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.