International projectionist (Oct 1931-Sept 1933)

22 INTERNATIONAL PROJECTIONIST February 1933 CHARACTERISTICS of Western Electric 262A , vacuum tube ■ — a three element tube having an indirectly heated cathode which permits operation of the heater element directly on alternating current. Rating Normal Filament Voltage . 10 v. AC or DC Heater Current 0.29 to 0.35 amp. Average Heater Current 0.32 amp. Maximum Plate Voltage 180 volts Grid Voltage — 4.5 volts Average Plate Current. .3.0 milliamperes Amplification Factor 15 Plate Resistance 16,000 ohms Dimensions Maximum Diameter 1-13/16" Maximum Length 5^/4" in the nickel cathode which is coated with the thermionically active material. The heater is operated at ten volts and .32 amperes, appreciably less current than is normally used for tubes of this class, and the combined effect of the reduction in current and the arrangement of the filament is to minimize the disturbance current due to magnetic fields. The third form of disturbance current occurs because of the conductance and capacitance between heater and grid, and heater and plate. The effect may be better understood by reference to Figure 2, where the resistance and capacity between heater and grid, and heater and plate are shown in dotted lines. The heater voltage causes a current to flow through the impedance Zg, of the grid circuit, and through the capacity Ci and resistance n in parallel, back to the other side of the heater. The voltage drop across Zg due to this current appears on the grid of the tube and produces a corresponding current in the plate circuit. In a similar way the resistance and capacity between the heater and plate are responsible for a disturbance circuit entering the plate circuit directly. Correctives Applied In the actual operation of these tubes, the cathode — instead of being connected to one end of the heater winding — is connected effectively to the midpoint. Under these conditions capacities and resistance exist between both ends of the heater, and the grid and plate, but the action is essentially the same. To reduce the disturbance currents introduced in this manner, the gridheater conductance and capacitance has been made lower than for the usual indirectly heated tubes. This reduction has been brought about by supporting the grid between two lavite blocks, evident in Figure 1. and in making connection to the grid through the top of the bulb. In this way both the capacity and conductance between beater and grid leads, usually existing in the common glass supporting press, have been eliminated. The only effective leakage path over glass in the new tube is down the stem and over the entire length of the bulb. This resistance is held greater than 100,000 megohms, and the grid to heater capacity is only about a thousandth of that of the more usual types of indirectly heated cathode tubes. S'uch values permit the use of resistances of several megohms in the grid circuit without materially increasing the disturbance output. Other Noise Sources Plate-to-heater capacities are not sufficiently large to contribute materially to the disturbance outputs, and insulation leakage between plate and heater sufficient to cause appreciable current is prevented by the electrostatic shield already described. This shield bells out over the glass press where the heater leads enter it. and prevents the deposition of material vaporized from the hot surface of the cathode. It is this deposited material that usually forms the conducting path between plate and f — 1I I -. A A , ^ -■>-'A.^ 2 HlilHiH FIGURE 3 The W. E. 262-A vacuum tube heater support wires. In addition to the disturbance currents discussed here, there are certain low level sources of noise, such as the shot effect, and the thermal noises due to the resistances. The level of these unavoidable disturbances forms a natural lower limit by which other disturbances may be judged. Measurements made of disturbance currents in the 262-A tube due to these causes show them to be from 118 to 127 db. below 1 milliampere for the shot effect, and about 105 db. for the thermal noise with 2.0 megohms in the grid circuit. Disturbance currents in the 262-A tube due to the alternating current supply of the heater are somewhat greater than the shot effect and slightly less than the thermal noise. They have thus been reduced until they no longer exist as factors limiting the application of the tube. 6 6 FIGURE 2 Possible disturbance currents in a vacuum tube flow from the heater to plate and grid through the interelectrode capacitance and conductance Science Forum THE STROBOSCOPE — WHAT AND HOW (Courtesy, General Radio Company) The quickness of the hand deceives the eye. But the eye knows a trick or two, and aided by ingenious mechanisms, it is not deceived by the gryations of machinery at far higher speeds than the trickster s hand achieves. Hence the stroboscope. THE stroboscope consists fundamentally of a device which permits intermittent observations, either visual or photographic, of a moving object in such a manner as to reduce the speed of, or stop, the motion. The slow-motion picture is a familiar example of the interesting and profitable information which may be derived from a leisurely study of events which necessarily take place at a a high rate of speed. The tennis player cannot slow the championship stroke to accommodate the laggard eye of the novice, but the camera can, and the motion picture camera is a stroboscope, but not all cameras are stroboscopes. The camera shutter, operating at high speed, chops up the action into a number of small elements, so short that movement is not apparent in any one. The film can then be projected at normal speed with results that are instructive, or even backward with results that are amusing. The function of the shutter is to exclude light from the film except for brief flashes. It seems reasonable that the same result can be obtained by shutting off the light from the object, except for brief flashes. Obviously, this type of stroboscope is well adapted for visual observations. Photography must still be used, if a nonrepeated event is viewed, to store the elementary views and to release them later