Control grid Control grid The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve (thermionic diode) to create the first amplifying vacuum tube, the Audion (triode). In a valve, the hot cathode emits
Valve audio amplifier pentodes or beam tetrodes, 6L6 or EL34 but occasionally KT88, 6550, or the lower-power EL84 in "Ultra-Linear" connection. The output stage is preceded by a voltage amplification stage (pentode or twin triode) and a phase-splitter (twin triode). Twin triodes with two identical sections in one envelope are used, usually the noval types 12AT7, 12AU7, or 12AX7 or equivalents, less usually the octal 6SN7. The earliest mass usage of valve audio amplifiers was for telephony. Valve amplifiers were critical in development of long-distance telephone circuits and submarine telephone cables. Radio applications followed soon after, where valves were used for both the