Thermoionic valve
The thermonionic valve, also called electronic valve, vacuum valve, vacuum tube or bulb, It is an electronic component used to amplify, switch, or modify an electrical signal by controlling the movement of electrons in a vacuum space at very low pressure, or in the presence of specially selected gases.
The original valve was the critical component that enabled the development of electronics during the first half of the XX century, including the expansion and commercialization of radio broadcasting, television, radar, audio, telephone networks, analog and digital computers, industrial control, etc. Some of these applications predate the valve, but experienced explosive growth because of it.
Throughout its history, many types of valves were introduced, but the basic operating principles are:
- Effect Edison. The vast majority of electronic valves are based on the property that hot metals have to release electrons from their surface.
- Ionized gas. In other cases, electronic driving characteristics are used in ionized gases, this is mainly important in voltage regulators, mercury vapor grinders, T/R switching valve, etc.
- Photoelectric effect In other cases, the operating principle is based on the emission of electrons by the photoelectric effect.
The decline of this technology began with the invention of the transistor and the subsequent development of solid-state components that were much smaller, cheaper, and more reliable than the tube. However, today it still survives in certain specific applications, where for technical reasons it is more convenient. For example, in high power radio frequency transmitters and radar systems, magnetrons, TWT traveling wave valves, thyratrons, etc. are used. Cathode ray tubes or image capture tubes are still used in television and medical imaging systems, and in the home it is the basis of operation of the microwave oven. They also continue to be widely used in microphone, guitar and bass preamps, as well as hi-fi sound equipment.
History
Although the thermionic emission effect was originally reported by Frederick Guthrie in 1873, it is the research of Thomas Alva Edison that is the most often mentioned work. Edison, seeing that with use the glass of incandescent lamps was darkening, looked for a way to reduce this effect, carrying out various experiments. One of them was to insert a plate-shaped electrode into the bulb of the lamp, which was electrically polarized in order to attract the particles that apparently came off the filament. Despite the fact that Edison did not understand how it worked on a physical level, and was unaware of the potential of his "discovery", in 1884 he patented it under the name "Edison Effect".
By adding a flat electrode (plate), when the filament is heated, an agitation of the atoms of the material that covers it is produced, and the electrons of the valence orbits are accelerated, reaching escape velocities, with which they are forms an electron cloud above it. The thermionic cloud, strongly attracted by the plate, due to the positive potential applied to it, gives rise to the circulation of an electronic current through the valve between the filament and the anode. This phenomenon is called the Edison-Richardson or thermoionic effect.
At this point, we have that the simplest thermoionic valve consists of a glass ampoule, similar to that of incandescent lamps, which has been evacuated and in which two electrodes are enclosed, called cathode and anode.
Physically, the cathode consists of a tungsten filament, covered by a substance rich in free electrons, which is heated by the passage of a current. The anode is formed by a metallic plate that surrounds the filament at a certain distance and to which a positive potential is applied. Because it consists of two electrodes, the valve described above is called a diode.
While the cathode function is performed directly by the filament, it is a direct heating valve.
When you want to obtain higher currents through the valve and an electrical isolation between the source of heating current of the filament and that of the anode-cathode, an independent cathode is used, made up of a small metallic tube coated or "painted" with some material rich in free electrons, such as thorium oxide, surrounding the filament, electrically insulated, but too close to it to be able to heat it properly. In this case, the valve is called indirect heating, and then the heating current can even be alternating. In this type of valves, the filament is only the heating element and is not considered an active electrode. As the filaments are insulated, the filaments of all the equipment's valves can be connected together (in series or parallel), which is not possible with direct heating cathodes.
If other electrodes are added between the anode and cathode (called grids), the flow of electrons that reach the anode can be controlled or modulated, hence the name valve.
Due to the fact that the current inside the valve can only flow in one direction, one of the applications of thermionic valves is its use as a rectifier. Likewise, and given that with small potential differences applied between the grid and the cathode, considerable variations in the current flowing between the cathode and anode can be produced, another application, possibly the most important, is as an amplifier.
Features
Although there is a great diversity of types of thermoionic valves, both in their application and in their operating principles (control of the number of electrons, in triodes, tetrodes, pentodes; modulation of their speed in klystrons; coupling between the flow of electrons and an electromagnetic wave in progressive wave tubes; etc.), most of them share a series of common characteristics that have been strengthened as their technological development progresses.
Filaments
The filament is the heating element that provides enough energy for the cathode to emit an adequate amount of electrons.
In early tubes, the filament also acted as a cathode (direct-fired cathode). Later the functions were separated, leaving the filament only as a heater and the cathode as a separate electrode (indirect heating cathode). Both forms coexisted, since direct heating improves the thermal transfer between the cathode and the filament, while indirect heating greatly simplifies the design of the circuits and allows optimization of each of the electrodes.
The filament, being hot, is subjected to the sublimation effect of the material on its surface, that is, its transition to the gaseous state, which gradually reduces its section at certain points that now get hotter than the rest, increasing the sublimation in them until the filament breaks. This effect decreases enormously if you work at low temperatures with materials with a high melting point (wolfram...). For this reason the temperature of the filaments has been decreasing.
Microphonic effect: this effect consists of the transmission to the filament of mechanical vibrations. When the filament vibrates, it transmits these oscillations to the cathode, varying its distance from the grid, which produces a modulation in the electron current. At the anode, the useful signal appears modulated by mechanical vibrations, which is especially unpleasant in the case of audio amplifiers, since the vibrations that are coupled come from the speaker itself.
Magnetic fields can also create oscillations in the filament, which is why some valves were enclosed in tubes with high magnetic permeability (mu-metal).
Cathodes
The cathode is responsible for the emission of electrons, which must be constant throughout the life of the valve. Unfortunately, this is not the case, and the cathodes wear out as they age.
To prolong the life of the filaments, the operating temperature of the cathodes has been getting lower and lower, thanks to the use of materials with a lower electron withdrawal potential (thorium alloys, lanthanide oxides...).
The cathodes must also be good conductors, which limits the application of some coatings to very particular applications. For example, calcium oxide often coats the filaments of vacuum fluorescent displays (VFDs).
Anodes
The anode receives the flow of electrons that, in most valves, have been accelerated until they acquire great energy that they transfer to the anode when they hit it. For this reason, the anodes of power valves are large, often massive, and are part of the valve body itself, and can be cooled directly from the outside, by contact with a cold surface, pressurized air, water vapor, etc. Previously, the cooling of the anode was carried out mainly by radiation, so the glass bulbs were large and separated from the anode, so that it could acquire a high temperature.
Secondary emission is an effect, normally undesirable, that occurs at the anode, when the incident electrons, of great energy, remove electrons from the metal. Although on some tubes this effect is exploited for gain, on most it degrades the signal and should be avoided.
Empty
A lower degree of vacuum implies the presence of a greater number of gas molecules in the valve, increasing the number of collisions with the electrons and decreasing the performance of the tube. In addition, a lower vacuum implies greater wear on the filaments, which is why, historically, progress has been made towards high-vacuum valves through a joint advance in all the other components. However, some valves such as thyratrons base their operation on the presence of certain gases filling the tube.
Metals and other materials have properties of absorption and adsorption of gases from the atmosphere, and when heated at low pressure they slowly release them. Therefore, even if all the air is extracted from a valve, with use, the internal vacuum is reduced. To avoid this, the getter is used, which is a material (for example, magnesium) that evaporates once the tube is sealed. The evaporated magnesium is deposited on the glass surface forming a shiny coating. The getter adsorbs any gas molecules that may be released into the tube, maintaining the integrity of the vacuum. When air enters the tube, the getter turns whitish.
Ceramics
The most widely used material in the construction of the "container" of the valve is glass, already inherited from the manufacture of light bulbs. But glass has a low melting point, is a good thermal insulator and is brittle, so for high power and radiofrequency tubes it is preferred to use ceramics, which are less brittle, have good thermal conductivity and a high melting point. Its Achilles heel has been the establishment of tight and durable unions between the ceramic and the metal (connections of the electrodes, anode, heatsinks). With the problem resolved, ceramic has displaced glass in power and microwave valves.
Typology
Based on the number of electrodes, valves are classified into: diodes, triodes, tetrodes, pentodes, and so on.
Other types of thermoionic valves are:
- Tiratrons: gas-filled triodes.
- Cathodic ray tube: TV screens, oscilloscopes, etc.
- Iconoscopios, orticones, vidicones, plumbicones: they are television camera tubes.
- Magic eye: tuning indicators, bridge balance indicator, voltmeters and multimeters, saturation indicator in magnetic tape recorders.
- Klistrones, magnetrons, progressive wave tubes, all of them microwave devices.
- Decatron and trocotron, tube counters.
- Selectron: digital memory.
- Tube Williams: Digital Memory.
- VFD: fluorescent vacuum displays.
Similar
Similar to thermionic valves, but without using the Edison effect are:
- Mercury rectifiers, Ignitrones, for high power handling.
- Nixie tubes, neon displays.
- Photoelectric cells, based on the Einstein effect.
- T-R cells or tubes for the protection of radar receivers.
- Detector Geiger-Müller, ionizing radiation detector.
- Stabs, voltage regulator tubes.