Radio broadcast .. (1922-30)

Selectivity and Gain Characteristics THE STEWART-WARNER SERIES 950 iimiimiimimimnmiiiiii inmiiiiimim iimniniimmmimimimi i miiiiiiiiiiiiiiitiiiiiiiiimiiiiiiiiiiiiiiMMMimiii imiiiimimimiiiimn By A. C. MATTHEWS Formerly Radio Engineer, Stetcart-Warner Corporation T I HE SET described in this article is known as the Stewart-Warner _|_ Series 950 Screen Grid. It utilizes eight tubes in all, including the rectifier. Three 224-type tubes are employed as radio-frequency amplifiers, two 227's, one as a plate-circuit detector and the other as an audio-frequency amplifier, and two 245's in push pull furnish approximately 3.2 watts of undistorted power output. ^ 35 R. F. Circuit H 30 From three stages of screen-grid r.f. amplification very high gain can be obtained. It was considered g < ); advisable, however, to sacrifice £" some of this gain by reducing the ci § 1( coupling in the r.f. transformers, c g ; thus gaining additional selectivity. With this thought in mind several methods of r.f. coupling were in- vestigated, i.e., tuned plate, 1 to 1 ratio transformer, 1.25 to 1 ratio transformer, etc. Although each type gave considerable gain, they did not give uniform amplification over the entire tuning range of the set. It was not until the high-inductance primary in conjunction with a special coupling con- denser was tried that it was possible to obtain uniform amplification throughout the broadcast band. The r.f. transformer primary coil con- sists of 625 turns of 0.005" enameled, cotton-covered copper wire wound on a \" diameter wooden form. Its natural period, with the output capacitance of the 224-type tube shunted across it, is slightly below the lowest frequency of the broad- cast band (it resonates at 490 kc.). Theoretically, such an arrangement should give us a very stable r.f. amplifier, for, since the primaries are resonant below I \^ \^ STEWART - WARNER SERIES950SCREEN GRID SET SENSITIVITY CURVE -STANDARD UUTPUT-4 METER ANTENNA 400~ MODULATED 30% SCO 1000 FREQUENCY IN KILOCYCLES 1200 Fig. 2 the broadcast band, the plate circuits will have a capacitive load in them at any point in the band. However, the lack of perfect shielding makes it necessary to take the usual precautions in order to in- sure stable operation. Between the plates and control grids of succeeding 224-type tubes small capacities of about 10 mmfd. are connected. These capacitors are mounted on the side of the tuning capaci- tor and are adjusted before being assem- bled on the chassis. In general, the resonant point of the lumped primary determines the shape of the sensitivity curve at the low-frequency end, the capacity of the coupling condensers largely determines its shape at the high-frequency end, and the coupling between primary and secondary determines the height of the entire sensi- tivity curve. These effects as de- scribed are not strictly true, of course; for example, the setting of the coupling condensers affects slightly the sensitivity at the low- frequency end, and so on. Considering the r.f. coil as a step-down transformer from plate to grid, it is readily seen that the impedance of the tuned circuit across the grid will be reflected back into the preceding plate circuit, thereby introducing an im- pedance comparable to the plate resistance of the tube. This, to- gether with the adjusting of the capacitive coupling and the resonate point of the primaries made it possible to obtain an amplification of more than 35 per stage uniformly over the band. This was reduced slightly in order to provide a better factor of safety and to obtain a higher degree of selectivity. Cross talk and local-station modulation of distant-station carriers is a serious matter with screen-grid circuits and neces- 1400 VVWpAIV^-O O—|||P Dynamic Fig. / — Complete schematic diagram of the Stewart-Warner Series 950 Receiver. • NOVEMBER 1929 • 55