Thermionic valves also known as vacuum tubes are a type glass tube device that has had its gas removed creating a vacuum space. It is comprised of electrodes that are designed to control electron flow between the electrodes sealed inside the thermionic valves.

Thermionic valves utilise a principal called thermionic emission in which a metal is heated allowing the electrons to be loosened up. This causes the positively charged anodes to attract the negative electrons emitted by the cathodes by causing an electric current to flow into the anode and out of the cathode.

Unlike transistors, these tubes can be easily replaced by the user. The advantage that they have over transistor circuits is that they have a wider dynamic range. This is mainly attributed to their higher operating voltages and overload tolerances.

Thermionic valves are widely used in equipment such as guitar amplifiers and other high end audio equipment.