There's a reason why the Brits refer to vacuum tubes as valves....it's a bit more visually comprehensible. The smaller signal input to the grid modulates the current flow from cathode to plate and through the output transformer. It's akin to mechanical leverage where a small action can be magnified. If you want a thorough understanding of how tubes work in various circuits, pick up a copy of Radiotron Designer's Handbook. It'll keep you busy for a while.
Can anyone explain what a power tube does inside an amplifier, eg kt88.
I know a tube is cool looking, and looks like a small lightbulb with many pins on one side and when it's turned on filaments glow inside a vacuum enclosed see-through curvy glass enclosure. I guess current flows in, goes on a journey, and then flows out.