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

Simple answer, tubes are valves. Small signal opens the valve and lets the power from your power supply go to your speakers. They are called valves for a reason.

Do a youtube search. Watch some videos if you actually care to know. Lots of info out there.

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.