Step 1: Tube Circuitry
Below is the electrical diagram for a typical tube. The filament heats up when current flows through it. When heated by the filament, the cathode emits electrons that are attracted by charge to the anode. Various grids insert signal charges, often fluctuating, to restrict or speed the flow of electrons to the anode.
Although there are a number of things that can go wrong in an electron tube, one of the most common is that the filament burns out.