Radio Broadcast (May 1928-Apr 1929)

THE PANEL IS SIMILAR TO THAT OF THE A. C. MODEL Building the D*C* Lab Receiver THE differences between the R. B. Lab Circuit receiver described by Hugh S. Knowles in the June Radio Broadcast (p. 93) and the model described in this article are few; the present receiver uses standard d.c. tubes of the 20IA or preferably the 112A type (because of their better detection and amplification characteristics), while the June receiver was designed to use a.c. tubes. The same placement of parts is followed, the parts themselves are the same; the circuit diagram has not changed. The only differences are those incidental to the use of the a.c. type of tubes. Some readers prefer the newer tubes, others prefer battery-operated tubes. It is probably true that thousands of Radio Broadcast's readers have d.c. tubes on hand to hundreds who have 226's and 227's, and there are many who still have faith in the old storage-battery-charger outfit — the Staff of Radio Broadcast Laboratory among others! Briefly, the circuit is the R. B. Lab that was first described in June, 1926, and brought up to date in the April and June issues of this year. Readers who are not familiar with its advantages and its theory are urged to get those copies. It is only necessary to state here that it is a fourtube receiver consisting of a stage of neutralized radio-frequency amplification followed by a grid leak and condenser type of detector with capacity feed-back, and a two-stage transformercoupled amplifier. It is the standard circuit to which all others are compared in the Laboratory. Any other receiver which, regardless of the number of tubes, can get signals of equal strength out of weak distant stations, is considered a good receiver. The chances are that such a receiver is subsequently described for the benefit of our readers. Fig. 2 is the circuit diagram of this receiver, operating with d.c. tubes. It requires less apparatus than the a.c. receiver, since the C-bias resistors which are used in the a.c. set to give the tubes the required grid voltages are not necessary. Provision is made for an external C battery to perform the same purpose in this model. In Fig. 1 is a top view of the d.c. set. It will be seen that exactly the same placement of parts as was used in the a.c. model is followed. Where the filament transformer of the a.c. set was placed, an output By Keith Henney Director of the Laboratory IF THERE is any way we can know exactly what the readers of Radio Broadcast want, we should like to find that way. For months we promised another article on the Lab Circuit receiver. For the June number we got Mr. Hugh Knowles to build us a receiver with all modern improvements, a.c. tubes, etc. Immediately we received complaints that what was wanted was a d.c. Lab Circuit article. Here it is. Almost the same list of parts is used, they are placed in the same postion on the baseboard, and their functions are the same. The differences in operation and dx getting ability are too slight to be noticeable. In favor of the a.c. set is the freedom from battery troubles; in favor of this receiver is the greater freedom from power noise. Battery operation also has in its favor the feeling shared by many radio experimenters that the a.c. tubes may not last as long as their older brothers, the battery operated tubes. ■ — The Editor. device, such as a condenser-choke or an output transformer may be located and, if desired, a C-bias resistor for the power tube may be placed beside it where the low voltage C-bias apparatus was installed in the a.c. set. Naturally, fourprong tube sockets will be used instead of fiveprong or Y type sockets. An on-and-off switch has been included and should be installed on the panel — above the phonograph switch is a good place. The phonograph switch, Swi, makes it possible, as Mr. Knowles has already explained, to throw the audio amplifier of the receiver from the set itself to a phonograph pick-up unit without the usual bother of removing the detector tube from its socket in order to place in it the plug which goes with all modern types of pick-up units. In other words, the plug is permanently installed in the extra socket, V5, the cabinet is closed, and all bother has been eliminated. coils for the lab circuit ALTHOUGH special coils are being manufactured for this receiver, many experimenters prefer to wind their own. The simplest method is as follows. Procure two coil forms 2 inches in diameter. Wind on both forms 75 turns of about No. 24 wire. The insulation and exact size of wire in not important. The coils will then tune over the broadcast band with 0.0005 mfd. condensers. For the antenna coil wind about ten or twenty turns about the exact center of one of the coils which has been tapped at the center turn for the C bias of the r.f. tube. This constitutes the coils, Lj. and L2, in Fig. 2. The other coil is tapped at about the 25th turn, or if other size winding forms are used with different numbers of turns, tap the coil at about one third of its length. The smaller part of the coil goes in the plate circuit of the r.f. tube; the detector input voltage is that appearing across two thirds of the coil. Experimenters who wish greater selectivity, with somewhat less amplification, can reduce the number of turns in this plate coil, say to 1 5 in a 75 turn coil, that is, increase the turns ratio to 5 to r. neutralizing the r.f. amplifier THIS amplifier differs from the usual stage of r.f. amplification in being neutralized by the Rice system. When the circuit has been properly wired up, make the detector oscillate by varying the regeneration condenser; tune-in a fairly strong station; then, when the carrier whistle can be easily heard, vary the tuning condenser setting of the r.f. tube. Probably this will force the detector out of oscillation with a thump or a swish or a squeal. Turn the neutralizing condenser until this stoppage of the detector oscillations does not take place. When the amplifier is properly neutralized, changing the amplifier tuning condenser setting will have little or no effect upon the detector circuit. It is not important to worry about this neutralizing business. It may be impossible to neutralize the amplifier exactly so that it has no effect on the detector, due to extraneous couplings between the two circuits — magnetic via the coil fields, resistive via common batteries, or capacitive, through the other elements in the receiver. The important adjustment is that which enables any frequency to be tuned to without the amplifier going into oscillation or preventing the detector from tuning properly. This adjustment is easy to 'find. 199