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

@tonydennison 

Chat GBT did a great job above. And your comment asking how it actually controls the valve activity is very key.

Also there was a document above from @tablejockey that looks very promising.

Ultimately the end answer, is likely very simple, amidst all the complicated fog.

When you ask a child, even in adult, how something works, like how things work when you turn on a sink faucet you'll get a good answer. But if you start going into the actual inner workings of a faucet, the child quickly gets very frustrated. Human nature is a challenge.

It is a valve that opens and closes in response to the input signal fed to it. Output side, (anode or plate), is connected to a high voltage power supply, that feeds the output transformer connected to your speaker through the tube. The input side, (grid), accepts the tiny input signal from your preamplifier, and turns the large power supply on and off in response to that input. The result is a much larger signal being applied to your speaker , than the tiny input signal being applied to the grid. Basically how amplification of the tiny input signal is achieved. 

Of course there are more parts to the tube that sets the amount of current bias is applied. But the above is a simple as I can attempt to explain it.

So does the plate collect the electrons until the charge is enough to push through the gate? And what prevents loss of  information? Once the tube gets warmed up, I guess it is just continous?

Read the beginning of this:  

https://worldradiohistory.com/BOOKSHELF-ARH/Technology/RCA-Books/RCA-Receiving-Tube-Manual-1975-RC-30.pdf

Oh my God, the document above is 760 pages long.

It looks pretty damn awesome and it has pictures.  I wonder if it was ever translated into another language.  Fortunately I only can read English and how lucky I am this document is in English. I failed German when I was in high school. What an awful experience that was.