Can anyone explain what a power tube does inside an amplifier, eg kt88.

Ever see or use a pantograph? This is a mechanical device that allows you to draw an enlarged copy on a piece of paper when you trace a small image. A vacuum tube (and also a transistor) are just electronic versions of this. A small signal to a control grid in between cathode (source of electrons) and anode (the output), makes a bigger copy (i.e., more voltage) of the original signal.

That's why they are called "amplifiers" -- they are amplifying a small signal, or again, making a bigger copy of a smaller signal.

@mlsstl 

Agree, it is making a bigger copy of the small signal. The small signal is just making it possible for a separate bigger signal to flow. Two signals. The small one is controlling the big one, it’s not that the small one is being magnified directly. 

What a convoluted pig trail of conversation. 

@ghdprentice 

Sorry, but no breaks for him. He's done this before.

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