Please clarify capacitor size effect for me.

[martinw]

I just want to get clear in my mind the effect of varying cap size in a circuit.
I understand the basics, i.e. large caps allow more bass frequencies through, due to fill time etc, so smaller coupling caps tighten bass, as do smaller preamp valve cathode bypass caps. Am I right?
What about treble frequencies though? Why does lowering the value of the treble pot cap from 470pF to 250pF on a Marshall type tone stack cut brightness?
Similarly, volume control bypass caps.
Is it an extension of the same principal, that works because the cap values are very small?

Thanks!

[Francis_Vaughan]

An easy way to understand capacitors in a circuit is to look at the two limiting cases. If the frequency is very high any capacitor looks like a piece of wire. If the frequency is very low the capacitor looks like an open circuit. If you apply this idea when viewing a tone-stack you can understand the circuit quite easily. Often a capacitor is used to shunt signal to ground, so the end result is less, not more signal coming out of the stack.

If you want to understand a circuit at a given frequency just calculate the equivalent resistor value for each capacitor.

R = 1/(2*pi*Freq*C) where C is in Farads.

Then you can apply the usual resistor combination rules along with Ohm's Law to work out what is going on. A good starting frequency to work with for a guitar amp is 400Hz. This is pretty much the middle of the useful range.