International projectionist (Oct 1931-Sept 1933)

THE WHAT, WHY AND HOW OF SOUND VACUUM TUBES James J. Finn II. Evaculating Process and Methods of Testing THE degree of vacuum to which the bulb is exhausted may be considered second in importance only to the proper preparation of the filament, and is one of the decided points of difference between a good and a poor one. The evacuating process from which the rather vague appelation "vacuum tube" is derived, consists of the physical exclusion, by a complicated process, of almost all of the gases from the inside of the glass bulb. The ideal condition under which the elements comprising a thermionic vacuum tube should operate is in a complete vacuum. Now, a total vacuum is practically unknown; consequently, it is not to be expected in a commercial article. Nevertheless, the nearer the approach to it that can be obtained commercially, the better and more efficiently the vacuum tube will be likely to operate. When gas molecules are present inside the glass bulb they interfere with the free passage of the electrons between the filament and the plate, and collision occurs. The free moving electrons move with great velocity, and when one of them strikes a gas atom with sufficient force, it may dislodge another electron from the atom. This electron follows the original electron stream to the plate, leaving behind a positive residue which, as in the case of the divided barium atom, form an ion. This ion, being positive, flies in the opposite direction to the electron stream towards the filament and does two things: (1) it bombards the filament violently, which tends to tear away its oxide coating, and (2) it acts as a positively-charged grid, thereby increasing the positive field and the current flow to the plate. The electron stream to the plate being thus amplified, bombards the plate surface more and more violently until the plate itself becomes red hot and acts as a separate emitter of electrons. When this occurs, the tube breaks down and distortion sets in. Extreme precautions are taken, therefore, to insure that as many of the gas particles as possible are withdrawn from the tube during evacuation. The proc ess followed in this regard is termed "pumping." Before the inside of the tube has been assembled in its glass encasement for pumping, all grease, oil, oxides and other impurities have been removed from the surface of the glass support and the nickel plate, grid, and wire supporting elements by suitable and careful treatment. All metal parts are pre-treated in high vacuum furnaces and in hydrogen furnaces to cleanse them "internally." This sounds like a strange process for solid parts, but the vacuum requirement specified for Western Electric tubes is so severe that not only does the space within the tube have to be free of gas, but the parts themselves must be free also, to be sure that no occluded gases will be freed during the operation of the tube. Evacuating Process The time interval between the furnace treatment and the pumping operation is kept as short as possible to avoid reabsorption of gases. In preparing the tubes for the pumping operation, the tubulation from the glass bulb is first sealed onto a glass manifold which is capable of accommodating six Fig. 3 Section of life testing rack for vacuum tubes [22] tubes. These tubes and the manifold are encased in an oven which is electrically heated, and the filament, grid and plate connections are connected to suitable sources of voltage. The pumping system for exhausting these tubes consists first of a two-stage, rotary oil beam capable of exhausting the tubes to a vacuum equivalent to a pressure of one-thousandth of a millimeter of mercury. Following the oil pump is a mercury diffusion pump capable of raising this vacuum to a pressure equivalent to one-millionth of a millimeter of mercury. Interposed between the mercury diffusion pump and the manifold is a liquid air trap for condensing the murcury vapor. Mercury at room temperature, approximately 20° C, has a vapor pressure higher than that which can be obtained with the diffusion pump and much higher than can be permitted in the vacuum tube. By interposing this liquid air trap between the diffusion pump and the manifold, the mercury vapor pressure at the outlet of the manifold is reduced to that at liquid air temperatures, which is lower than the required pressure in vacuum tubes. Consequently, an extremely low pressure area is created at the outlet of the manifold, which gases from inside the tube attempt to raise by flowing out toward this low pressure area. These gas particles are carried along by the mercury vapor stream and are drawn out by the oil pump, leaving behind the condensed mercury vapor for further use. After a good vacuupi has been obtained by the pumping system alone, the temperature of the oven surrounding the tubes is increased as high as possible without causing a softening or collapse of the glass bulb, in order to expand and liberate as much of the occluded gases in the glass and metal parts of the tube as possible. After thoroughly baking the tubes for a while with the pumping system still in operation, the filaments are lighted and voltages are applied to the grid and the plate elements of such a value as to create an electron bombardment sufficient to heat the anodes to a temperature of from 800 to 1,000 degrees C. As a final operation, a small amount of