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Thermionic Effect
Valves and vacuum tubes work according to the Thermionic Effect, which Thomas Edison discovered in 1875. When Edison experimented with light bulb designs, he found that an electric current flowed in a vacuum between a hot filament and a metal plate; current did not, however, flow in the opposite direction. The device was a one-way valve for electricity, which is a desirable ability. Subsequent work led to more complex designs including metal filaments, screens and plates in various arrangements. A screen placed between the filament and plate varies the current flow, making a control valve capable of amplifying weak signals.
Radio Frequency Oscillator
A valve radio employs a circuit called a variable-frequency local oscillator; when you adjust the radio's tuning dial, you tune the oscillator's frequency. Radio broadcasts operate at a frequency range of between 530 kHz to 108 MHz. A mixer circuit combines the local oscillator with the antenna signal, producing an output that has a fixed intermediate frequency containing one radio station's broadcast. The radio sends the intermediate frequency to other circuits that decode sound from the signal.
Rectifier
AM radio carries a broadcast through a process called "amplitude modulation" -- the sound waves of music and speech vary the strength of a high-frequency signal. To demodulate AM, a radio uses a rectifier circuit built around a diode -- the simplest kind of valve, in which current flows in one direction. The diode removes the intermediate-frequency radio signal, leaving only the original audio broadcast, although it is not yet powerful enough to drive a speaker.
Amplifiers
The radio contains a valve amplifier that boosts the power of the demodulated audio signal. The screen of one valve uses the weak audio to control a stronger current from the radio's power supply; this current passes through a volume control and speaker. It takes only a few watts of power to drive a tabletop radio speaker to comfortable listening levels. In addition to the audio amplifier, the radio has high-frequency valve amplifiers to increase the power of signals coming from the antenna.
Transformers
Some valves produce a high-voltage direct current at their outputs, which can interfere with other circuits connected to the valve as well as damage the speaker. To solve this problem, radio engineers use transformers in various parts of the radio; the transformer passes the radio and audio signals while blocking any DC. Radios have transformers between amplifier circuits and the local oscillator, and the audio amplifier drives the speaker through another transformer.