Radio Amplifier for the LOFTIN-WHITE

Radio Record, Volume IV, Issue 18, 14 November 1930, Page 16

Radio Amplifier for the LOFTIN-WHITE

Systems Using ScreenGrid R.F. Stages

Loftin- White Series A unique series of articles has been arranged. These will probably extend over five weeks, the topics for each week being;:ist Week-General introduction and construction. hints. 2nd Week-The "‘Loftin Three."’ Using the existing amplifier but rebuilding it into a compact set with one radio stage. 3rd Week-The "Loftin Three.’ The American system, for those who do not wish to interfere with the: existing amplifier. . 4th Week--The Revised "‘Loftin Three," for those who wish to start from the beginning and incorporate certain alterations in the value of the amplifier resistances. 5th Week-The ‘‘Loftin Four:’’ Various adaptations of the amplifier and the use of the power pentode,

S$ an amplifier the directly coupled Loftin-White system has aroused world-wide interest. Commencing in America with the original researches of two young radio | engineers, Loftin and White, published in "Radio "News," interest has spread; Australia, shortly afterward followed by New Zealand, were soon to realise its possibilities and published descriptions led to «a -period of construction unparalleled in recent years. Probably its simplicity, faithfulness in reproduction and cheapness made it admirably suited to the amateur constructor and _ the ‘surprising fact was made known shortly after its appearance in this

country that first the resistances. and then the valves were unobtainable. Judging by the correspondence we have received, constructors have been singularly fortunate for the complaints could be numbered .on the fingers of

one’s hand.. And these invariably were traced to neglect of instructions or defective resistances, particularly the .5 megohm leak. It is only comparatively recently that the construction has spread farther afield, and we notice that foreign journals, particularly the French, are giving it attention. No sooner was the amplifier popular than inquiry was made to its possibilities in a multi-valve receiver. For some considerable time the system defiel attempts to add a suitable tuner and in some quarters it was actually condemned. It was unstable when associated with certain detector. systems and so it was considered that its usefulness would end as a gramophone and local station amplifier, The application of considerable ef-

fort has resulted finally in a workable receiver which is both stable and sensitive. This radio amplifier about to be described has been applied to various commercial receivers. with considerable success. Outstanding .in this field are the midget- sets, many . of which are equipped with the Loftin White system. Although technical data for the construction of-the radio stages has been to hand for some time, we have. re: frained from publishing it until our experiments satisfied us that it could be undertaken with confidence by the average constructor.: ‘The result has been that now we dre in a position to

publish a unique series of articles which should fill the needs of constructors who have already constructed their amplifiers, those who, although they have constructed, will not mind rebuilding, using the same parts, and those who wish now to build the complete amplifier and tuner for the first time. There is little difference in the efficiency of the system, though the complete model made up with t adaptation in the resistances is per haps slightly the better, though the difference is not sufficient to warrant the owner of a Loftin-White dismantling his set and discarding some of the resistances, Some will be debating the question as to the number of radio frequency stages they will add. That must be decided by their particular ~ requirements. The one valve, as in the Lof-tin-Three, will give excellent results from the local station and. from the best of the New Zealand stations. It is not primarily designed as a distancegetter and although many will claim that it is good on Australian stations, it must be regarded-primarily as a New Zealand set. The extra stage of radia that will be described in the course of these articles under the name of "The Loftin-Four" is the one for (d.x.ers to construct. Other radio~ valves can be added so that .the experienced experimenter may obtain ali the amplification he requires. Selectivity is a feature of the set. No Inferier Parts, I setting out the construction of any of the circuits one must be guarded against the introduction of inferior parts. Watch particularly the resist-

ances, as their accuracy ‘will greatly affect the results. There is no need to worry about first-class articles. Pay & good -price and obtain the best. It \.. Will bring in good results and be much beaper in the end. Take care of the ! shielding. It is simple, though if not adequate will introduce instability and dissatisfaction. It will be noticed that the coils and the under wiring are alone. shielded, the system of DRY QIYYYYYIYYYYI EIA III IEA ho SINISE

shielding being different from that. before described in these columns. This is strictly in accordance with modern practice and not only is it more simple than the older style of having shield boxes, it is decidedly more satisfactory. But take care that the shielding aluminium is sufficiently heavy. Only recently we learned of the difficulties that an experimenter had had through ‘Yanot using sufficiently heavy aluminium. |) No matter what he tried the set would

not stabilise, and finally he used heavier shields, with the: remarkable results that the set became stable and workable, Aluminium not lighter than 18 gauge should be used, and foreseeing the difficulties in coil covers we have made arrangements whereby coils will be available from dealers. Their cost will be very small. It need not be stressed that the condensers need be. of: the right working test; for the average constructor knows enough: about the game to realise that he is asking for real difficulties if he neglects this precaution. Many a valve ‘has been blown out and not a few power packs burnt out because of the neglect of this simple precaution. It is wise to .test them before building up. Making the Coils. WE have’ now traversed the main points to consider before the receiver is tackled. The moral is obvious and the foregoing remarks are intend-

ed not to frighten away the constructor but to impress upon him the very real need to follow the instructions and the specifications to the letter. Odd parts will frequently do in -constructing quite good receivers of the ordinary type, but not the LoftinWhite. Although we publish the first circuit of our series this week, we do not advise anyone to attempt to make up until the layout description is given ‘ week. ‘We will, however, describe the coils for those who wish to make them, ‘They must be carefully matched, as one-dial control is used, though two dials can be employed if preferred. When the three coils are to be used in the "Loftin Four," the greatest of care must be taken in their winding or they will not be matched. Make a winding jig and take a little time and there should be no difficulty. Here are the specifications :- First coil: Primary, 60; tap, 30; secondary, 90. Second coil: Primary, 65; secondary, 90. Third coil; Primary, 65; secondary, Coil formers, primary lin. and secondary 13in. Wire, primary 34 gauge d.s.c., secondary 26 d.s.c. The tapping on the aerial coil provides for a more selective circuit than would otherwise

be the case. It may be necessary to vary the number of turns on the primaries between 65 and 80 to. obtain the best results. If the set oscillates on full strength the ‘number of turns on the primary of the radio transformer or transformers must be lessened. A large number of turns on the primaries increases the sensitivity, but makes the set prone to oscillation when pushed. There is little instruction necessary before one can construct the coils: That ° is self-evident when the diagram -ac: companying is studied. Do. not atBah mnnadhonnanrnannmannans

tempt to alter any of the specifications given for the coils. A small former’ is necessary so as to reduce the field zsated by the current in the coil; furthermore, a large former will not fit in the shield-cans. The wire has Lon selected for many reasons, and constructors will be well. advised: to use that specified if good results are to lt. obtained, Both coils must’ be wound in the same direction. If the three-valve set is to be used two coils are necessary, but for the larger set three. Further stages reauire a proportionate number of similar coils. In next week’s article we are presuming that the amplifier is readymade, but that the constructor has no ,objection to rebuilding it.