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With the tone control on zero the cap is right across the pickup. If the cap responds non-linearly to an applied voltage, the voltage should appear across the output with some attenuation depending upon the frequency. In general this is kind of like being in the signal path.
There are reasons why the cap would not distort very much in a guitar circuit. Bateman used ac voltage of several volts in his tests; A smaller ac signal level generates less than what he measured. In general he put a dc bias across the capacitor that was a significant fraction of the rated voltage. The dc bias was important in generating distortion in most cases. So for a guitar, with no dc bias and a lower signal level, the distortion would not be nearly as high as he measured. Also, he measured very low levels. A good capacitor had maybe 0.00006%. The worst capacitors had about .15%. Both these percentages would be much lower in a guitar.
The capacitor with .15% is a kind of ceramic. Not all ceramics are bad; in fact the material called COG, sometimes labeled as NPO, makes some of the lowest distortion capacitors. Another material, usually called a class 2 ceramic, is the worst. These ceramics use barium titanate and make high value capacitors that are small. This material should be avoided in any audio circuit. You probably cannot hear it in a guitar, especially with the tone control on 10 for the reasons stated above, but there is no reason to use something so inherently bad when better stuff is available.
There are reasons why the cap would not distort very much in a guitar circuit. Bateman used ac voltage of several volts in his tests; A smaller ac signal level generates less than what he measured. In general he put a dc bias across the capacitor that was a significant fraction of the rated voltage. The dc bias was important in generating distortion in most cases. So for a guitar, with no dc bias and a lower signal level, the distortion would not be nearly as high as he measured. Also, he measured very low levels. A good capacitor had maybe 0.00006%. The worst capacitors had about .15%. Both these percentages would be much lower in a guitar.
The capacitor with .15% is a kind of ceramic. Not all ceramics are bad; in fact the material called COG, sometimes labeled as NPO, makes some of the lowest distortion capacitors. Another material, usually called a class 2 ceramic, is the worst. These ceramics use barium titanate and make high value capacitors that are small. This material should be avoided in any audio circuit. You probably cannot hear it in a guitar, especially with the tone control on 10 for the reasons stated above, but there is no reason to use something so inherently bad when better stuff is available.
Originally posted by David Schwab
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