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A vacuum tube (also called an electron tube or, in the UK, a valve) is a device sometimes used to amplify electronic signals. In most applications, the vacuum tube is obsolete, having been replaced decades ago by the bipolar transistor and, more recently, by the field-effect transistor. However, tubes are still used in some high-power amplifiers, especially at microwave radio frequencies and in some hi-fi audio systems.

Tubes operate at higher voltages than transistors. A typical transistorized amplifier needs 6 to 12 volts to function; an equivalent tube type amplifier needs 200 to 400 volts. At the highest power levels, some tube circuits have power supplies delivering several kilovolts.

Vacuum tubes are making a comeback among audiophiles who insist that tubes deliver better audio quality than transistors. These old-fashioned components are more electrically rugged than their solid-state counterparts; a tube can often withstand temporary overload conditions and power-line transients that would instantly destroy a transistor.

The major disadvantages of tubes include the fact that they require bulky power supplies, and the high voltages can present an electric shock hazard.