Vacuum Tube Principles

In the previous chapters we have seen the manner in which an electric current flows through a metallic conductor as a result of an electron drift. This drift, which takes place when there is a difference in potential between the ends of the metallic conductor, is in addition to the normal random electron motion between the molecules of the conductor.

The electron may be considered as a minute negatively charged particle, having a mass of 9 X 10~28 gram, and a charge of 1.59 X 10~19 coulomb. Electrons are always identical, regardless of the source from which they are obtained.

An electric current can be caused to flow through other media than a metallic conductor. One such medium is an ionized solution, such as the sulfuric acid electrolyte in a storage battery. This type of current flow is called electrolytic conduction. Further, it was shown at about the turn of the century that an electric current can be carried by a stream of free electrons in an evacuated chamber. The flow of a current in such a manner is said to take place by electronic conduction. The study of electron tubes (also called vacuum tubes, or valves) is actually the study of the control and use of electronic currents within an evacuated or partially evacuated chamber.

Since the current flow in an electron tube takes place in an evacuated chamber, there must be located within the enclosure both a source of electrons and a collector for the electrons which have been emitted. The electron source is called the cathode, and the electron collector is usually called the anode. Some external source of energy must be applied to the cathode in order to impart sufficient velocity to the electrons within the cathode material to enable them to overcome the surface forces and thus escape into the surrounding medium. In the usual types of electron tubes the cathode energy is applied in the form of heat; electron emission from a heated cathode is called thermionic emission. In another common type of electron tube, the photoelectric cell, energy in the form of light is applied to the cathode to cause photoelectric emission.

4-1 Thermionic Emission

Electron Emission of electrons from the Emission cathode of a thermionic electron tube takes place when the cathode of the tube is heated to a temperature sufficiently high that the free electrons in the emitter have sufficient velocity to overcome the restraining forces at the surface of the material. These surface forces vary greatly with different materials. Hence different types of cathodes must be raised to different temperatures to obtain adequate quantities of electron emission. The several types of emitters found in common types of transmitting and receiving tubes will be described in the following paragraphs.

Cathode Types The emitters or cathodes as used in present-day thermionic electron tubes may be classified into two groups: the directly-heated or jilament type and the indirectly-heated or heater-cathode type. Directly-heated emitters may be further subdivided into three important groups, all of which are commonly used in modern vacuum tubes. These classifications are: the pure-tungsten filament, the thoriated-tungsten filament, and the oxide-coated filament.

The Pure Tung- Pure tungsten wire was used sten Filament as the filament in nearly all the earlier transmitting and

Figure 1 ELECTRON TUBE TYPES

The new General E lectric ceramic triode (6BY4) is shown alongside a conventional miniature tube (6265) and an octal-based receiving tube (25L6). The ceramic tube Is designed for rugged service and features extremely low lead Inductance.

Figure 1 ELECTRON TUBE TYPES

The new General E lectric ceramic triode (6BY4) is shown alongside a conventional miniature tube (6265) and an octal-based receiving tube (25L6). The ceramic tube Is designed for rugged service and features extremely low lead Inductance.

receiving tubes. However, the thermionic efficiency of tungsten wire as an emitter (the number of milliamperes emission per watt of filament heating power) is quite low, the filaments become fragile after use, their life is rather short, and they are susceptible to burnout at any time. Pure tungsten filaments must be run at bright white heat (about 2500° Kelvin). For these reasons, tungsten filaments have been replaced in all applications where another type of filament could be used. They are, however, still universally employed in large water-cooled tubes and in certain large, high-power air-cooled triodes where another filament type would be unsuitable. Tungsten filaments are the most satisfactory for high-power, high-voltage tubes where the emitter is subjected to positive ion bombardment caused by the residual gas content of the tubes. Tungsten is not adversely affected by such bombardment.

The Thoriated- In the course of experi-Tungsten Filament ments made upon tungsten emitters, it was found that filaments made from tungsten having a small amount of thoria (thorium oxide) as an impurity had much greater emission than those made from the pure metal. Subsequent development has resulted in the highly efficient car-burized thoriated-tungsten filament as used in virtually all medium-power transmitting tubes today.

Thoriated-tungsten emitters consist of a tungsten wire containing from 1% to 2% thoria. The activation process varies between different manufacturers of vacuum tubes, but it is essentially as follows: (1) the tube is evacuated; (2) the filament is burned for a short period at about 2800° Kelvin to clean the surface and reduce some of the thoria within the filament to metallic thorium; (3)

the filament is burned for a longer period at about 2100° Kelvin to form a layer of thorium on the surface of the tungsten; (4) the temperature is reduced to about 1600° Kelvin and some pure hydrocarbon gas is admitted to form a layer of tungsten carbide on the surface of the tungsten. This layer of tungsten carbide reduces the rate of thorium evaporation from the surface at the normal operating temperature of the filament and thus increases the operating life of the vacuum tube. Thorium evaporation from the surface is a natural consequence of the operation of the thoriated-tungsten filament. The carburized layer on the tungsten wire plays another role in acting as a reducing agent to produce new thorium from the thoria to replace that lost by evaporation. This new thorium continually diffuses to the surface during the normal operation of the filament. The last process, (5), in the activation of a thoriated tungsten filament consists of re-evacuating the envelope and then burning or ageing the new filament for a considerable period of time at the normal operating temperature of approximately 1900° K.

One thing to remember about any type of filament, particularly the thoriated type, is that the emitter deteriorates practically as fast when "standing by" (no plate current) as it does with any normal amount of emission load. Also, a thoriated filament may be either temporarily or permanently damaged by a heavy overload which may strip the surface layer of thorium from the filament.

Reactivating Thoriated-tungsten fila-

Thoriated-Tungsten ments (and only thoriated-Filaments tungsten filaments) which have lost emission as a result of insufficient filament voltage, a severe temporary overload, a less severe extended overload, or even normal operation

Figure 2 V-H-F ond U-H-F TUBE TYPES

The tube to the left in this photograph Is a 955 "ocorn" trlode. The 6F4 acorn trlode Is very similar in appearance to the 955 but has two leads brought out each for the grid and for the plate connection. The second tube Is a 446A "lighthouse" triode. The 2C40, 2C43, and 2C44 are more recent examples of the same type tube and are essentially the same in external appearance. The third tube from the left is a 2C39 "oilcan" tube. This tube type is essentially the Inverse of the lighthouse variety since the cathode and heater connections come out the small end and the plate is the large finned radiator on the large end. The use of the finned plate radiator makes the oilcan tube capable of approximately 10 times as much plate dissipation as the lighthouse type. The tube to the right is the 4X150A beam tetrode. This tube, a comparatively recent release, Is capable of somewhat greater power output than any of the other tube types shown, and is rated for full output at 500 Mc. and at reduced output at frequencies greater than

1000 Mc.

may quite frequently be reactivated to their original characteristics by a process similar to that of the original activation. However, only filaments which have not approached too close to the end of their useful life may be successfully reactivated.

The actual process of reactivation is relatively simple. The tube which has gone "flat" is placed in a socket to which only the two filament wires have been connected. The filament is then "flashed" for about 20 to 40 seconds at about l'/2 times normal rated voltage. The filament will become extremely bright during this time and, if there is still some thoria left in the tungsten and if the tube did not originally fail as a result of an air leak, some of this thoria will be reduced to metallic thorium. The filament is then burned at 15 to 25 per cent overvoltage for from 30 minutes to 3 to 4 hours to bring this new thorium to the surface.

The tube should then be tested to see if it shows signs of renewed life. If it does, but is still weak, the burning process should be continued at about 10 to 15 per cent overvoltage for a few more hours. This should bring it back almost to normal. If the tube checks still very low after the first attempt at reactivation, the complete process can be repeated as a last effort.

The Oxide- The most efficient of all

Coated Filament modern filaments is the oxide-coated type which con sists of a mixture of barium and strontium oxides coated upon a nickel alloy wire or strip. This type of filament operates at a dull-red to orange-red temperature (1050° to 1170° K) at which temperature it will emit large quantities of electrons. The oxide-coated filament is somewhat more efficient than the thoriated-tungsten type in small sizes and it is considerably less expensive to manufacture. For this reason all receiving tubes and quite a number of the low-powered transmitting tubes use the oxide-coated filament. Another advantage of the oxide-coated emitter is its extremely long life — the average tube can be expected to run from 3000 to 5000 hours, and when loaded very lightly, tubes of this type have been known to give 50,000 hours of life before their characteristics changed to any great extent.

Oxide filaments are unsatisfactory for use at high continuous plate voltages because: (1) their activity is seriously impaired by the high temperature necessary to de-gas the high-voltage tubes and, (2) the positive ion bombardment which takes place even in the best evacuated high-voltage tube causes destruction of the oxide layer on the surface of the filament.

Oxide-coated emitters have been found capable of emitting an enormously large current pulse with a high applied voltage for a very short period of time without damage. This characteristic has proved to be of great value

Figure 3

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