The control grid is, simply stated, an electrical conductor used in vacuum tubes. It is an electrode that modulates the flow of electrons in a plate circuit, or more specifically in a cathode-to-anode circuit. The control grid is an integral part of vacuum tubes which are used in a multitude of electronic applications, such as in radio broadcasting, computers, televisions, industrial processes, radars, telephones, and in sound reproduction.
A vacuum tube is any device that controls the movement of electrons in order to create, increase, transfer or modify electric signals. The electrons are contained in a low-pressure, vacuum-like space, hence the term “vacuum tube.” There is an anode or metal plate that is positively charged, and a cathode or filament that is negatively charged. Electrons flow from the “hot” filament to the “cold” plate or from cathode to anode. Because of this one-directional flow, a diode is formed. The control grid, as mentioned, is the mechanism that modulates the electron flow in the diode.
In North America, vacuum tubes are also called electron tubes, and in the UK, they're called thermionic valves. The simplest vacuum tubes look like incandescent light bulbs; these have a filament in a glass envelope, a vacuum-like space, and a metal plate. Over the years, variations to the vacuum tube have been made to make it more efficient. There now are “soft tubes” that are filled with low-pressure gas.
A control grid is placed in a vacuum tube to adjust the anode current. With the addition of the grid in the diode, the assembly now becomes a triode. Because the control grid is an electrode or conductor, it acts as the bridging mechanism between the anode and the space in the tube. It can cause variations in the anode current by adjusting its voltage supply. Even very small changes in voltage can cause significant current fluctuations.
A simple control grid usually consists of a zigzagging piece of wire which runs from the filament to the anode. More sophisticated types can look like a helical mesh that connects a cylindrical cathode to a cylindrical anode. The grid is often made up of very thin wires that are highly resistant to heat. These wires are usually plated with gold to withstand heat and to prevent them from emitting electrons as well. The control grid may be placed closer to the cathode, which results in greater and faster amplification of current. There may also be more than one control grid in a vacuum tube.