The Birmingham age-herald., 1922-11-05
By JACK BINNs


 ments of this epoch making year in radio receivers is the so-cailed re fIex system" or double function cir cult This is the circuit which per mits the same amplifying vacuum tubes to act as magnifiers of radio and audlo frequencies without Inter ference in such manner that three tubes do the work of six, or two tubes do the work of four.
 There has recently come on to the market completed sets built upon the fundamentals of this system, Wnieh hermit the reception of signals on loop aerial over reasonable distances and the reproduction of the speech and music on loud speakers.
 The secret of this system lies In the use of suitable condensers at certain strategic points in the circuit, so that the very high frequencles will have a pathway of law impeno- ance on which to travel their ap- pointed way, while the lower tor audible frequencies) are compelled to go through the coils where they can be utilized to the best possiDle advantage.
 Transformer are Important
 Although the condensers, therefore are the most important part of the system, the transformers also play an important part. and only the most efficient should be used, both as re- gards those designed for radio fre- auency amplification, and those for audio.
 The tuning element fin this system can be of any standard design, that Is vario- coupler with a variable sRH0nd8TX. or even a simple variometer, as shown in the diagram.
 The input from the tuner is upon the grid of the first tube. The plate circuit of this tube is then joined to the primary of the raido frequency transformer. The other side of this primary Is then adjoined to the pri- mary of the second audio frequency transformer. The other side Of the latter is joined to the positive term. inal of the "B' battery in the usua. manner.
 Across the primary of the audio- frequency transformer there IS joined a condenser, and this conden- ser acts as a bypass for the high frequency currents in the K. F. transformer, while the audible cur rents must pass through the audi transformer. Another condenser or similar character is placed across the secondary of the audio-fre- quency transformer for the same purpose. It will be seen that this system is followed throughout.
 Crystal Detector ia Hest
 For the best results with the sys- tenn the crystal detector should be used for rectification. The amount of energy supplied to the crystal will be quite large, because of the fact that there Is considerable amplifi- cation of the aido-frequency cur rents by the two tubes shown in this particular diagram. The output from the crystal detector Is passed through the first audio-frequency transformer shown connected to 1t, which in turn transfers 1t to the grid of the first tube for further amplification
 So far as the amplification of the voice currents Is concerned the amount of wire in the radio fre- quency transformers Is negligible in effects, so there is nc interference from them. and amplification goes on in the same manner as in the standard sets.
 There Is slight difference in the placing of the transformers. The radio frequency transformers follow the tubes in both cases. The first audio transformer follows the crystal detectcr and the second follows the first vacuum tube. The telephones are placed in the plate circuit of the second tube, in series with the prl- mary of the second radio transform CT. A condenser is placed across the telephones as bypass for the radio frequency currents from the second tube.
 Fixed Values Must he Found --
 While the circuit can be assem- bled by any amateur, it will require quite a little experimentation before It is placed into suitable operation. This will be because of the different characteristics of the vacuum tubes and the transformers. It is for this reason that absolutely fixed con- stants can not be given for the cir cult.
 It will be found however. that the capacities of all the bypass e nden- sers can range from .0005 to .001 microfarads, the exact amount of the capacity of course will have to be determined by experiment.. The con densers used for this purpose can either be on the ordinary ole type or of the small fixed type which will permit easy variation in small steps,. The latter are the best, however. The exception in capacity is the condenser across the teie- phones which should be of .0005 mil crofarad.
 The important thing 10 be kept in mind IS the fact that capacity effect is very pronounced in handling high frequencies such as those used on the amateur and broadcast wave-:engths Therefore it is imperative that the wires connecting the transformers and the tubes should be kept as short as it IS physically possible to keep them. This point cannot be too strongly emphasized, as success de- bends absolutely upon At.
 In radio frequency amplification it is very often the case that re- suits can be greatly improved by changing the vacuum tubes around in the circuit. This is due tL' the variation of characteristic in the tubes. no two of which are exactly alike, and changing them around aids in getting a better match with the particular transformer. The same Is true of the reflex system.