Radio Broadcast (May 1928-Apr 1929)

HOW THE HEW A.C. LAB RECEIVER LOOKS— BEHIND THE PAHEL IF THE experience of those who manufacture both battery and a.c. operated receivers is any criterion there is no question about the present being an "all electric" season. This demand for a.c. receivers has been reflected in the custom set-building field where experimenters are looking askance at any circuit which cannot be modified to permit operation from the lighting circuit. The use of a.c. tubes does not alter the fundamental operation of a circuit and for this reason it is quite natural to look for receivers using wellknown and tried circuits which have been modified slightly to make the use of these tubes possible. In this connection the receiver we are about to describe should be of particular interest to the readers of Radio Broadcast. It uses the familiar R. B. Lab circuit, with its possibilities for excellent performance with a minimum number of tubes together with selected parts which make possible the realization of this performance. The circuit itself needs no introduction, since the original and several variations have appeared in Radio Broadcast. Those who are interested in an exposition of the special features of this circuit are referred to the June, 1926, and April, 1928, issues. The principal advantage of this over other similar four-tube arrangements lies in its "gain" or sensitivity and in the fact that the balancing circuit of the Rice type gives accurate neutralization over the whole frequency range. The grid "suppressor" or losser method of stabilization is not used in the radio-frequency stage. This element of design improves the selectivity of this circuit and makes it more uniformly selective over the whole frequency range. The advantages of a.c operation have been obtained together with an actual improvement in performance, due to the fact that a.c. tubes in general are somewhat better amplifiers than the standard 201 -a type. Direct comparative tests between this receiver and one of the battery types .indicates an improvement in gain or sensitivity, better stability and negligible hum or a.c. modulation even on the more distant stations. One addition has been made to the circuit; a dummy socket has been connected in parallel with the detector socket. This makes it possible to plug-in a phonograph pick-up permanently. The small switch between the drum dials permits an instantaneous changefrom receiver to pick-up. This convenience will be appreciated by those who have had to open the cabinet, remove the detector tube, plug-in the pick-up and then perform the inverse operation to operate the receiver again. An inspection of the schematic wiring diagram in Fig. 2 will show that two minor changes have been made in the radio-frequency circuits. The resistance R2 has been substituted for the radiofrequency choke previously used in the mid-tap of the first coil, and the condenser C3 has been added. The resistance prevents very high frequency oscillations which would block the first tube. For this purpose it is just as satisfactory as the choke and less expensive. This resistance is not e<©(^^"*— i%i CI HE set described here is a straight jour-tube ■*■ "Lab" circuit receiver, arranged to operate entirely from the a.c. line. The results achieved are rather better than those from the average fourtube set, due to the high gain in the radio frequency circuit. Mr. Knowles' receiver is a well arranged set based on exactly the same circuit, with the exception of provision for a.c. operation for the filaments, that was described on page 423 in this magazine for April, 1928. That unusually interesting story dealt with the engineering design which went into this receiver and exact measurements of its performance. — The Editor. in any sense a "grid suppressor" at broadcast frequencies since a high-impedance choke may be used, or the circuit left open, for that matter. In practice the mid-tap connection is used to provide a means of biasing the first tube. the circuit '"THE principal circuit modifications are those made necessary by the a.c. tubes. Three 227 type tubes are used and a 171-A type in the last audio-frequency stage. Heater type tubes have been used in the radio-frequency, detector, and first audio-frequency stages because of their comparative freedom from hum or ripple. Tubes of the raw a.c. type may be adjusted to give very little hum but any change in the effective plate voltage increases the a.c. modulation considerably. The filament current for the tubes is supplied by a step-down transformer T3. To insure long heater life, care should be taken to see that this transformer is one of the new types designed to give 2.25 volts. The old type supplies 2.5 volts. Bias for the radio-frequency and first audiofrequency circuits is secured by taking the drop across a resistor in the plate circuit of the tubes. This method is quite satisfactory where a single stage of radio-frequency amplification is used. No B socket-power device has been used in the receiver since it was felt that many constructors would have ones which were previously used with battery-type receivers and others would have power amplifiers in which such a Bpower supply was included. Where this is not the case any good quality power unit may be used and the receiver made "all electric." In most cases it is advisable to use a C battery for the power tube. There is really little or no objection to this practice since the life of the C battery is determined by its "shelf life" which may be in excess of a year. Details for avoiding the use of this battery will be given later. (See Fig. .). 93