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 negatively charged electrons, which are attracted to and captured by the anode, which is given a positive voltage by a power supply. The control grid between the cathode and anode functions as a "gate" to control the current of electrons reaching the anode. A more negative voltage on the grid will repel the electrons back toward the cathode so fewer get through to the anode. A less negative, or positive, voltage on the grid will allow more electrons through, increasing the anode current. A given change in grid voltage causes a proportional change in plate current, so if a time-varying voltage is applied to the grid, the plate current waveform will be a copy of the applied grid voltage.

A relatively small variation in voltage on the control grid causes a significantly large variation in anode current. The presence of a resistor in the anode circuit causes a large variation in voltage to appear at the anode. The variation in anode voltage can be much larger than the variation in grid voltage which caused it, and thus the tube can amplify, functioning as an amplifier.

The grid in the first triode valve consisted of a zig-zag piece of wire placed between the filament and the anode. This quickly evolved into a helix or cylindrical screen of fine wire placed between a single strand filament (or later, a cylindrical cathode) and a cylindrical anode. The grid is usually made of a very thin wire that can resist high temperatures and is not prone to emitting electrons itself. Molybdenum alloy with a gold plating is frequently used. It is wound on soft copper sideposts, which are swaged over the grid windings to hold them in place. A 1950s variation is the frame grid, which winds very fine wire onto a rigid stamped metal frame. This allows the holding of very close tolerances, so the grid can be placed closer to the filament (or cathode).

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