Thermionic Valves

THERMIONIC VALVES The term "thermionic valve" or "vacuum tube" is applied to a class of device that utilizes a cathode emitting electrons by vir tue of its being heated. It is essentially an evacuated envelope containing, in addition to the thermionic cathode, one or more additional electrodes used for controlling the current through the tube. It is made in a wide variety of sizes and types, ranging from a small radio receiving tube consuming a fraction of a watt to a radio transmitting tube several feet long and delivering more than i oo kw. of high-frequency power.

All of these devices may be classified by the combination of features falling under the headings: (I) The content inside the envelope, whether high-vacuum or par tial pressure of a gas or vapour, (2) The design and nature of the surface of the cathode, (3) The number of electrodes; from a minimum of two to as many as eight.

Envelope.

This must be absolutely airtight and glass has for years been the material usually employed. Within the past few years, however, steel has also been used and for some applications has met with much favour. In the case of glass, the parts of the envelope are joined by fusing in a flame, whereas in the case of steel, seam and spot resistance welding is the common method employed. The envelope is either highly evacuated or contains a relatively low pressure of an inert gas like argon or a vapour, usually mercury.

Seals.

It is necessary to have several electrical conductors through the envelope. Glass is almost universally used to insu late these conductors one from another and to furnish a vacuum tight joint. This requires glasses and metals having special char acteristics. The metals most commonly employed are copper, tungsten, and alloys including nickel and iron.

The making of satisfactory seals for large units requires a highly developed technique.

Cathode.

The thermionic cathode is the heart of the device. It is usually in the form of an incandescent filament or a surface heated from a tungsten filament. The latter is termed an indirectly heated cathode or a heater type cathode. For high anode voltages a pure tungsten filament is usually employed. For the lower volt ages the surface of the tungsten is covered by another material, usually thorium, that emits electrons more copiously and at a lower temperature. For the still lower voltages, as in the case of radio receiving tubes, a nickel surface is used, coated with a thin layer of barium and strontium oxides. In this case, either fila mentary or indirectly heated types are employed.

In high-vacuum tubes where the repelling forces between elec trons (space charge effect) is a factor that increases voltage loss, the cathodes are not surrounded by any enclosures. In the case of gas content tubes where positive ions result from gas molecules and these neutralize the space charge, the cathodes are often more or less surrounded by heat-conserving shields to increase the cathode efficiency. The more complicated indirectly heated types are used in high-vacuum receiving tubes to eliminate the effect of alternating filament current in causing "hum" interference in radio receivers. In gas or vapour-content tubes, indirectly heated cath odes are employed to increase the amount of usable electron emission.

Anode.

The anode receives the electron bombardment and must, therefore, dissipate heat which, depending upon the size of the device, may range from a small fraction of a watt to i oo kw. or more. Many different materials are used depending upon the ratings of the tube. The most common are molybdenum, nickel. tantalum, iron, and carbon. Such materials are used because they have relatively high melting points and, therefore, can be freed from occluded gases and yet be normally operated at a fairly high temperature which is below this degassing tempera tore. Copper is also frequently used where it can be water-cooled and, therefore, the operating temperature kept low.

Grids.

The electrodes, termed grids, are the control elements, directly or indirectly. In the case of high-vacuum tubes, they are usually made up of nickel, tungsten, or molybdenum wire wound in various shapes. In the case of gas or vapour tubes, the grids are more often sheet metal enclosures with one or more holes through which the discharge passes. Such grids are usually made of nickel or an alloy including nickel and iron. It is just as im portant that the electron emission from the grid be minimized as that it be copious from the cathode. To this end grids are fre quently of special design to conduct away the heat received as radiation from the cathode and coated with a dark surface like car bon or oxide to increase radiation or minimize electron emission.

Processes and Methods.

In the manufacturing procedure there are certain unique processes employed and some that must be carried out to an unusual degree. Cleanliness is of paramount importance, particularly as regards dust, oxidation, and corrosion. The mere fingering of electrodes will often be the cause of a de fective tube. For similar reasons the purity and uniformity of the raw materials used are of unusual significance. Resistance welding is almost the universal method used for fastening to gether the metal parts of the electrode structure. If parts of the electrode structure must be supported one from another, the preferable materials are mica, glass, natural or synthetic lava, or quartz. These can be readily freed from gas at a high temperature.

In many tubes, particularly receiving tubes, the various elec trodes must be held in space relation to one another within a few thousandths of an inch to assure satisfactory performance.

Many other details must be watched, such as vibration of elec trodes when mechanically shocked, current leakage over glass or insulators, metallic or carbonized dust or lint particles between electrodes and electron emitting materials on a grid. Any of these things and many other details may render a tube unsatisfactory in performance or life.

The evacuation is much more than a mere pumping out of the air from the envelope. Extremely small amounts of some gases will prevent satisfactory operation. This means that all parts of the interior must be subjected to a high temperature during pumping to allow gases in the materials to diffuse rapidly to the surface for pumping out. The heating of these inner metal parts by currents induced in them from an external coil excited by a high-frequency current is commonly employed. This method has been developed to a high degree.

Pumps for evacuation are usually connected in series, the one for the highest vacuum being of the vapour diffusion type, while the other one is of the mechanical piston type.

In order to expedite exhaust, various "getters" are used. A "getter" is a material which under certain conditions has the property of "cleaning up" ; that is, absorbing one or more gases by means of chemical change. For instance, the metal magnesium will rapidly combine with traces of water vapour or oxygen to form a stable oxide. Other metals used are barium, strontium, and aluminium. These are vaporized from the solid metal by heat ing during or after exhaust. They are also formed by heating a mixture of their chemical compounds inside a capsule so that the desired metallic vapour is released as desired. These getters can be seen in a glass tube as a dark or metallic coating inside part of the bulb.

As the mere inspection of the finished product even in opera tion reveals little, elaborate testing equipment has been developed to simulate the various conditions under which the tube must operate. In the early history of tube manufacture, the processes and machinery were largely patterned after the practices employed for lamps. To an increasing extent the past few years, special methods have been developed.

As in the case of other manufactured products, there has been a definite and rather rapid trend toward labour-saving machinery and the introduction of schemes to eliminate the necessity for high degrees of skill. Also automatic control has been introduced for as many of the variables involved as possible.

Vacuum tubes are constantly finding new fields of application and this sometimes results in radically new types which in turn bring into being new materials and processes.• BIBLIOGRAPHY.-E. D. McArthur, Electronics and Electron Tubes Bibliography.-E. D. McArthur, Electronics and Electron Tubes (1936) ; K. Henney, Electron Tubes in Industry ; J. H. More croft, Principles of Radio Communications, 3rd ed. (1933) ; W. G. Dow, Fundamentals of Engineering Electronics (1937) ; H. J. Nolte, J. E. Beggs, and T. A. Elder, "All-metal Tubes for Radio Receiving and Industrial Power Purposes," General Electric Review, vol. 38, No. 5 (May , pp. 212-218. (W. C. WH. )