A vacuum tube is a device that uses a small input voltage to control a larger output fed by the resources of the power supply. Typically, the high voltage low amperage output is then sent through a transformer to become a lower voltage, higher amperage output for driving a speaker.
Because of the limits of the technology, usable amplifiers are developed in 2 or more 'stages', each optimized for voltage or current gain, or for specific applications like the pre-pre amps which provides magic loudspeakers in