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<< back to Kens column Trainwreck thinking by ken Fischer Chapter 2.1 I will now talk about controls for pickups. These are potentiometers, commonly called, "pots". I will use the term pots, in this article, to keep it simple. Most people pay little attention to the pots, used for controls, in their guitars. Pots have a number of parameters we should know about. The obvious ones are the physical size of the pot. Also, we want to know the mounting bushing length, and diameter, to insure it will fit the hole it will be mounted in. We also want to know the length of the shaft, and the type of shaft, to be sure the pot will accept the control knobs we plan to use. This is simple if you keep in mind that there are two main standards for these parameters. There is the American, (really English), and the metric standards. The unseen parameters are wattage, taper, ohmic value, degrees of rotation, and the material the resistive element is made of. The typical pot in a vintage American guitar is a half watt carbon type. Some guitars use a linear taper, and some an audio taper. A linear taper is just that, linear. For every 10% of rotation, you get 10% of the pots value. If you rotate a 100 ohm pot 60%, you will get 60 ohms. There are several audio tapers. For my example I will use a true audio log taper. A true audio log taper will give you 10% of the pot's value at 50% of shaft rotation. For example, a 100 ohm true audio log taper will give you just 10 ohms at 50% rotation, compared to 50 ohms at 50% rotation for a linear pot. This is done because the human ear is not linear in response to increasing volume levels. The audio taper tries to mach the volume curve to the human ear. Perfect! Then why the use of linear pots in some guitars? Pickups do not have a linear output. Most guitar amps and speaker do not have a linear output. Linear pots are sometimes used to sound like you are getting an even audio response with certain amplifiers, when these amps are played clean. Compression changes everything! I should note here that common wattage values for metric pots, as used in guitars, are, 1/2 watt, 1/4 watt and, 1/8 watt. The lower the wattage, usually the smaller the physical size of control. here are some terms you will encounter, relating to ohmic value. "K" equals 1000,"M" equals 1,000,000. For example, a 500K pot is 500,000 ohms. A 1M pot is 1,000,000 ohms, or 1000K. The ohmic value is most critical parameter for choosing a pot, followed by taper. A typical vintage Gibson guitar would have 500K pots. Fender used 250K pots in most of their vintage guitars. Here is where it gets interesting. The first Fender Jazzmaster guitars used 250K pots. In the sixties, the pots in the Jazzmaster were changed to 1M (1000K). The change gave the Jazzmaster much brighter, and a much thinner voice. The pickup being a tuned circuit is also further tuned by the value of the control, and how the control is used in the circuit. As a general rule, a higher value pot gives more highs and less mids. A lower value pot gives less high end, but more mids. This is not the case 100% of the time, but a useful general guide. When the pot is used for a tone control, the general rule is, the higher value of the control, the more high end you will get. The down side of this is, the control will seem to have less effect, until you turn it down a lot more than a lower value control. When the control is turned down all the way down, all values sound the same, as now capacitors is directly across the pickup. There is one more parameter, I have not yet mentioned in regard to potentiometers. The resistive element has a tolerance specification, which is plus or minus a percentage of the ohmic value. For example, a 500K pot, with a 20% tolerance can be as low as 400K, and as high as 600K. This can be a downfall, to getting consistent results, when wiring a guitar. It can also be a fine tuning trick, if you measure each pot, and know how to use the results. You should also know, that a pot tends to increase in value with wear. An old trick used for vintage guitars, with a worn volume pot, is to switch the volume pot, with a little used tone pot, from the same guitar. This way, all the pots remain original. Of course the sound will change a bit, as the unused tone pot will be a lower value than the worn pot it is switched with. Choosing pots as a tuning tool is really great way to fine tune your guitar. You can test various for your guitar using test leads. BE AWARE THAT: touching any original solder connection in a vintage instrument will reduce the resale value. There are two common ways to hook up a volume control. There are also two common ways to hook up a tone control. In diagram V.1, we see the most common volume wiring. This method, keeps the resistive load, of the pot, constant. The only downside is in a two pickup, two volume control guitar, with both pickups on. In this case, shutting off one pickup with a volume control will also shut off the other pickup. In diagram V.2, as used in a Jazz Bass, and some brands of guitars, the hot lead is connected to the pots slider. The signal is taken from, the "hot", terminal of the pot. This type of wiring has the advantage of not requiring a selector switch when using two pickups. This is because, turning off and one the controls will not turn off the other pickup as in diagram V.1. A selector switch can still be used with this circuit if desired. The main downside to this wiring type is, as you turn down the volume, you also decrease the resistive loading of the pickup, "loading it down". In affect, you change the tuning of the pickup, depending on the volume setting. Some people like the tonal variation with volume setting, and some do not. These two methods also affect the amplifier's input loading in different ways when using the control.  (click here for bigger picture) As if things could not get more complex, there are two main ways used to hook up a tone control. In diagram T.1, the tone capacitor is put between the pickup and the control. This is common on Gibson guitars. In diagram T.2, the control is placed between the pickup and the capacitor. This is common in Fender guitars. Not to get too technical, a capacitor creates a phase angle shift. In a circuit with a resistor (pot) in series with a capacitor, this angle lags or leads, depending on the order of hook up. A personal note, never use a disc type cap in a guitar. They do not resonate right in a guitar circuit. A proper tone control should also add resonant tone as it is used, not simply flatten the high end. Well, this voodoo is all beyond my personal understanding, so until next time, All the best, Ken Fischer << back to Kens column |