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

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

Yes.

An attempt at Simplification. Perhaps someone else can add clarification while still keeping it simple.

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tubes are sometimes called ’valves’.

think of it like a garden hose. open the valve a little, a little more, much more.

low strength signal from component

to/thru controlled volume control, varies signal strength to the tube

sends a little or a little more, or much more signal to the tube

tube is like a water valve that varies the high power circuit that goes to the speakers: high power signal is proportional to tube’s low power signal strength: a little, a little more, much more.

Simple explanation.  It's magic!  electricity goes in-music comes out!  Lots of people will tell you that tubes are magic.

All tubes and transistors can be simplified by thinking of them as 3 pin devices:

Input (music), Output (Music), power.

Between the input and the output a device increases voltage, or current, or both.

Amps tend to have multiple stages, up front there’s a voltage amplification, at very low currents. The standard is about 20x from input to output, then at the end is a current stage, which may have no voltage amplification but can deliver the current the speakers need. That’s usually what a power tube or transistor does.

This  is why tube amps will usually have at least 2 different types of tubes.  The smaller one's doing voltage related work, the larger one's current. 

While voltage goes up by around 20x, current must go up by say 40,000 : 8 or 5000:1 and this is very difficult to do with single stage amplifiers.