacitance which is effectively in parallel with the inductance of the coil - this is caused by the addition of the very small capacitance between each turn on the coil.

Looking at the combination of these factors, one can consider a pickup to be an electrical circuit comprising an inductor and a capacitor connected in parallel, with a resistor in series - this is effectively a tuned circuit with a well defined resonant frequency, determined by the inductance and capacitance and a damping factor, determined by the DC resistance.

Equivalent circuit and frequency response of a guitar pickup
Equivalent circuit and frequency response of a guitar pickup

In practice the range of values found in typical guitar pickups result in a fairly smooth resonant peak in the upper midrange, values can be anything from one or two kHz (kilohertz) for high output humbuckers up to around 8 kHz for "vintage" single coils. It is this resonant peak which gives each pickup its own individual character.

3 - Controls and Cable

Many guitarists do not realise how much their tone is affected by the guitar's controls (not to mention the cable!). A little thought and a small amount of cash can make an enormous difference to the sound of the instrument.

  1. Volume Control(s)

    A volume control is a component called a potentiometer (or "pot"), which functions as a "potential divider" - this can be considered as two resistances in series which effectively divide the signal from the guitar's pickups in the same ratio as that of the resistances. Commonly found values are 250k (250 000 ohms), 500k (500 000 ohms) and 1M (1 000 000 ohms). The other thing that must be considered is the "taper" of the pot, ie the way in which the resistance ratio changes with the angle of rotation. Typical pots have either a linear taper, where the graph of resistance against rotation angle is a straight line, or an audio, or log taper, where the graph of resistance against rotation angle is a logarithmic curve - this latter type is usually used in volume controls, as it gives a smoother variation in volume over the range of the control.

    The total resistance of the volume pot is connected directly across the pickups, and acts as a "load" on them. Because of the nature of the pickups complex internal impedance, this will have the effect of attenuating the extreme ends of the tonal range (usually more noticeable with high frequencies) and reducing the sustain (ultimately the energy has to come from somewhere - in this case from the vibrating string!).

  2. Tone Control(s)

    A Guitar's tone control is actually a very simple low pass filter comprising a capacitor, and a potentiometer wired as a variable resistance in series. This type of circuit is called a "first order" filter, and has the effect of gently rolling off frequencies above a certain point determined by the combined AC reactance of the capacitor and the DC resistance of the potentiometer. Typical capacitor values range from 10nF (0.01µF) to 100nF (0.1µF), usually the resistance of the pot is the same as that of the volume pot. Tone pots usually have a linear taper to give a smoother range of control. Some manufacturers make a "one size fits all" audio taper pot which gives acceptable results when used for tone or volume.

    By the nature of the circuit used, a tone control will always filter off some high frequencies even when turned fully clockwise, and indeed many players who find that they never touch their tone controls will disconnect them to add a bit of extra "sparkle" to their sound.

  3. Selector Switch and Jack Socket

    While these are inherently simple devices that normally have very little effect on an instrument's tone, they are very vulnerable to contamination from dirt, grease, congealed Gaffer Tape and corrosion. When the contact areas of these parts do become contaminated, it can cause loss of volume, reduced high or low frequency content, increased noise and hum pickup and intermittent failure.

    I regard paying attention to keeping these components clean as part of any guitarist's essential maintenance. My own "recipe" for cleaning them is a combination of a quick squirt of Isopropyl Alcohol followed (after letting it dry for 30 seconds or so) by a SPARING squirt of a good quality switch cleaner spray - on no account use one of the products recommended for spraying on your spark plugs on a damp morning; these will leave a sticky residue which will only cause further problems. My favourite is Kontact 60, you can obtain this and Isopropyl Alcohol from Rapid Electronics in Europe and the UK.

  4. Your Guitar Lead!

    This seemingly innocent length of cable can be a major culprit in the area of tone loss! Any cable will exhibit an inherent capacitance - if you check out any bulk cable manufacturers catalogue, you should see this quoted as so many pF per meter. This can vary from as little as 100pF/m or less for high quality cable up to some 400pF/m for cheap cables. This additional capacitance, when combined with the pickups own internal resistance, will form an additional low pass filter which will roll off a good proportion of the high frequencies that your instrument produces. The effect is exacerbated when the volume control is backed off, as the increased resistance will effectively lower the filter's cutoff frequency.

    Many Guitar leads use so-called "noiseless" cable, this cable has a thin (usually black) conductive plastic coating over the insulating sheath of the inner core, but underneath the outer copper braid. This coating is very effective in reducing pickup of interference and handling noise. The advantage of this construction is however, quite often negated by shoddy assembly - I have seen many instances where the person who has assembled the cable has not bothered to strip this conductive coating back from the inner core: this results in a comparatively low resistance being connected across the output of the guitar, resulting in a muddy indistinct sound.

    Another area that can make a big difference is the plugs used. I would recommend avoiding solid brass plugs, although they function very well when new, before very long the metal will become covered in a very thin layer of copper oxide, which, in addition to not looking very good, also happens to be an insulator! I do not have a very high opinion of gold plated plugs - although these will give superb performance in equipment that is cabled up and left undisturbed (ie hi-fi, home studio or installation), when gold plated connectors are used for equipment that is continually being plugged in and out, the layer of gold (a rather soft metal) will gradually wear away, revealing the base metal underneath.

    I would recommend using good quality nickel plated connectors, my own personal preference is for Neutrik connectors - I was convinced after seeing my guitar lead being driven over by our tour bus just before a gig - it lived to tell the tale!

We could carry this chain of factors further into the territory of effects pedals, true bypass, input impedance and so on, however I am going to stop here and concentrate the focus of this series of articles on what is happening in your guitar and the ways in which you can increase the range of tonal colours on its palette...

Home | Repairs & Prices | JFET Preamp Kit | Tips & Tricks | Links | Contact Info