International projectionist (Jan-Dec 1947)

Bell Labs' New Traveling-Wave Tube Amplifies Over a Band 40 Times Wider Than Present Tube IT looks like an oversize thermometer. But this entirely new and yet surprisingly simple vacuum tube amplifier, known as the traveling-wave tube and developed by Bell Telephone Laboratories, may be of far-reaching significance in television and long-distance telephone transmission. It may even make it practicable to send television all the way across the country. Preliminary tests indicate the tube may amplify dozens of full color or black-and-white television programs simultaneously. Or it could theoretically handle more than 10,000 simultaneous cross-country telephone conversations, or over 100 million words a minute by telegraph! Over the past three decades in which amplifier engineers have had the vacuum tube to work with, considerable progress has been made in developing tubes and associated circuits capable of amplifying wider and wider bands of frequencies. For example, a band width of about 10,000 cycles at the beginning of the period has been broadened to one of about 20.000,000 cycles (20 megacycles) today. Breaking sharply with the past, this magic traveling-wave tube promises to amplify voice or television signals over a frequency band 40 times as wide as that of the best tube now in use and to give many times the amplification. Enormous Amplifying Power Conservative figures for the new tube show a power gain of 10,000 over a band width of 800 megacycles! By comparison, the present pentode tube can give a power gain of only 10 over a band width of 20 megacycles ; while a velocity modulation tube, operating in the microwave range, gives the same amplification over a band width of 10 megacycles. Engineers believe that even these SUCCESSFUL 16-MM SHOWINGS (Continued from preceding page) ready to insert a new one immediately. Arrange with someone to be responsible for turning off and turning on the lights. The lights should be turned off the instant the picture starts (not before) and should be turned on again as the end title fades out. A few minutes before the film is to be shown, be sure to turn on the sound in order that the amplifier will be "warmed up" and ready to go when the time comes. Do not take down the equipment until the meeting is over. As soon as the film is completed, be ready to proceed immediately with the next item on the program. Avoid any undue delay. figures for the new tube can be improved, for it has by no means reached its full development. In addition to these contributions to wideband amplification, the tube is remarkably simple. It is only about 18 inches long and only a few inches across. An entire amplifier hook-up, of which the tube is the heart, occupies a space less than two feet long and a few inches square, and it can do the work of a maze of tubes and circuits. The idea underlying the tube was proposed and was worked on during the war by a British scientist, R. Kompfner of Oxford University's Clarendon Laboratories. Dr. John R. Pierce, of Bell Telephone Laboratories, with Dr. L. M. Field, has solved the electronic problems of the new tube and overcome effects which rendered earlier efforts of little practical value. Together with F. H. Best, also of Bell Laboratories, they have produced a practical, workable device. Electrons "Blow Past" Wave The principle of the tube's operation is entirely different from previous type amplifiers. It does not even look much like an ordinary tube for it has a narrow, glass stem about a foot long, and then on one end it flares into a bulb. Inside the stem is the essence of the tube, a long coil of thin wire, or helix. Bell Lab's entirely new yet surprisingly simple vacuum tube (traveling-wave) amplifier. Holding the tube is Dr. J. R. Pierce, who developed it. running from one end of the stem to the other. The wave to be amplified is fed onto the coil at the bulb end through a wave guide and then drawn off at the other end in the same way. The wave travels along the coiled wire at the speed of light, but because of the winding of the wire, it moves along the length of the tube at only 1/13 of this speed. Meanwhile, from the bulb — actually, an electron gun — a beam of electrons is shot through the inside of the coil down the stem in the same direction the wave is moving and at approximately the same speed, i.e., 1/13 the speed of light. Wind, Water Analogy The speeds are not exactly matched. however, and on the average the electrons go faster than the wave. They tend to slow down, though, and in so doing, they g!ve up some of their energy to the wave. As a result the wave gains a tremendous amount of energy and becomes many times amplified. The picture is not unlike that of a breeze blowing past ripples on a pond: the ripples grow larger as the breeze blows them along. In the tube, the electrons "blow past" the wave on the coil and reinforce the wave by transferring energy to it. RCA 16-mm Equipment Used on C. & O. DeLuxe Trains The regular showing of 16-mm motion pictures on Chesapeake and Ohio Railroad trains features RCA projection and speaker equipment. Overcoming space limitation problems encountered on two dining cars initially doubling as "theatres-on-wheels," C. & O. set up a tiny projection booth 45 inches wide and 72 inches long. Dual projectors, standing side by side, permit uninterrupted showing of feature-length films with maximum convenience for the projectionist. The projection booth is complete with automatic changeover, monitor speaker, rewinds, film cabinets, etc. Four-inch lenses are used in the projectors. The beaded screen is approximately 60 inches wide. The speaker system includes the standard RCA 16-mm speaker in combination with a directional horn unit to insure complete distribution of sound throughout the length of the car. Dining car tables fold out of the way and disappear under drapes which are pulled across the windows. Seats are set up across the width of the car. Normal movement of trains is reported to have no disturbing effect on the projection equipment. Standard Projectors Employed Projectors are standard commercial models in all respects, including 750-watt lamps and 4-inch coated lenses. One righthanded and one left-handed projector are teamed to economize on space. Throw is 50 feet, with speakers positioned 4 feet above the car floor. Speakers, consisting of one standard and one special h-f unit to overcome the 60 db. noise level of extraneous noises when the car is in motion, are connected to amplifiers by a shielded cable carried through existing duct work. Pre-heating of amplifiers and projection lamps is automatic with the changeover control. Ports in the soundproof projection booth are glazed with ^-inch optical glass. Monitor speaker is mounted above ports. Projectionists are I.A.T.S.E. members under contract to the C. & O. R. R. for this specific type of work. INTERNATIONAL PROJECTIONIST February 1947 23