The most basic tube has two elements: a cathode (also known as the emitter), and an anode (also known as the plate). The cathode is connected either to ground or a negative voltage; the plate is connected to a high positive voltage. As the cathode is heated, electrons jump from the cathode to the plate.
In a basic triode tube, a control grid sits between the cathode and plate. A voltage applied here either encourages or inhibits the flow of electrons from cathode to plate. In this way, a small voltage (such as an audio signal) applied to the control grid results in a much larger change in voltage at the plate, causing that input signal to be amplified.
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