In the attempt to make some sense out of passive components and their function in valve operation, we’re going to come across a lot of capacitors, so we need to know what they are, what they’re for, and what they do.
The ‘what they are’ bit is straightforward enough. A capacitor is two metal plates that have no electrical connection between them. The stuff that seperates (insulates) them so that they don’t short together, is called a dielectric, and there are a lot of different materials used for this. Mica, polycarbonate, polyester, polystryrene, are a few.
If we put a d.c. voltage across a capacitor nothing (apparently) happens. It behaves a lot like an open switch; but not quite. So what’s the point of the two circuits above, which should according to that, do nothing at all? Well, although a capacitor won’t pass d.c. it does pass a changing voltage electrostatically, so a signal from, say, a guitar pickup or a mic will be passed through the capacitor, but d.c. will not. This is one of the prime functions of a capacitor in all electronic circuitry, but especially in valve circuits.
The schematic above is of two triode valves, the output of one feeding the input of the other. It could be two halves of, say, an ECC83. This circuit arrangement is called a CASCADE circuit. The idea of this arrangement is the basis of all preamp circuits whether valve or solid state of some sort. The first valve (v1) amplifies the input by some factor, lets say 20 times; and then the second valve v2 amplifies that again. So if they both had the same gain of say 20, if for instance we put in a voltage of 1 volt onto v1 valve the output would be twenty times that, so 20 volts. Then that 20volts is applied to the input of v2 which is multiplied by the gain of that valve so the output would be 20×20 volts which is 400 volts, give or take. (Note that it’s not 20+20 which would be 40. It’s 20×20)
This all seems fine, except that the d.c. voltage at the output (anode) of v1, for it to operate might be anywhere between say, 100 and 300 volts, but the voltage at the input of v2 would normally need to be very low, almost zero in fact. This is where we get to the point (at last). We can’t put the output of v1 straight onto the grid (input) of v2, because the 100+ volts would be very likely to destroy the valve. So we make this connection via C2 which will block the d.c. we can’t have and couple the a.c. signal that we need to pass onto the second valve. In this function C2 would be called a coupling capacitor. The reason C1 and C3 are there is to block any d.c. that might be at the input or output.
Going back to our thread and the first diagram. Without going into values of capacitive reactance and such, in general, whereas the resistance of r2 will stay the same whatever signal we put onto it, the situation with C1 is different. The higher the frequency the easier it will pass through the capacitor (that’s any capacitor). The resistor will have some resistance value in ohms. The capacitor has a property called impedance, that we can think of as being the same thing. The resistance of a resistor is represented by a letter R (or r). The impedance of a capacitor is also in ohms but is referred to as Xc. The big difference between R values and Xc values is that, depending on the frequency of the signal applied R will be a constant and Xc will vary.
This property means that the circuit at the top behaves as a HIGH PASS FILTER. That means that it will pass high frequencies and tend to block lower ones. If we double the frequency onto C1, Xc1 (that’s it’s effective impedance) will be halved, and the output increased.
That was hard going. Time for tea. The next post will deal with low pass filters and how these are used in tone/treble/bass controls.