Radio Broadcast (Nov 1926-Apr 1927)

Radio Broadcast Photograph DRESSED UP IN A CABINET Befitting its worth, the R. B. "Lab" Receiver presents a thoroughly workmanlike appearance Constructing the R* B» "Lab" Receiver Four Tubes? in a Circuit Employing Rice Neutralization) Provide Sufficient Volume for houd Speaker Opt Technical Editor, Radio Broadcast FOLLOWING Keith Henney's articles in the June and September issues of Radio Broadcast on the R. B. "Lab" circuit, this third article is published with the intention of placing before the reader constructional data which will enable him to duplicate such a receiver with a minimum of trouble. The design does not follow orthodox paths but incorporates an unconventional feature not attempted in many receivers. It will be noticed, from both the circuit diagram and panel view photograph accompanying this article, that unlike most receivers, the progression of the signal as it enters the antenna circuit follows from right to left instead of from left to right. In other words, the antenna and detector tuning units are situated at the righthand end of the receiver instead of, as is more usual, at the left side. There are several good reasons for following this procedure. First, the connections from socket to coil, from socket to transformer, etc., are extremely short; in fact, in the audio channel no wire is used for connecting the sockets and transformers because these units are close enough so that the lugs on the terminals of each may be soldered together. Secondly, practically all of the A, B, and C battery wires are located behind the sockets and at the rear of the base board so that it is possible to arrange these wires in the form of a many-wired cable. Thirdly, the fact that this cabling is possible insures against feed-back between units comprising the receiver. The worth of the circuit has been established previously, and this constructional article makes it possible to make use of the circuit at its utmost efficiency, simply because time, energy, and thought have been expended in devising for the circuit the best possible layout. In the model described here, a panel shield has been used which completely eliminates any hand The Facts About This Receiver" ■ R. B. "Lab" Receiver. One stage tuned neutralized radio frequency amplification, regenerative detector, and two stages of transformer audio frequency amplification, followed by an output device. Four; aoi'A's for r.f. stage, detector, and first audio stage; uv-171 in last audio stage. 500,000-ohm variable resistance shunted across the secondary of the first audio transformer. Condenser feedback. Rice. Jsfarne of Receiver Type of circuit ~Humber and Kind of Tubes Volume control Regeneration Neutralization Utilizing all of the high gain produced by the peculiar interstage coupling feature, without waste or undesirable feedback effects, it is possible with the R. B. "Lab" receiver to attain a degree of selectivity and sensitivity hard to approach with other types of circuits. By employing a most efficient audio channel, the tone quality and volume of the loud speaker signal is above reproach. capacity effects which ordinarily would be noticed when Rice neutralization is used without any shielding. The photographs show how the homemade shield is installed with cut out places to fit around the apparatus on the panel. A piece of sheet copper, quite thin, is best for the shield. Neutralization in the Rice circuit is practically independent of frequency, or, said in another way, one adjustment of neutralization will be sufficient no matter whether the receiver is tuned to the longer or shorter waves within the band covered. This is ob'"" ""'"1 viously an advantage because the satis| factory performance and successful operation of a tuned radio-frequency amplifierat its highest point of efficiency depends entirely upon the degree of perfection of neutralization. The theory governing the proportioning of the inter-stage coil has been dealt with at length by Mr. Henney in his previous articles, and will not be repeated here. Suffice it to say that commercial coils suitable for immediate inclusion in the circuit have been made available. For those who desire to make their own coils, specifications are given in Figs. 2 and 3. To tune the two coils, Cardwell taper plate type 169E condensers are used. Although the plates are semi-circular in shape, the tuning chart, Fig. 1, shows practically a straight frequency-line characteristic due to the special design of these plates.