International projectionist (Jan-Dec 1936)

July 1936 INTERNATIONAL PROJECTIONIST 21 FIGURE 4 Testing tubes for shock circuit ceases to be sufficiently linear with the input voltage, and causes distortion in the output. How the inter-electrode spacing has been decreased and the areas increased is indicated by Fig. 3, which shows a cross-section plan of the 300A tube in dotted lines, superimposed on that of the 252A tube which is an earlier tube of approximately the same bulb size and developed for plate potentials up to 500 volts. This latter tube is capable of delivering only about one-half of the power of the 300A tube under similar operating conditions. Important Structural Changes The plate of the 300A tube has been widened in the direction of the plane of the filament and the transverse spacing made less. To obtain the increased width of the plate without using a larger bulb, the plate construction has been changed. In the 252A tube the two halves of the plate are joined by flanges at the ends, and four support wires are fastened to the plate near the corners. In the 300A tube, the two halves are joined by wide flanges on the sides, to which the support wires are secured. This allows the plate to occupy the entire distance in width occupied by the plate and flanges in the 252A tube, and has the additional advantage of stiffening the middle portion of the plate. The grid has been correspondingly widened in one direction and narrowed in the other to decrease the distance between the grid and filament. Besides these changes, it has been essential to have the filament extend the full length and width of the plate, so that the plate current may be drawn as nearly as practicable over the entire plate area. This has been accomplished in the 300A tube by arranging the filament in a double M instead of the single M usually used. This is shown in Fig. 2. The two M's are connected in parallel across the filament voltage supply. In bringing the filament, grid and plate closer together, the danger of short-circuit from accidental physical contacts of the parts has been increased. Design features — such as the side ribs on the plates, and the rigid fastening of both ends of the support wires — improve the stiffness and offset this danger. The inherent stiffness of the tube ele ments is tested by attaching the tube to a cord and allowing it to swing as a pendulum against a stop in the center position. Such a test is shown in progress in Fig. 4. The tube is drawn back to increasing angles and then released, and the angle at which failure occurs, as indicated by a relay circuit sensitive to short-circuits of very short duration, is a measure of the stiffness of the elements. Maximum Plate Voltage Typical characteristics are shown in Fig. 5. The output of the tube will depend, of course, on the plate and grid voltages selected. As the result of extensive life tests, it has recently been possible to raise the maximum operating plate voltage for the 300A tube from 350 to 450 volts. With 450 volts on the plate and a plate current of 60 milliamperes, a single tube will deliver 12.5 watts into a 400-ohm load with second and third harmonics 26 db and 39 db, respectively, below the fundamental. Two such tubes in push-pull may be expected to deliver 25 watts with harmonic levels of the order of 40 db below the fundamental. 90 eo FIGURE 5 Plate current and grid voltage characteristics 2 < -j 60 50 Z40 10 FILAMENT VOLTAGE AC = 5 / ' FILAMENT CURRENT = 1.2 AMPERES ' PLATE „„ j VOLTAGE-450/ 350/ 250/ 150/ -130 -120 -IIO -90 -80 -70 -60 -50 -40 -30 -20 -10 0 GRID POTENTIAL IN VOLTS Sarnoff, RCA Head, Discusses Television Possibilities FORMAL opening of the New York City television field test station atop the Empire State building produced an authoritative statement on the commercial possibilities of this baby art by one best qualified to give it, David Sarnoff, president of RCA, sponsor of the test. Said Sarnoff, in part: "As a commercial operation, television has not yet arrived, it is not around the corner, nor even around the block. But we have advanced sufficiently far to say that the technical obstacles are not insuperable. RCA has invested millions of dollars and an incalculable sum of scientific genius in bringing television to the point where it may be moved from the seclusion of the laboratory to the freedom of an experimental service in the field. Larger Image Vitally Necessary "The test period may be a long and costly one. Critical technical problems must still be solved. The present range of television transmission from a single station is scarcely more than 25 miles, and existing wire facilities are not suitable, for inter-connecting television stations. Radio relays or a modified wire system must be developed to enlarge the range and potential audience for a single costly program. "There is also the problem of further increasing the size of the television picture without loss of clarity. The experimental television observer today watches the stage from the backrow of the second balcony. We want to give him a seat in the front row of the orchestra. When television broadcasting reaches the stage of commercial service, advertising will have a new medium, perhaps the most effective ever put at its command. Will Not Supplant Broadcasting "The new medium will not supplant or detract from the importance of present-day broadcasting. Rather, it will supplement this older medium of sound. Television will add little to the enjoyment of the symphony concert as it now comes by radio to your living room. Sound broadcasting will remain the basic service for the programs particularly adapted to its purpose. On the other hand, television will bring into the home much visual material — news events, drama, paintings, personalities — which sound can bring only partially or not at all. Service Free; Advertising Will Pay "The benefits which have resulted from the industrial sponsorship of sound broadcasting indicate that our major television programs will come from the same source. It requires little imagination to see the advertising opportunities of television . . . Commercial announcement can be expanded through television to include demonstration and informational services that will be of value to the public as well as to the advertiser. "Broadcasting has won its high place in the United States because — unlike European listeners — American set owners have had their broadcasting services free. Despite the greater cost of television programs, I believe that owners of television receivers in the United States will not be required to pay a fee for television programs. That is an aspect of the television problem in which the advertising fraternity will doubtless co-operate in finding the solution."