Post by JohnH on Feb 19, 2014 2:47:18 GMT -5
This simple component can cause confusion, and expense for those who pursue vintage mojo. Personally I think the type of cap makes very little difference, only the value is relevant, and that is all we can analyse here.
One of the things we know about caps in a guitar tone control is that at maximum settings of the tone pot, the actual cap value makes very little difference. How little? with 250k pots and a typical single coil pickup, less than 0.25db of difference anywhere over the full spectrum of an electric guitar, even over an extreme range of values from 0.002uF up to 0.1uF.
Let's consider a typical 0.022uF cap, at a frequency of say 3kHz at which we expect tone controls to be doing something. The impedance of that cap at that frequency is 2.4k, which is much less than that of the 250k pot that it might be connected to. And the way of combining the two is such that we take the root of sum of squares of the two values, like on a right angle triangle. So the net impedance of the cap and pot at max is 250.01k, ie the same 250k to all intent.
But once we turn down the tone pot, the cap starts to be more significant. But when and how much?
Here is the first output. It is for a guitar with a single coil pickup and two 250k pots for tone and volume, testing the most common range for tone caps of between 0.01 and 0.047uF (10nF to 47nF). The key to the chart shows knob setting/cap value. Knobs are assumed to be on log pots with 10% at midway, over a range 0 to 10. The chart shows decibels v frequency, of the guitar acting as an electrical filter:
The top two traces relating to 10 and 8 on the knob, are in fact, each two traces with 10nf and 47nF, and for reasons described above, the results from different cap values effectively coincide.
At 6 on the knob, there is a slight visible difference, shown by two pale blue lines. Below about 5 in this case, the difference starts to become more marked, and very significant at tone-0. Below about 2, the low midrange peak starts to emerge, which is where the cap value makes most difference. The three red lines relate to 10, 22 and 47nF, at minimum tone setting (10nF is highest).[/p]
So tone cap values are best picked to find your favourite sound at low tone settings, knowing that they have almost no effect at high settings
Here is the same set of readings using 500k pots, and the PAF humbucker. With this, there is even less effect at high tone setting and the knob needs to be below 5 before the cap value has an effect. also see how all the peak frequencies shift down the scale, due to greater pickup inductance.