Basic Theory of Guitar Amplifier Electronics: A Comprehensive Guide
Block Diagram of a Guitar Amp
Power tubes need to be driven in "push-pull" pairs. One tube amplifies the positive half of the wave, the other the negative half. The phase inverter takes the single-ended signal from the preamp and creates two identical signals that are 180 degrees out of phase. The long-tailed pair (used in Marshall and Fender) is the standard topology explained in any advanced basic theory PDF.
Potentiometers: Variable resistors that act as your Volume, Tone, and Gain knobs.