Radio Broadcast (May 1928-Apr 1929)

Interesting Design Data on AN ECONOMICAL BATTERY-OPERATED SET By HOWARD E. RHODES Technical Editor AN INEXPENSIVE receiver with a /-% plate-current consumption low enough to permit economical operation from dry-cell batteries should appeal to those living in districts so remote from power lines that light-socket-operated receivers cannot be used. Those who would like to build this type of receiver also require that it provide good quality, sensitive, and selective enough for ordinary reception, and that it cost not more than about $40, exclusive of tubes, batteries and loud speaker. Such a set has been built up in Radio Broadcast's Laboratory and is illustrated and described in this article. The feature, which more than anything else contributes to the low current consumption of the receiver, is the resistance-coupled audio amplifier, since the plate current drawn by the high-mu tubes is not more than about 0.2 milliamperes per tube. The power tube is a 112a and the r.f. amplifier and detector tubes are 201a's. The plate-current consumption of the entire receiver is 10 milliamperes, and with this load the three heavy-duty B batteries required for the operation of the set should have a life of about 500 hours, the equivalent to about a year's operation if the set is used a couple of hours each day. The total drain of 10 milliamperes required for the operation of the set is divided between the various tubes as indicated below. 201a r.f. amplifier 201a detector 240 first a.f. amplifier 240 second a.f. amplifier 112a power tube Total PLATE CURRENT 2.0 mA. 0.5 mA. 0.2 mA. 0.2 mA. 7.0 mA. 9.9 mA. The circuit diagram of the receiver is given in Fig. 1. The set consists of a stage of tuned radio-frequency amplification, a regen Vieiv of receiver installed in an attractive ivalnut cabinet erative detector, and a three-stage resistancecoupled amplifier. The two tuning condensers are Ci and C2, and C4 is the neutralizing con This receiver, which has more to recommend it than neat appearance, should interest those who are unable to use the power lines as a source of A and B potential. The A potential must be supplied from a storage battery, and with this design it is really economical to operate this outfit from B batteries. The total current consumption is not more than 10 mA. With average use this means that a set of B batteries should last about a year, The cost of the essential parts does not exceed $40 which should make this set even more interesting ! — The Editor. Top view shows arrangement of parts on baseboard denser. Regeneration is controlled by the tickler coil, L5. Coil specifications, which will enable those who so desire to build their own coils, are given in Fig. 2. The circuit of the resistance-coupled amplifier is perhaps somewhat unusual. Such amplifiers frequently have a tendency to " motorboat" and to prevent this filter circuits have been placed in the plate circuits of the detector tube and the first and second-audio tubes of this receiver. In the detector plate circuit the filter consists of C7 and R3, in the first audioamplifier circuit the filter is Cs and R3, and in the second amplifier circuit, C9 and Rj comprise the filter. In these filter systems, the condensers C7, Cs, and C9 provide low-impedance paths directly from the plate circuits to the filaments, so that all the currents flow through these condensers, rather than through the resistors Ri and into the B batteries where they might cause common coupling which would result in oscillations or "motorboating." These filter systems will prove especially advantageous when the B batteries become old and their resistance increases as this tends ordinarily to produce "motorboating." In the Laboratory it was found possible to place a resistor of 1000 ohms in series with the negative B lead before the amplifier began to "motorboat." This value of resistance would correspond to a resistance of about 333 ohms per battery and when the resistance reaches this value the batteries have long since passed the end of their useful life. The plate resistors, Ri, used in the amplifier each have a value of 250,000 ohms and the grid resistors, R2, all have a value of 2 megohms. The coupling condensers, C6, have a value of 0.005 mfd. These values of resistance and capacity yield a satisfactory frequency response. However, those who feel that the decrease in response at 60 cycles is too great may improve the response at this frequency by using larger coupling condensers. When a detector is followed by a resistancecoupled amplifier it is quite important that • February, 1929 page 257 •