Radio Broadcast (May 1928-Apr 1929)

Coupling Methods for the R. F. Amplifier One of the Coils Tested By BERT E. SMITH Aero Products, Inc. WHEN the screen-grid tube was first introduced it was heralded by many as a panacea for all the ills associated with r. f. amplifiers. That such is not the case is becoming more and more evident. The tube undoubtedly is valuable, but just what can be expected of it can be determined only from the data obtained from carefully done laboratory investigations. In this article is presented some such data indicating the comparative gain and selectivity that can be obtained from the tube when it is used with different types ofr.f. transformers. — The Editor. FOR several years the development of radiofrequency amplification has been practically dormant, with little, if anything, new in sight at the present time. Recent improvements have been confined to the sonic end of the receiver, with the result that radio has been lifted from a fad to an art, but the old thrill of "distance" has passed! No longer do eager commuters rush for the morning train to brag about the dx of the night before, and no longer do "Radio Widows" get divorces because their husbands desert them to spend the nights with the radio set and the thin elusive signals from a transmitter three or four thousand miles away. For, strangely, in spite of the fact that receivers now average six or seven tubes where they used to have perhaps three, and broadcasters use ten or a hundred times their former power, it is harder and harder to get distant stations. xMany theories have been advanced to explain this, but in the final analysis, it becomes increasingly evident that the truth must be that in the rush for selectivity and quality, designers have lost sight of sensitivity. The science of radio-frequency amplification has seen retrogression rather than progress. Nothing of any real value has been introduced since the Hazeltine neutrodyne system several years back, and even that, insofar as the principle of neutralization by external capacitative reactance is concerned, was only a variant of the earlier Rice system. In the years immediately following the introduction of the neutrodyne, many schemes have been advanced purporting to produce the full theoretical amplification of the tube and transformer, or to enable the tubes to be stabilized without any loss of efficiency, but all have proven impractical, and 100 percent, efficient radio-frequency amplification continues to be a vainly sought chimera. A surprising number of manufacturers have returned to the oldest method of stabilizing known — potentiometer grid control. Some are cutting down the plate voltage applied to the tubes. Some use variable leaks across the tuning condenser, broadening the tuning and losing all the well known advantages of low-loss coil and condenser construction. Practically every manufactured receiver to-day on the market employs one of the Mosser" systems which were so violently, although justly, condemned a few years ago! Small wonder, then, that receivers do not reach out for distance now as then ! That there is still a call for sets which will bring in distant stations was distinctly evidenced by the sudden rush when the screen-grid tube was announced, and it is decidedly unfortunate that this tube was heralded by so much misleading publicity, which led builders to expect much that has proven impossible. Many of the leading publications carried editorial matter describing the great amplification obtainable from a tube which would not oscillate, and even the largest tube manufacturers and best engineers in the country allowed statements to appear such as "a voltage amplification of 200 per stage is obtainable, but at broadcast frequencies the resonant impedance is lower reducing the amplification by 25 per cent, of this value." It may be possible to get such gain from the tube in laboratory apparatus, constructed by competent engineers, and under ideal conditions, but performance of this kind cannot by any means be secured from the ordinary broadcast receiver. It has become extremely doubtful, in the writer's opinion, whether the tube in ordinary use in tuned radio-frequency receivers operating at frequencies of 500 kilocycles and higher can produce great deal better all around results than the 201 a type tube which has been standard for so long a time. With all these facts firmly in mind, an investigation was recently undertaken with the object of determining two things: First, whether some existing method of stabilization could not be so modified as to give really passable results by permitting r.f. amplifiers to be built without the intentional introduction of losses except as Transformer under test FIG. I 36, desired for a volume control; and secondly, whether this amplification might be obtained within appreciable limits of selectivity, regardless of the type of tube used. However, all the tests described below utilized the 222 type of tube. No new ground was gone over, and nothing was developed which did not bear out previous empirical design, but there had been so much theoretical data published, and so little of the results of actual quantitative tests, that it seemed that figures obtained through concrete experimentation might be at least refreshing. Prior to commencing any actual work, certain limits were laid down as essential if the results were to be of any value in designing a receiver which could be constructed by the kit builder. First, standard apparatus, obtainable by anyone, must be employed. Second, the need for any complicated balancing, by means of expensively accurate apparatus after construction, must be avoided if possible. Thirdly, the use of shielding, while not barred, was considered undesirable as introducing superfluous expense and trouble. Last, no involved or critical adjustments of any kind were allowable, as a receiver must be infallibly sure to give good results if the connections are properly made, in the hands of the most inexperienced operator. THE TEST OF COUPLING METHODS THE first actual operation was the determination of the method of coupling tubes, and the optimum values to be used in the coupling device. For this the set-up shown in Figure I was utilized originally, but since the conditions in the grid circuit of the amplifier tube were not identical with those which would be encountered in actual practice, it was found advisable to add a second stage, coupled to the grid of the test stage tube by a special radio-frequency transformer having an absolutely flat amplification-against-frequency characteristic. The use of this stage allowed the test stage to be adjusted to operate in a manner exactly identical with its performance in an actual receiver, permitting oscillation to take place in the same way and at the same point. The output of a modulated oscillator, variable in frequency over the broadcast spectrum, was led through an adjustable attenuator to the grid of the first amplifier tube,