Thermionic valves are non solid-state devices which amplify, switch, modify or create an electrical signal by controlling the movement of electrons in a low-pressure space. Thermionic valves have largely been replaced by solid-state devices such as transistors and diodes, but are still popular in pre-solid state applications, high-frequency transmitters and retro audio equipment. Thermionic valves available from Rapid include Edicron beam tetrode valves, pentode valves, double triode valves and rectifier valves.