International projectionist (Jan-Dec 1936)

22 INTERNATIONAL PROJECTIONIST November 1936 rates the a.c. and d.c. components) and then measuring the harmonic voltages separately. In standard amplifier circuits, a pushpull output stage improves quality because the second and other "even order" harmonics meet in the primary winding of the output transformer 180 degrees out of phase, and cancel each other. Third and other "odd order" harmonics are in phase in the primary winding of the output transformer and do not cancel, but go on to the loudspeakers. They are heard as a form of distortion not easy to identify by ear, but having the effect of an unnatural quality in the sound. Now in the circuit of Fig. 1 the harmonic distortion as well as the fundamental output in the plate circuit of V-3 is fed back, in part, to the screen grid of V-l, 180 degrees out of phase with the control grid of that tube. So far as the fundamental is concerned, the effect is merely to reduce volume. So far as the harmonic content is concerned, the effect is to introduce into the amplifier the identical harmonic content, 180 degrees out of phase. In so far as the design of the feedback circuit can maintain a precise balance of phase and volume, the harmonic content, even order and odd order both, is cancelled entirely, and no harmonic distortion at all appears in the speakers. Scanning Compensator To compensate for losses in scanning which occur at the higher frequencies even with the best possible focus, a highfrequency compensator has been introduced into the reverse feedback circuit. In tracing that circuit it was seen that the screen grid and cathode of V-l are coupled across the output of V-3, and form part of the a.c. load upon that tube. But there is still another parallel load. Just right of V-l is R-2, and right of that R-3. From the screen grid of V-l trace up, right, down and right to the upper end of R-3; thence down through C-3, left, down through C-6 to the negative bus, and back to the cathode of V-l. Through this circuit some of the high-frequency feedback is by-passed around the screen grid and cathode of V-l. Consequently, the overall reduction in volume resulting from reverse feedback is somewhat reduced at the higher frequencies; in other words the h.-f. response of the amplifier is increased, the circuit that accomplishes that result being included in the reverse feedback circuit. ANEW portable projector arc lamp designed so that it may be supplied with the various controls and systems required for use with either 35 mm. or 16 mm. film is announced by Strong Electric Corp., Toledo, Ohio. To be known as the Strong Junior High, it is 22" long, 12" wide and 14" high, which is larger than the Strong Portable but smaller than the Utility series. It is 6%" high from the base of the optical center. Screen brilliance equal to that attained with regular theatre equipment is provided when supplied with either highor low-intensity burner for use with 35 mm. film. When used with 16 mm. film the Junior High is supplied as a highintensity arc which permits projection of a theatre-size picture and a snowwhite light having eight times the brilliancy of incandescent light. The arc control is built as an integral part of the lamp, which has been designed for simplicity of operation as it usually will be used by lay projectionists. The Junior High is equipped with arc imagers, ammeter, automatic pilot light, manual controls and an inside dowser system. RCA INDIANAPOLIS PLANT Substantial increases in the sale of RCA motion picture sound equipment has necessitated the opening of a new and larger manufacturing plant in Indianapolis. RCA manufacturing facilities in Camden, N. J., have been greatly taxed this year by a 100% increase in theatre installations of Photophone sound reproducers. The signing of recording license agreements recently with several major motion picture producing com' panies has also been a large factor in the expansion move. Efficient coordination of the engineering and manufacturing divisions is assured by the establishment of a complete engineering staff in Indianapolis. DEVRY REEL RECOMMENDED The new DeVry clock-spring steel reel lengths is recommended for use with the NOTES from the SUPPLY FIELD • 2000-foot film lengths now standard in the projection field. The outstanding feature of this new reel is its indestructabibty, with even the hardest usage (including repeated jumping upon it) failing to disturb its perfect alignment. This new DeVry reel can be threaded in the dark, so cleverly arranged is the slot, over which the film need only be drawn in order to catch tightly. It bas replaceable sections, although the need for this feature is difficult to understand in view of the indestructability of the reel. G. E. MAN TO COAST H. W. Hale, automotive products field representative for General Electric, has been transferred from the Boston office to the San Francisco office. He will be responsible for sales and service of tungar chargers and copper-oxide rectifiers for motion picture machine projectors. APPROVE NEUMADE CARINET FOR DOUBLE REELS A new Sealtite film cabinet, designed for use in storing the new double reels in theatre projection rooms, is announced by Neumade Products Corp., 427 W. 42nd St., N. Y. City. This cabinet has fixed capacities for five, six, eight, ten and twelve reels, with a double-walled compartment for each reel, and is of heavy metal construction. The compartments are closed by gravity, the reel being about half surrounded with a permanent metal container attached to a vertical door, which has a full hand grip. Formal approval of this cabinet has been given by numerous city inspection bureaus, including Chicago. NEW UNIT ELIMINATES THE MOTOR CONTROL BOX Elimination of the motor control box on theatre reproducing systems is accomplished through the use of a constantspeed, condenser-type motor now being distributed by Co-operative Sound Service Supply Co., 3315 Olive St., St. Louis, Mo. The motor (110 v. A. C, 60-cycle, single-phase, 1200 r.p.m.) has a key number of DR-429-C and is warranted to give complete satisfaction. Other sound projection units, notably for power supply, are made by this company, headed by Clyde Weston, well-known I. A. man. MAZDA PRICE REDUCTIONS Further price reductions on several Mazda projection lamps used in portable motion picture projectors have been announced. New prices are 10 per cent below previous prices and 25 per cent below introductory prices. Lamps and classes of service affected include the standard-voltage, 100-watt and 200-watt T-8 bulb lamps with bayonet base, for 8-mm. and slide-film projectors; the standard-voltage, 400-watt and 500-watt T-10 bulb and 750-watt T-12 bulb biplane-filament lamps with medium base, for 16-mm. projectors; and the standardvoltage, 1000-watt short T-20 bulb biplane-filament lamp with medium base, for 35-mm. projectors. S. O. S. CORP. NEW ADDRESS S. O. S. Corp., handling a complete line of visual and sound projection equipment and general theatre accessories, has taken a long-term lease on new and larger quarters in 622 11th Ave., N. Y. City. Change effective in January.