Radio age (May 1922-Dec 1923)

I II nil 1 n 111 I 1 1 M I II L I [ M II I in II I I I I I 11 I 1 1 1 I I II I III fm I I I I I II II I I I I in 1 II I I ] 11 II I II I I I I I M I M I I I L 1 HI 1 1 I III! I I III HIM in nir RADIO JIGE Tfie Ma^a^ine of tfte Hour PUBLISHED lJIONTHi.Y n 1 1 ij 1 1 1 1 1 Ml 11 1 1 1 1 II I I I 1 I 1 1 1 1 1 1 1 I I 1 1 1 1 1 1 1 I III 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 n 1 1 1 1 1 1 1 1 1 I II 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I n I 1 1 I i_i 1 1 1 1 , I , i-TT The Grimes Inverse Duplex System By FRANK D. PEARNE PERHAPS no circuit, since tlie days of the Armstrong superregenerative system, has attracted so much attention as the Grimes Inverse Duplex. The name, "Inverse Duplex," applied to this newarrangement by the inventor, Mr. David H. Grimes, has a rather foreboding sound to the timid amateur, but, nevertheless, it has looked inviting enough to engage the attention of thousands jof enthusiastic fans all over the United States, and the different reports received are indeed interesting. This circuit is nothing more or less than a combination of radio frequency, detector, and audio frequency amplification cleverly arranged, so that each tube used will do the work of two. In other words, the tubes amplify at both radio and audio frequency simultaneously without overloading them. The greatest difficulty experienced in the ordinary reflex circuit is caused by some of the tubes doing more than their share of the work, and the others not being worked to their full capacity. Mr. Grimes has so arranged his circuit that the work is more evenly divided among the tubes, each carrying about the same load, those carrying the heaviest audio frequency load where the lightest radio frequency currents are passing. In the beginning I will say that this is no circuit for the really green amateur to attempt to master. Any radio frequency circuits, and especially those of a reflex circuit, are complicated, to say the least, and it will require much patience and experimenting before the set can be pronounced perfect. In the description given in this article I have tried to give the prospective builder the benefit of much experimenting and experience of those who have worked it out with different types of transformers and different condenser values, ,so that some of the difficulties experienced by them will not have to be overcome by our readers. The "Inverse Duplex" when properly constructed will have a wonderful range when used with a small loop aerial. Distances of 2,000 miles have been covered with a threefoot loop aerial, according to reports received, but unless the greatest care and patience are exercised in its construction, it will be found quite difficult to control. Figure 1 is an isometric drawing of the rear of the panel and base. In order that the circuit and parts may be easily traced these parts are shown spread out over considerable space, but in practice they must be mounted close together so that all connecting wires will be as short as possible. Every inch that can be cut off the wires connecting the different parts will greatly add to the efficiency. The reason for this is the fact that these wires and parts are all carrying high frequency currents and if the wires are long, or run too close together, they are very apt to affect each other on account of their capacity. All parts such as transformers, sockets, condensers, etc., must be mounted on a bakelite or hard rubber base. This may sound peculiar to the fan, but any slight leakage between the parts will give no end of trouble, and it has been found that a wooden base contains enough moisture to cause considerable leakage. The fixed condensers shown should be mounted directly on the transformer terminals if possible. ooos H|.||||||{||.{|||| + PTOp — ' V/9J3 KflOJO Jit-MC ■ Figure 3. Circuit diagram showing the radio frequency component of the Grimes circuit in lighter lines, with the heavier lines representing the addition of the audio frequency. The numbers refer to the connections for use when the radio frequency part is connected up as provided for in the text.