Radio Broadcast (May 1928-Apr 1929)

198 RADIO BROADCAST JANUARY, 1929 0 tt a: OU150 a| 100 28 §s 70 ►= 0 60 < a. i± 0 50 40 : 3 30 300 500 1000 FREQUENCY IN CYCLES FIG. I the thought was to employ as many stages of suitable r.f. amplification as were needed to give the desired degree of amplification, rather than to obtain the highest possible amplification per stage. This decision made, the designers were left free to concentrate upon the problem of selectivity rather than amplification in each stage. The wisdom of this policy is indicated by the fact that the selectivity of each r.f. stage is practically that of the tuned secondary circuit alone without the deleterious effects of coupling a preceding amplifier tube into this circuit through a primary large enough to obtain the highest possible value of amplification which always halves the selectivity factor. Lest, however, the casual reader should be inclined to regard the amplification of the Sargent-Rayment receiver as being of a very low order, it is interesting to compare the r.f. amplification of typical six-tube, one-dial receivers averaging about 1000 times between antenna and detector grid with the r.f. gain of the Sargent-Rayment, neglecting entirely its tuned antenna input circuit with its large potential amplifying possibilities. The r.f. gain of the Sargent-Rayment, operating in a perfectly stable manner has a value adequate to allow the receiver to go down to the lowest noise level; this is equivalent to many times the gain given by many of the receivers of the type mentioned above. An over-all amplification curve for the twostage audio amplifier employed in the SargentRayment is shown in Fig. 1, this curve being combination of tone quality, sensitivity, and selectivity. ANTI-MOTOR BOATING FILTER I JNFORTUNATELY, the set is ^ not without its one drawback, though this drawback is in 3000 5000 10,000 itself the accompaniment of the extremely high amplification developed by the receiver. When used with standard B -power units, there occasionally develops a tendency for the receiver to " motorboat," particularly at one setting of the volume control regulating the screen-grid voltage to the r.f. tubes. Messrs. Sargent and Rayment have recommended a non-motorboating filter which they have termed a "stabilizer." It consists of a small choke coil, similar to the S-M 251 output transformer, connected in the 45-volt screen-grid lead and a 50,000-ohm resistor connected in the positive B lead to the detector plate. Each of these circuits is then bypassed back to the receiver chassis, a 4-mfd. condenser being used for the screen-grid bias and a i-mfd. for the detector-plate lead. The circuit is shown in Fig. 4. 500 600 700 800 900 1000 1100 1200 1300 1400 FREQUENCY IN KILOCYCLES FIG. 2 made with I i2A-typetubesin thedetectorand first audio positions, and a iyiA-type tube in the second audio position, with recommended operating voltages. It is, however, quite feasible to employ a 210 or 250-type power tube in the output stage of the receiver through the use of a high-voltage power-supply unit. The standard S-M 675ABC kit is especially suitable for this purpose providing, as it does, 7.5 volts for filament lighting through an adapter plug inserted between the power tube and the second audio socket, together with B voltage and C bias for the entire receiver including a 210 or 250-type power tube. With this combination and the substitution of the i-mfd. 600-volt condenser, as specified in the circuit diagram on page 355 of October, 1928, Radio Broadcast, the SargentRayment receiver provides an unusually fine INCREASING THE SENSITIVITY AN EXAMINATION of the receiver circuit diagram (October Radio Broadcast) indicates that the input from antenna to first r.f. tube is through a rejector type of wave filter which, in its general characteristics, is essentially similar to that of the tuned r.f. stages. The thought has occurred to some builders that through shifting the screen-grid connection of the left-hand screen-grid tube from the antenna connection (2) of the coil Li to the free end of the tuned secondary coil (3) of the wave filter, an increase in amplification may be obtained. Whether or not this is necessary is for individual decision, for the performance of the receiver is perfectly stable with the recommended connection, and the amplification is high enough to go down to the most favorable low-noise level. The change in connection to utilize the rejector circuit as a tuned r.f. stage will usually tend to make the receiver oscillate, with oscillation controlled by the volume knob. The effect, however, of the regeneration introduced through this change is to reduce the effective repeater amplification, so that only a very slight actual gain in sensitivity results. Nevertheless, under extremely favorable conditions, as, for instance, early in the morning when listening for Japanese or Australian stations, this connection has sufficient merit to justify its trial, at least, for it will result in some boost of a very weak signal. As stated above the writer is of the opinion that the receiver possesses ample sensitivity, as well as selectivity, in its present form. This view is more or less substantiated by letters from many experimenters who have built the receiver. Two of these communications are particularly interesting and excerpts are printed below. The first is from Frank McDonell, president of Rossiter, Tyler and McDonell, a well-known engineering and service organization located in New York. After testing the Sargent-Rayment receiver in a steel-frame building in the heart of the lower New f45 O York business district, Mr. $ McDonell write as follows: "You will be interested in a word of comment on the Sargent (2 Rayment receiver. I want to put B Omyself on record right now assaying that it is without =0+22 0 question the best receiving set of any type or description that we have ever demonstrated. During the evening demonstrations, we were able to tune-in at will almost anything in the country that we desired, getting such stations as Fort Worth, Atlantic City, Atlanta, Ga., and literally hosts of others, with as much volume as any ordinary receiving set receives weaf in this locality. Incidentally, our receiving conditions here are most abominable." FIVE JAPANESE STATIONS HEARD TURNING again to the opposite Coast for _ confirmation of such performance, the report of Kenneth G. Ormiston, the technical editor of Radio Doings, a Los Angeles publication, is interesting. Mr. Ormiston commented as follows: "We are impressed with the very obvious sensitivity of the Sargent-Rayment receiver, due to its ability to reach the noise level with the sensitivity control but half on. The volume of wgn, when he signed off, inspired us to set the 1.84 J 1.23 Select vity 6.3 500 600 700 800 900 1000 1100 1200 1300 1400 1500 'frequency in kilocycles' FIG. 3 alarm clock for 4 a. m., and, when we turned the set on at that hour, five of the "Japs" and 4QG in Brisbane were received with good volume. Also, wfaa, wmmj, kma and some Easterners were heard on the air with their early morning programs. "In all, we were very well satisfied with the performance of the receiver. Not alone satisfied, but considerably surprised! No repeats or harmonics, very fine tone quality, on both local and dx, and its ease of operation are factors which are bound to make the set popular with those fans who believe that d.c. tubes, operated from either batteries or a socket-power unit, give peak performance." Apparently Mr. Ormiston started the ball rolling, for immediately after receiving word of his reception, reports came in from many different West-Coast builders of reception of Japanese and Australian stations, not to mention a large number of eastern American stations. In particular, E. W. Gardner of Del Monte, California, reports the reception of six Japanese broadcast stations. Considering the fact that the comments quoted from and referred to herewith are but a very few of the large number of favorable reports which have been received from builders, it may be assumed that the Sargent-Rayment receiver provides an unusual degree of selectivity and amplification (as we write this, a Chicago experimenter reports reception of what were apparently Japanese programs upon the wavelength of joak on November 6.) 4i" ~1 -g* 1 To T*mfd Jl mfd. Supply -O-B FIG. 4. AN EFFICIENT FILTER CIRCUIT