Radio Broadcast (May 1929-Apr 1930)

While the values of r.f. gain may appear low, as compared to the maximum that may be had from a single stage, they are about as high as may safely be obtained stably in production, and result in an overall gain of as high an order as can be used in practice. The r.f. circuits are isolated by means of by-pass condensers, resistors, and chokes to a point where the only coupling existing to cause oscillation is the input to output coupling, and this has been effectively eliminated by grounding, not only the set chassis, but the loud speaker frame as well. In isolation, the matter of defining accurately all r.f. current paths, and avoiding the use of the metal chassis as a common path, was found most important. The r.f. tubes are operated at plate potentials of about 170 volts, with screen voltage variable from zero to 67 volts for volume control. C bias is obtained automatically by means of individual resistors common to grid and plate returns. The r.f. amplifier does not oscillate, being completely stable under all conditions of operation. Power Detection In the past the 227-type tube operated at a high plate voltage and with a highly negative bias (so-called "power detector") has been considered the most generally satisfactory detector available. In this set, a new detector is used — a screen-grid power detector of such high efficiency that it is probable that within a short time it will replace all other types of detectors. Its conversion efficiency (r.f. signal modulated 30 per cent, at 400 cycles to a.f. signal at first audio grid) is illustrated clearly in Fig. 6 at a as compared against a typical 227-type power detector at b. A comparison of the respective slopes indicates the great superiority of the screen-grid detector, which was operated with a 60,000-ohm bias resistor, a total B potential of 170 volts, and a plate resistor of about 300,000 ohms. A particular point to be noted is the flattening off of the curve at increasing signal voltages. This occurs at such a point as to prevent serious overloading of the audio amplifier, for it has been found that such "distortion" as exists is of a volume limiting nature, and is far less annoying than overloading of the power audio stage — the detector characteristic, therefore, serves as a means 3t a p_tt \;ni Tir.r r. a tm c ILVE R Rj> DIO I I JY-22 :b=18 1 3 Ec=-1.5 Esg=67.5 500 600 700 800 900 1000 1100 1200 1300 1400 FREQUENCY KILOCYCLES Fig. 7 — Curve showing r.f. gain of receiver. of automatically regulating volume to prevent annoying audio-overload distortion (easily possible, for the sensitivity is so great that a fair signal can easily be built up to far more than the 3-watt undistorted output of the power stage). While the 224-type detector has been used in the power "detector" circuit, no need exists to take advantage of its maximum output because of the use of two a.f. stages. As the detector tube has a very high plate resistance, about the only really practical method of coupling it to the first audio tube is by means of resistance coupling, which is used. The "overtone switch" previously mentioned cuts a 0.001-mfd. condenser into the detector plate circuit to diminish the high tones when desired. As the phonograph pick-up jack is in the first a.f. grid circuit, an external transformer is needed to produce maximum volume. The push-pull stage is coupled to the dynamic loud speaker through a transformer having an excellent transmission characteristic. The loud speaker head itself shows very uniform conversion from 50 to 5000 cycles. It is baffled in order to avoid cabinet resonances, and likewise to avoid howling due to mechanical vibrations, being coupled to the detector tube. The Power Supply The power supply unit employs but one choke, of very high inductance, the dynamic loud speaker field acting as the second choke. The field is connected just after the rectifier tube, at a point where about 100 mA. is flowing through the filter. A portion of this current is bypassed around the field by a 1500-ohm resistor, the actual voltage drop across the field being about 70 volts, a value producing the desired excitation. This allows the second choke to be of unusually high inductance with corresponding filtration. The detector and r.f., first a.f., and second a.f. plate voltages are all taken off the voltage-divider system at different points, in order to provide isolation sufficient to obviate any possibility of the "motor boating" which is apt to occur in audio amplifier circuits shoving as good low-frequency transmission as do those of this receiver. For convenience, the filaments of the 245-type tubes are excited from a separate filament winding on the power transformer, which is provided with an electrostatic shield to cut out r.f. noises which might otherwise get into the receiver through the power lines. RADIO IN N. Y. SCHOOLS The first demonstration of a new system of "centralized radio" for schools took place recently in the New Utrecht High School of Brooklyn, before 2500 students, music supervisors, educators, architects, and officials of the New York City Board of Education. The demonstration was arranged with Stanley & Patterson, Inc., a sales agent of the RadioVictor Corporation of America, at the invitation of Stephan A. Thomas, chief of the electrical division of the Board of Education, and with the cooperation of Dr. Harry A. Potter, principal of New Utrecht, to provide educators with an opportunity of observing at first hand, the operation of a radio system especially adapted to school use. The auditorium of the high school was wired with a single-channel control panel, hooked up to a master receiver combined with an electric phonograph, and an arrangement of ten dynamic loud speakers located in the wall organ recesses. Additional loud speakers were installed in Dr. Potter's office, and in the boy's gymnasium. As many loud speakers as are desired in the various classrooms may be connected to the master control panel, without any mechanical changes in the centralized radio system. A time clock automatically starts and stops the programs. • JULY 1929 • • 163