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

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.

It's all here...

taste_of_tubes.pdf (ken-gilbert.com)

Best understood with some electronics 101 reading.

@emergingsoul   Someone mentioned using CHTgpt, so I decided to access it and a pretty good and simple explanation comes out. 

Here is what it says: 

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I have often wondered this.

The tube makes a larger copy of a signal, yes.

But how?

I imagine they are called valves because they act as gates, only opening to release the larger copy when it is large enough based on specs.

Again, how? How does it increase the size and  know when it has accomplished its goal? How does the gate open?

I can understand and explain how a gas engine does what it does, but not a tube.

Ive read some technical explanations but they always seem to avoid the actual answer to the actual question.

T

@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.