Valve operation and theory primer

OK, so you have an amp with a load of hot glowing bottles in the back, but how it works is most likely something of a mystery to most people, and many guitar shops and techs either can't explain it, or can't explain it in any understandable way. This guide aims to cut through the treacle out there and give a concise explanation of the basics of valve amp operation which will help you understand your equipment much better.

Theory

Sorry to have to start with a dull bit, but we'll make it quick. Understanding even vaguely some of the principles here make all the rest so much simpler, so it makes sense to get it out of the way.

We'll start with what electricity is. Dead simple really. Its the movement of electrons from one place to another. There are two measurements associated with it, current and voltage. Current tells you how many electrons flow past a point in a circuit in a second. If you have a current of 1 amp, then in one second, 6.242 × 10^18 electrons have gone past the test point. Voltage then tells you how much energy these electrons have, in Joules per Amp. So if you have a voltage of 15V, then every 6.242 × 10^18 electrons have 15Joules of energy between them.

Electrons have come up rather a lot, so you're probably guessing they have something to do with valve operation, and you'd be right. A valve controls the flow of electrons from one place to another. How? Well the first little snippet we need to know is that electrons are negatively charged, thus are attracted to positive things. That being said, lets look at valve construction...

The heater heats the cathode to a temperature where it emits electons, and they sit in a little cloud around the cathode. All well and good, but when we apply a large positive voltage to the plate, those negatively charged electrons rush towards the plate.