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Show Radio Club Notes Conducted by G. S. Morris l esson 3, Part 1 Tladio Tubei All substance contains electrons and, a; the flow of electrons in-j creases with an increase of heat. it may be readily seen why a "filament" "fil-ament" is employed in a radio tube (DC), as a source of electrons; elec-trons; also why increasing the brilliancy of the filament increases increas-es the flow of electrons up to a certain point, beyond which the increase in-crease of electron flow drops off and the "life" of the filament drops off. In addition to the filament fila-ment wire which usually is constructed con-structed of tungsten there are added certain chemicals either during dur-ing construction of the filament wire or cemented thereon, some of these chemicals being known as strontium, barium or thorium. A tube containing a filament upon which barium or strontium has been cemented is called an "oxide" coated filament and burns rather brightly, a filament having thorium thor-ium drawn in with the wire during dur-ing construction operates at a lower low-er temperature and gives off a small degree of light. Such a tube is called a "dull emitter", the word "emitter" meaning of course an emitter of electrons. Just as water will boil more enthusiastically en-thusiastically if the generated steam is pumped away and a vacuum maintained, so will more electrons be emitted if those free ones are constantly drained away from the filament by the attraction attrac-tion of a positively charged "plate". The "plate" is constructed construct-ed of nickel and surrounds the filaments and is insulated from it. The positively charged plate attracts at-tracts the negative electrons and they are transported in space between be-tween and from filament to plate, thus providing a path over which the plate current may travel from "B" positive to filament negative. Between the plate and filament is the "grid" or control valve for flow of electrons which is much closer to the filament than the plate. This grid is given a negative nega-tive charge by connecting to the negative end of a "C" battery and, as it is close to and has a similar charge as the filament, it serves to repel and thereby govern the flow of electrons to the plate. In view of this a small charge on the grid negatively would have a great effect on the amount of current passing iroin piaie lu inauieiii,; as the grid charge (NEG) is increased increas-ed the plate current (POS) is decreased. de-creased. It is possible to place such a heavy charge on the grid by increasing the negative potential poten-tial that no current whatever will flow from plate to filament. The tube then is not functioning. Placing Plac-ing a positive charge on the grid would of course increase the flow current. Thus the grid serves as a valve to control the amount of plate current. The tube described above is known as a 3-element tube for use in circuits where a DC supply is used on filament, grid and plate, such as the 199, the 201, etc. The source of supply indicated corresponding corres-ponding to the "A" "B" and "C", battery receiver. iShould the above tube filament be connected to an "AC" source of supply there would exist in the headphones or speakers, by reason of the constantly reversing of supply sup-ply current, a heavily audible "hum" and, as this hum would in most cases exceed in strength the incoming signal, it would prove impracticable for normal use. To offset this and permit the filament fila-ment and source of electrons to receive re-ceive their supply from AC a small strip of metal is placed close to and heated by the filament and therefore becomes the source of electron supply with no AC component com-ponent from the filament of any consequence. A tube containing such an emitter is called a "heater" "heat-er" type tube by reason of the fact that the filament is now nothing more than a heater for this strip which is called the "cathode" of the tube. We now therefore have a 4-element tube, heater type, similar to types 227, 224, etc., and the receiver may now operate with no "A" battery, receiving its filament source of supply through a step-down AC transformer, the potential of which is governed by the type of tube in use. |