Radio Broadcast (May 1929-Apr 1930)

500 600 700 800 900 10001100 12001300 14001500 FREQUENCY, KILOCYCLES Fig. 3 — Overall sensitivity curve of receiver; (A) with self-contained antenna, (B) with outside antenna. issue. As both the sets to which measurements apply use the band-selector method of tuning for preservation of high tones, some idea can be gained of the terrible extent to which the average, ordinarily selective t.r.f. set (not employing band tuning) cuts sidebands; i.e., suppresses high musical notes. It is the writer's personal opinion that 999 out of every thousand radio listeners have no idea of what faithful reproduction of high audio tones in radio reception is — in other words, that no single receiver available in the past season reproduced tones in the neighborhood of 4000 cycles at even 50 per cent, of the value at which they were transmitted. This opinion is based upon many measurements and practical competitive tests between the set here described and other types. The holding up of the high end of curve A is, it is felt, as good an argument for band-selector tuning as could be asked for, particularly when the high sensitivity and selectivity indicated in Figs. 3 and 5 are considered. Curve b of Fig. 4 indicates a possible concession to bad radio reception weather or locations. It shows the resulting overall fidelity of the receiver after an "overtone cut-out switch" has been set to cut a 0.001-mfd. condenser into the detector plate circuit to diminish response to high audio frequencies to a level comparable with that of ordinary commercial receivers. This provision is made for two reasons; individual listeners may prefer drummy, bass-accentuated reproduction, and in bad weather static, usually found in the higher audio tone ranges, may be diminished to make reception more enjoyable. In Fig. 5 appears an overall selectivity curve, taken at 550 kc. It indicates that the frequency band passed is 10 kc. wide, at a level at which an interfering station would have to be ten times as strong as the desired station to produce equal volume (or, at which level the interfering one of two equally powerful stations would be only one tenth as loud as the desired one). At the level at which the interfering station would be reduced to one-one-hundredth of the volume of the desired station, the band width is 24 kc. To the average reader used to tuning sets of claimed "10-kc. selectivity," "knife-like sharpness," etc., such a curve is far from the ideal rectangle 10 kc. wide at its base. Again the dearth of overall measurements is the unfortunate reason for possible hasty misjudgment, for the fact remains that the degree of "apparent" selectivity indicated by Fig. 5 has, in practice, proven considerably greater than that of any commercial receiver so far tested. It should be noticed that the effective selectivity is independent of antenna size; i.e., the set does not "go broad" on a large antenna. The Silver Chassis The stock Silver Radio chassis upon which the above measurements were made is illustrated in Fig. 2 and diagrammed in Fig. 8. Mechanically, it consists of a cadmium-plated steel chassis carrying, at the left, an r.f. shielding case, with removable cover, divided into four compartments. In the left compartment is the antenna coupler, first r.f. tube, and first section of the four-gang tuning condenser. The second, third, and detector tubes, with their tuning condensers, are, respectively, in the next three compartments to the right. The detector compartment also houses the first-stage a.f. tube. Beneath the chassis, in four separate sections under the r.f. shielding case, are the coils, condensers, and resistors necessary to the r.f. circuits. At the exact front center of the chassis, is an illuminated vernier drum dial, with translucent scale marked directly in "telephone numbers" (kilocycles) for easy tuning. At the right rear is a steel case housing the power trans ;J§f2° 100 80 70 60 50 40 30 20 JVER ■ \Ll SfcLLLT SII.VRR 1 VITY lADIf 550 y C 1 r 1 -40 Fig. -30 20 -10 0 10 20 30 40 550 KC. OFF RESONANCE 5 — Overall sensitivity curve taken at 550 kc. former which furnishes all A, B, and C power for the set. In front of this case are, left to right, two 245-type power tubes, a socket for an automatic line-voltage regulator, and the 280-type rectifier tube. On the rear edge of the chassis appear a hum adjuster, a jack for a phonograph pick-up unit, speaker plug socket, and two sets of primary fuse clips. A single fuse, shifted from one set of clips to the other, allows omission or inclusion of the automatic line regulator, if required by power line fluctuations in any given territory. The dynamic loud speaker chassis is a 1 2 3 4 R.M S.VOLTS AI 600 KC MODULATED 30 PER CENT AT 400 CYCLES Fig. 6 — The efficiency of a screengrid detector as compared with that of a typical 227 -type detector. * r' >Z A 1 Fidelity ofType 30 Silver t><. a,., R.miuar itinnn v r 1 R.F.InpuUI7.8uv ted 30%,throu£h 25 Dummy Antenna I 1 Ho W i? 30 50 100 300 500 1000 4000 FREQUENCY IN CYCLES Fig. 4 — (A) Overall fidelity curve of the receiver. (B) How the fidelity may be changed with the "overtone cut-out switch." separate unit, connected to the set by means of a cable and a five-pin plug. Provision is made for but three external connections — antenna (if the selfcontained screen collector housed in the cabinet is not used), ground, and power cord. The general mechanical construction is of a solid and substantial nature, reducing, and where necessary, simplifying, servicing problems to a minimum. Electrically, the receiver consists of three stages of tuned radio-frequency amplification, using 224-type a.c. screengrid tubes, the first and second stages coupled by a band selector. The r.f. amplifier is followed by a screen-grid power detector, resistance coupled to a 227-type first a.f. tube, which feeds a pair of 245type power tubes in push pull through a 1:1 audio transformer. The power unit supplying all A, B, and C potentials is selfcontained with the set and consists of a power transformer, a 280-type rectifier tube, filter choke, condenser bank and voltage-dividing resistors. The more interesting points can be explained most easily in reference to Fig. 8, the schematic diagram. Uniform Sensitivity At the left appear the antenna and ground binding posts, and the input coupler. This coupler is a small choke coil, so proportioned as to resonate with the self-contained screen antenna, just above the broadcast band (about 650 meters). This characteristic gives a voltage transfer curve from antenna to the first r.f. tube sloping downward from 550 to 200 meters. This is made much steeper than the gain curves of the following r.f. stages in order to compensate the cumulative steepness of the gain curve of the balance of the r.f. amplifier. This is one method, and the simplest, of evening up gain over the entire broadcast wave band (it is well known that the average t.r.f. amplifier gives greatest gain at the lowest wavelength in its range, and least gain at its highest wavelength). While other systems might have been employed to attain this end, the simplest method is always the most desirable in practice. The efficiency of the method is illustrated by curves A and b of Fig. 3, where the sensitivity is seen to vary in a ratio of less than 2:1 throughout the broadcast band under the most favorable conditions, and less than 3:1 under the most unfavorable — a not serious variation. The first r.f. tube is coupled to the second through a band selector consisting of two separately tuned circuits so coupled as to produce a "humped" resonance curve of extremely steep sides and broad top. The second r.f. tube is coupled to the third, and the third to the detector, through transformers having untuned primaries and tuned secondaries. The secondaries of these transformers are identical with two band-selector inductances, and all are tuned by a four-gang condenser of extremely wide spacing, having individual compensators. The gain of all r.f. stages has been made equal, so that the curve of Fig. 7 is representative. 162 • • JULY • 1 929 •