Sin It's sort of like feeding a bias voltage to a plate and a variable low voltage to the grid, which regulates the voltage exiting a triode amplifier vacuum tube or valve (as the Brits say). But in a digital system you only need on/off, or on full/on low. 1 and 0. My brother has designed such digital circuits for interplanetary probes. Circuits which use coded streams of instructions to produce a usable output string.
In a vacuum tube the regulation is of the variations in the voltage, which is much stronger than the regulating voltage in the grid. A lit up amplifier tube amplifies because the strong voltage of the grid is partially blocked or not blocked by the low input signal, say, from a microphone, phono needle crystal or other pickup, such as in a guitar.
A lot of the circuitry is about producing an absolutely smooth voltage to the emitter plate and other noise reduction circuitry. The signal from a microphone isn't anywhere near strong to move the coil of a speaker. But the bias voltage is. If you have big speakers, you use a series of tubes to keep amplifying the signal until it will drive a BIG coil and a magnetic field that is attached to the cone, which vibrates the air, which vibrates the eardrum. Cool, huh?
Harry, I hope I got that right, because that triode model has helped me understand this stuff a little better. The transistor is sometimes called a gate that's open or closed, vs a valve where the amount of flow produces the output signal. Kindly correct me if I have this wrong.