Thermionic valve

Article

May 25, 2022

The thermionic valve (or vacuum tube) was the first "active" electronic component invented. By "active" we mean a component which, thanks to an external source of energy, outputs an amplified signal. The general operation is simple: the cathode emits electrons by thermionic effect, ie by heating; the flow of electrons, ie the current, passes between the cathode and another electrode, the anode, controlled by the voltage at which some metal parts (grids) are placed between the two electrodes. Since the current flow is due to electrons (not ions), some call the device thermoelectronic valve.

General

Until the sixties, thermionic tubes of various types were used in large quantities in electronic equipment such as radio receivers and transmitters, televisions and, in general, in all types of electrical signal amplifiers. The first electronic computers were also made entirely of thermionic tubes. The invention of the thermionic valve made the transition from radiotelegraphy to radiophony possible, since, by amplifying electrical signals, it made it possible to transmit not only telegraphic impulses, but also voices and sounds, thus inaugurating the era of mass media. Although today transistors, in their various forms and types, have supplanted tubes in almost every application, they remain the only means of amplifying signals at high frequencies with powers of the order of kilowatts or higher. They are still used by a small niche of "high fidelity" audio device enthusiasts. The semiconductor photodetectors can be advantageously used instead of the vacuum photodiodes in the presence of exposure to ionizing radiations, since they are immune to it. A vacuum tube, the magnetron, is still present in every common microwave oven. The cathode ray tube that was in common use in televisions and oscilloscopes is also a special type of thermionic tube.

Principle of operation

The thermionic tube consists of a glass casing (in some models it is metal or ceramic), in which a vacuum is made, containing a metal filament brought to incandescence (between 1,000 and 3,000 ° C) making it pass through by a electric current. Unlike the light bulb, it contains one or more metal elements (in the shape of a grid or screens), which can be connected from the outside. The metal filament, or rather a metal tube that surrounds it, in the case of indirect heating, is called a cathode. The outermost metal element is called the anode. Any intermediate elements are called grids. The operating principle of the thermionic tube is that of thermionic emission: every metal, especially at high temperatures, emits electrons, elementary electrical charges with a negative sign. If the cathode is negatively polarized with respect to the anode, i.e. if the cathode is connected to the negative pole of a battery and the anode to the positive one, a flow of electrons, i.e. an electric current, will be established between the cathode and anode (because the electrons are attracted to the anode). If the polarization is opposite, no electric current will pass between cathode and anode, because the anode will repel electrons. The result is that of a device capable of passing the current in one direction only, called a diode used mainly as a detector or as a rectifier. In the first types of valves, the cathode was directly heated, i.e. the cathode was made up of the filament itself. The system was abandoned, given the problems related to the need to make the cathodes work at different voltages (see multiple valves, double triodes, triode-pentodes). The task of heating the cathode is today entrusted to a filament similar to that of low voltage light bulbs, inserted inside a nickel alloy tube coated with e