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by Dirk Wacker
This month, Dirk Wacker, of singlecoil.com, explores your guitar's pots and tries to shed some light on these mysterious creatures.
Potentiometers ("pots") are the most common electrical parts on guitars, no matter if it's an active or passive system. Put simply, a pot is an adjustable resistor, nothing more and nothing less. We use pots on our volume and tone controls to control the current in our guitars, and change our sound.
The Basics
With a pot you can control resistance; they are normally used as a volume and tone control on guitars. You can divide them into two different taper types: "linear" and "audio" (aka "log" or "logarithmic"), which indicates how the resistance of your signal is affected by the pot. In earlier days, audio pots were marked with an "A" and the linear pots with a "B," but today you can also find this lettering system used vice versa, so the best way to identify what type of taper you have is to use a digital multimeter (DMM) and measure them with the taper exactly in the middle position - if both sides show the same value (exactly 50% of the pot's value) it's a linear pot, otherwise it's an audio pot (which is logarithmic).
Besides the type of tapering, you can also divide them into two different groups of shafts - one group has a solid shaft (in picture) and the other group has a splinted (aka "knurled") shaft. You can put knobs for solid shaft pots on splinted shaft knobs (e.g. Telecaster "dome" knobs"), but not vice versa. There are also a lot of special pots you can also find on guitars: pots with longer shafts (for Les Pauls), pots with smaller bodies, push/pull or push/push pots with a built-in switch, no load pots, reverse pots, dual taper pots, concentric pots, stereo pots, etc. Your choices are virtually endless, allowing you to customize your guitar's circuitry to your heart's content.
Tone Shaping
The most common tone pot values (which refers to a pot's resistance) are 250k for passive single coil systems and 500k for passive humbucker systems - for active systems you need completely different values, similar to what stompbox pots. There are also guitars with 1 Meg pots (e.g. late '60s Telecasters) but few people seem to use this value today.
The value influences the tone, because the pots add a little load to the complete circuit. In short, the higher the value of the pot is, the less treble roll-off they have. That's the reason why 250k pots are common for singlecoil pickups - singlecoils have a lot of treble bite and it's not necessary to give them more, otherwise the tone can become pikey and harsh. Humbuckers don't have that much treble, so 500k pots are used keep more of the treble in the signal. The 1Meg pots I mentioned in combination with a Telecaster's lead pickup produces a very trebly and harsh sounding tone; I recommend changing them with the standard 250k pots to smoothen out things a bit, but it is certainly personal preference. The modern values of 220k and 470k are more or less identical to the old values, as pots generally have a tolerance of up to 20%.
Deciding to use an audio or a linear pot for the volume control is simply a matter of taste and you have to try it on your own. The problem with pots as a passive volume control in general is that, in most cases, it's not very helpful, because it functions more as an on/off switch than a useable control. You have a lot of hot spots during the taper's travel and it's difficult to dial in the desired level because the working taper is very, very small.
Normally an audio pot is made to solve this problem and back in the good old days it was - modern audio pots normally have a ratio of 80:20, which is not very useful, while older pots from the '50s and '60s (e.g. the famous CENTRALAB audio pots) had a ratio of 70:30, or an even better 60:40, which is ideal for guitar use. You can buy custom audio pots from Hamer with a very good ratio of approximately 65:35, which is very good and an excellent volume control. You can also try to get some old pots, but they are vintage parts and very expensive.
You can also use a normal, modern audio pot and do a little trick to smoothen out the taper and to prevent the typical treble loss when rolling back the volume, which is very common in singlecoil systems. There is a complete project for this at singlecoil.com with all the technical details, but all it really entails is soldering a 0.001uF cap with a 150k resistor in parallel between the first and the second terminal of the pot. This works well and you can experiment with the values to change the effect - changing the value of the resistor will change the working of the taper, and changing the cap's value will change the amount of treble that is saved when rolling back the level. Keep in mind, for the tone control you must use an audio pot to have a useable taper!
It's possible to mix different values in a guitar; you simply have to try it out and decide what you like best. A big problem is the horrible tolerance of the modern pots, as even high-quality pots from CTS or Mouser have a tolerance of 20% or more. I have seen 500k pots with values of 380k up to over 700k! It's a big improvement for your tone to use matched pots with almost no tolerances. There are different ways to get matched pots: you can buy military-grade pots with almost no tolerances, but it's very difficult to get those and it's hard to find the right values for guitars. There are also some technical tricks to change the value, but this is difficult and requires special knowledge and good electrical skills. The easiest route is to buy a matched set from our web shop or any other supplier. We simply buy large quantities of top-notch pots and measure them, and match them up, which will drastically improve your tone.
Remember to take care of your pots!
Dirk Wacker has been addicted to all kinds of guitars since the age of 5 and is fascinated by anything that has something to do with old Fender guitars and amps. He hates short scales and Telecaster neck pickups, but loves twang. In his spare time he plays country, rockabilly, surf and Nashville styles in several bands, works as a studio musician and writes for several guitar mags. He is also a hardcore DIY guy for guitars, amps and stompboxes and also runs an extensive webpage (singlecoil.com) about these things.
Matched sets of pots are available at Singlecoil.com webshop!
SingleCoil.com
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