Before the start of the twentieth century, scientists knew that electrons could carry a current through a vacuum. They also knew that hot electrodes would emit electrons more easily than cool ones. These phenomena were put to use in the first electron tubes, known as diode tubes, for the purpose of rectification.

Cathode, filament, plate
In any tube, the electron-emitting electrode is called the cathode. The cathode is usually heated by means of a wire filament, similar to the glowing element in an incandescent bulb. The electron-collecting electrode is known as the anode or plate.

Directly heated cathode
In some tubes, the filament also serves as the cathode. This is called a directly heated cathode. The negative supply voltage is applied directly to the filament. The filament voltage for most tubes is 6 V or 12 V.

Indirectly heated cathode
In many tubes, the filament is enclosed within a cylindrical cathode, and the cathode gets hot from infrared radiation. This is known as an indirectly heated cathode. The cathode itself is grounded. The filament normally receives 6 V or 12 Vac.

In either the directly heated or indirectly heated cathode, electrons are driven off the element by the heat of the filament. The cathode of a tube is thus somewhat analogous to the source of a field-effect transistor, or to the emitter of a bipolar transistor.

Because the electron emission in a tube depends on the filament or “heater,” tubes need a certain amount of time—normally 30 seconds to a few minutes—to “warm up.” This waiting period can be an annoyance, and it seems bizarre at first to people who haven’t dealt with tubes before.

Cold cathode
In a gas-filled voltage-regulator tube, the cathode might not have a filament to heat it. Such a device is called a cold-cathode tube. The solid dot indicates that the tube is gas filled, rather than completely evacuated. Various different chemical elements are used in gas-filled tubes; mercury vapor is probably most common. In this type of tube, the “warmup” period is the time needed for the elemental mercury to vaporize, usually a couple of minutes.

The plate, or anode, of a tube is a cylinder concentric with the cathode and filament. The plate is connected to the positive dc supply voltage. Tubes typically operate at about 50 V to more than 3 kVdc. Because the plate readily attracts electrons but is not a good emitter of them, and because the exact opposite is true of the cathode, a diode tube works well as a rectifier for ac. Diode tubes can also work as envelope detectors for AM, although they are no longer used for that purpose.

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