Radio broadcast .. (1922-30)

RADIO IN 1929? By KEITH HENNEY ENGINEERING ENGINEERING advance in 1929 centers around the development and introduction of new tubes. The most important of these is the a.c. screen-grid tube: others are the humless-heater and quick-heater types; finally the 245-type tube which has already been adopted by the industry. The advantages of the screen-grid tube are two, greater inherent stability, and great er inherent amplification. Strangely enough neither of these advantages, nor the two together, is sufficient to produce an entirely new type of set or to make new receivers incomparably better than old. Before the intro- duction of the new tubes there were stable, high-gain, neutral- i/ed receivers. Screen Grid Sets: The advantages of these sets lie in other directions than in greater sensitivity and stability. The by-products of the new tube are more important than the chief raison d'etre. These by-products of the screen-grid tube were discovered; not thought out in advance. The greater stage gain of the tube led some to believe a radio set could be made with less stages of amplification than was possible in 1928. Unfortunately just as many stages were required for selectivity's sake, and so the r.f. amplifier of 1929 has at least two screen-grid tubes, just as the 1928 set had at least two stages employing 227-type or equivalent tubes. This made a set which had much greater overall amplification than the set of previous years, but this amplification was not neces- sary ; receivers already went down to the noise level in the average locality. And so some manufacturers reduced the a. f. amplification — and gained considerably there- by. Low Hum: The re- ceiver with one stage of a.f. needs less care in design to prevent undue hum. This is probably the greatest single advantage of the high gain r. f. amplifier— it makes possible a hum- less receiver which has good low-frequency re- sponse. Here it must be said that receivers with but a single a.f. stage (and hence a "power detector") were in use before the advent of screen - grid tubes, but not in general use in standard radio circuits. Circuit Changes: This single a.f. stage and a high-gain r.f. amplifier made other changes in design which are ad- vantageous. The volume control in some sets, for example, may now be made to operate on the a.f. end of the circuit so that when the volume is down, the hum is down too. Thus the ratio between signal and hum is constant and does not de- crease when the volume is turned down as in the older sets where all the volume control was in the r.f. amplifier. Screen-grid receivers may be somewhat more selective than .'MK. y triode sets with the same number of stages. However, these receivers are still not selective enough, or rather, they are not selective in the proper manner. They are too sharp at 5 kc. off resonance, too broad at 10 kc. off resonance. In some sets sensitivity has been sacri- ficed in favor of reducing the cost of shielding; in some sets the newer tubes have been used for benefit of the sales department, or for some mechanical reason. Linear Detectors: High-output r. f. amplifiers have required detectors with a high overload limit. Such detectors have some portion at least of their characteristic which is linear, the advantage being less distortion on high modulation and somewhat greater selectivity. More research is necessary either to make truly linear detectors or to extend the range over which the detection is linear, or to develop new detectors which are more linear or more efficient. Power: The 245 tube in pushpull makes possible a power output of about 3 watts which is sufficient for the aver- age home. Some users require more power output and can get it from the few sets which use 250-type tubes in push pull. It is probable that power output will not change greatly in 1930. It may be secured somewhat more efficiently by the use of new tubes but it will not be lowered—unless vastly more sensitive loud speakers make their appearance—and power output need not be appreciably in- creased. The new heater type of tube which does not crackle or hum is a distinct ad- vantage; the quick heater is an advance provided life, or freedom from noise, or both, are not sacrificed. At present the 5-10-second tube seems a good com- promise between all the essential and desired char- acteristics. Volume Control: This year saw two developments in volume control, the local- distance switch which changed the sensitivity of the receiver in a big jump, say 40 to 1 in voltage, and the variable type which reduces the coupling or gain to the antenna at the same time the sensitivity of the receiver is de- creased. Either of these seems satisfactory but those which vary the coupling alone or the sensitivity alone are not generally satisfactory due to cross talk, overloading, etc. Greater use of the automatic volume control circuits was evidenced during the year. A control of about 300 to 1 in voltage due to automatic control, and a 40-to-l control due a (Concluded on page 162) JANUARY 1930 • 131