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

@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

“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.

Emerging Soul already knows how they work "current (sic) flows in, goes on a journey and then flows out". What more is there to say...