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Vacuum tubes are also called thermionic valves. Thermo meaning heat, ionic meaning charged ions, and valve meaning it acts like an electrical valve similar to the valve of a sink or hose.

The cathode is connected to ground and heated with a filament, like a light bulb. This allows the thermionic or Edison Effect to take place, where the electrons in a heated wire boil off. The plate, or anode, is connected to a high voltage supply so it is positively charged. This charge draws a large current, or many electrons from the cathode. A grid, or metal mesh is placed between the plate and cathode.

The charge at the grid controls how much current flows through the tube. The amount of current is proportional to the charge at the plate and the charge at the grid. Because the charge at the plate is much, much higher than the input signal at the grid, the output from the plate is a much larger signal. This addition of energy is called gain, or amplification.

It's not exactly proportional however, which means the signal is distorted. Tubes are also big, inefficient, and need high voltage supplies (hundreds of volts) which compares to the ten to twenty volts needed for similar gain with a transistor which also is more efficient and low distortion. They're also small, and cheap. Hence why we don't use tubes outside niche applications.
 
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