Control grid

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.

A more negative voltage on the grid will repel the electrons back toward the cathode so fewer get through to the anode.

The grid in the first triode valve consisted of a zig-zag piece of wire placed between the filament and the anode.

The grid is usually made of a very thin wire that can resist high temperatures and is not prone to emitting electrons itself.

By placing the control grid closer to the filament/cathode relative to the anode, a greater amplification results.

A phenomenon known as the Miller Effect causes the input capacitance of an amplifier to be the product of Cag and amplification factor of the valve.

This, and the instability of an amplifier with tuned input and output when Cag is large can severely limit the upper operating frequency.

These effects can be overcome by the addition of a screen grid, however in the later years of the tube era, constructional techniques were developed that rendered this 'parasitic capacitance' so low that triodes operating in the upper very high frequency (VHF) bands became possible.