Radio Broadcast (May 1928-Apr 1929)

A Nine-Tube Screen-Grid Super By ROBERT BURNHAM FIG. i. THE RECEIVER WITHOUT THE CABINET Compact construction ma\es this nine-tube super -heterodyne occupy a smaller space than most receivers of the same number of tubes. All the tubes except the antenna input and audio tubes are housed in the shield cans in the rear. ITH the steady increase in the number of broadcasting stations operating in the frequency band of 550 to 1500 kc. that has taken place in the last several years, the efficacy of the super-heterodyne system of reception has gradually dwindled, for the characteristics of the typical super are such that the effective selectivity of the oscillator dial is practically halved due to the basic principle of the system. This and the accompanying fact that the average super-heterodyne, besides repeating stations at two or more points on one of the two tuning dials, will bring in a multiplicity of heterodyne squeals, has militated against the advantages of this system, such as the high amplification possible, until to-day it is quite safe to say that a good 6-tube screen-grid t.r.f. receiver will outdistance an ordinary 7 or 8-tube super-heterodyne in the matter of actual distance reception, primarily due to the greater effective selectivity rather than to the greater sensitivity of a t.r.f. set. The usual method of obviating repeat points upon a super-heterodyne is to use an intermediate frequency so high that the separation upon the antenna tuning dial of two stations which may be heterodyned by a single given oscillator adjustment is so great that the repeat point is either beyond the required oscillator range (for the majority of signals) or so far separated as to impose no undue selectivity requirement upon the antenna circuit. This system in a measure vitiates the principal advantage of the super-heterodyne which is the higher amplification, obtainable at low radio frequencies, than may be had at such high intermediate frequencies as are encountered in the r.f. amplifier of a t.r.f. set or as are necessary in the i.f. amplifier of a successful one-spot super. This factor makes some one-spot superheterodynes actually inferior to a good t.r.f. set of two less tubes.' Bearing in mind that a low intermediate frequency is necessary to realize the full amplification possibilities of the super-heterodyne system and that the frequency changing feature of the super-heterodyne must in no way be relied upon to provide adequate selectivity under present broadcast conditions, the receiver pictured and described herewith was developed. As it has been developed, this receiver does not depend upon the selectivity usually obtained in the intermediate-frequency amplifier at the expense of tone quality, or upon the apparent selectivity resulting from the frequency changing action. To provide a high degree of selectivity, the input to the first detector has instead been designed to provide in itself practically all of the selectivity required for this rather unusually sensitive set. This end is attained through the use of a 3-stage radiofrequency amplifier, tuned by a 3-gang condenser to any desired wavelength. This amplifier is sufficiently selective in itself to provide effective selectivity and at the same time a high degree of r.f. amplification. By this means the superheterodyne system has been freed of any drawback resulting from oscillator dial repeat points attendant upon the use of the low intermediate frequency necessary to provide really high amplification; and at the same time the disturbing effect of heterodyne squeals has been practically done away with, even when the receiver is operated in such congested centers as New York and Chicago. THE DESIGN OF THE RECEIVER IN FIGURE 1, the receiver is seen with the cabinet removed. The set has been designed to be enclosed in a console or table cabinet to suit the builder's fancy, though it is particularly adapted to the new S-M metal cabinet which adds to the effectiveness of the shielding in the set. Figure 4 shows the details of the receiver assembly with the copper stage shields which enclose all r.f. circuits removed, and with all parts labeled as in the parts list. In Figure 2 is the schematic circuit diagram of the receiver, while Figure 3 shows the set with tubes in place but shields removed. The receiver consists essentially of a three-stage broadcast band t.r.f. amplifier employing three screen-grid tubes and a screen-grid first detector. Coupled into the screen lead of the first detector is the oscillator, which is of conventional type. Following the first detector is the two-stage 65-kc. intermediate amplifier and the second detector. All of these circuits are individually shielded in small copper cans with removable sides and tops for easy access. Following the second detector is a single stage of audio amplification utilizing the new Clough system. The receiver is intended to operate with an external power amplifier in order to provide a high degree of tone quality. Because of the desirability of a 2 10 or 250 type output tube to prevent overloading it has been thought best to omit this last stage tube from the receiver assembly. Despite the apparent complication of the r.f. amplifiers involved, the control of the receiver is simplicity itself, for the 3-gang t.r.f. amplifier condenser is controlled by the left-hand drum dial of Figure 1, while the oscillator condenser is controlled by the right-hand dial. Sensitivity of the t.r.f. amplifier is controlled by the small knob at the lower left of the control panel escutcheon, Rj. This adjustment takes the form of a potentiometer which varies the screen-grid potential of the ^KA^NY of us have probably operated a super-heteroiVJ dyne wbicb would tune-in a single powerful local station at four and sometimes six or more points on the dial. This condition is due to two factors: (a) insufficient selectivity in the tuned circuits preceding the first detector, and (b) the generation of harmonics by the oscillator. There are two methods of overcoming these difficulties. One of these methods is to raise the frequency at which the intermediate amplifier is designed to operate to a value such that the second point {and the harmonics) of the oscillator cannot, for practically all dial settings, beat with any station except the one desired to produce the proper frequency to be amplified by the intermediate amplifier. This method was used in the receiver described by Mr. W . H. Hollister in the September issue. The second method of making fool-proof the operation of a super, uses one or more stages of ordinary r.f. amplification ahead of the first detector, so that practically all the required selectivity (as well as some gain) is obtained in this amplifier; the intermediate amplifier then functions to amplify greatly the signal without being also called upon to supply all of the necessary selectivity. The receiver described in this article employs this second method. — The Editor. 334