RADIO AND ITS RECEIVERS

Sun (Auckland), Volume III, Issue 717, 17 July 1929, Page 14

RADIO AND ITS RECEIVERS

Conducted fcr THE SUN by

R. F. HAYCOCK.

Screen-Grid Valves

AMPLIFICATION WITH STABILITY

Advantages Explained

A CIRCULAR, finely woven metallic “screen” which radio research has added to the vacuum valve has given the netv 1929-30 receiving sets an amplifier of nearly a hundredfold sensitivity and far greater volume than heretofore possible. The delicately woven mesh of wire actually “screens” one valve element from tile other, making it possible for the first time to obtain high amplification without unstable operation of the set.

QUESTIONS BY SET OWNERS What is the screen-grid valve? Will it make the new sets superior to receivers equipped with other types of valves? Can the screen-grid be used in any type of set? What are its advantages over the standard type of detector and amplifier valve? Can it be used in any socket? Can it be used as a detector? These are some of the questions asked by prospective set buyers and radio fans. The answers are sketched, as follow, from a series of tests with the new valves made by valve makers to determine the conditions under which they may be operated with best results. “In the first place,” said D. P. Schmitt, the chief engineer, under whose direction the valve tests were carried out, “the four-element or screen-grid valve operates exactly like any other vacuum valve, but does it to a much greater degree. The screen-grid element is placed between the original grid to which the radio signals are conveyed from the antenna and the valve’s plate. Its sole function is to reduce the ‘capacity’ between these parts. Thus what engineers call ‘feed-back’ may be eliminated and the valve made an excellent amplifier without danger of stirring up howls. “Besides being a very good amplifier, the screen-grid valve makes an excellent detector when used for faint signals," said Mr. Schmitt, “or when used for large signal inputs. It is not used with the conventional grid leak and condenser circuit but is best as a ‘grid-bias’ detector. When applied in its special circuit, the screen-grid voltage and con-trol-grid ‘C’ bias should be chosen, so that the incoming radio signal cannot cause excessive current to flow in the detector plate circuit. On weak signals it is best to feed the detector output into a first audio transformer and to operate the screen-grid at about 35 to 45 volts positive potential and the control-grid at about 3.5 to 4.5 volts negative. “For large, loud signals the valve

works best as detector with about 75 volts positive potential on the screen-grid and a negative controlgrid bias of 7.5 volts. In this latter case the audio signal output is large enough to directly operate a large power valve, of either the 345 or the 350 type, without any audio transformers whatever, merely some convenient valve coupling device. The audio amplification possibilities in this line are immense. One can visualise powerful one-valve audio amplifiers consisting merely of the power valve, fed directly from the detector valve.” The screen-grid valve cannot be used in the socket of any set unless the receiver is especially designed for it. “It would be like trying to harness lightning,” said one radio engineer, “without making any plans whatever to use its force. It is like a powerful engine, capable of immense force when the hand of a child opens the throttle, but with no work to be done. If placed in one socket of a modern set built to use the conventional radio valve, even with special precautions taken to supply it with the proper filament and plate voltages and to shield it thoroughly in a metal compartment, the power of one screen-grid valve added to the power of the other valves in the set would overload the set’s detector and ruin reception. In the simple set with one radio frequency amplifier stage it can be more easily adapted, but even then the radio amplifier should be completely reconstructed, using special coils and shielding, without which its power cannot be harnessed.” While the average three-element valve now in general use needs a balancing or “neutralising” device to keep it from howling, the screen-grid valve is free from such difficulties until high amplification a stage is reached, or between 80 and 100 times a screen-grid valve stage. This is scarcely comparable with the average of 12 times a stage obtainable with the three-element valve in its proper circuit. However, the plate and grid circuit parts of the screengrid stage must be thoroughly shielded in metal containers to obtain the best results. NEW ERA FOR RADIO The general opinion of radio engineers and designers is that a new era in radio receiving apparatus will be ushered in by the widespread use of this valve. The predictions go so far as to forecast that the A.C. screen-grid valve will lead to the wholesale production of radically simplified sets giving a much higher plane of performance. It is pointed out that sets having many radio tubes and stages to highly amplify the signal are more than likely to cause distortion by a slight defect in some obscure part of the apparatus. This is not so likely to occur in the simple sets of fewer valves, the radio designers contend: hence the advantage of the screen-grid, in which one radio amplifier does the work of three or four ordinary valves. Engineers have said that the new screengrid sets will possess, if the valves are properly used In the circuit, a degree of selectivity and sensitivity which will be a revelation to the user, and all this with fewer valves. “The possibilities of this radio valve seem to be inexhaustible,” said one radio leader. “It takes very little foresight to imagine that the radio sets or five or ten years hence will be instruments of wonder. The radio

valve has given the voice of man seven-league boots. Few of us have any conception of what the everyday radio valve, as we know it, has already accomplished in this highly scientific age, or of the tremendous significance which it is destined to play in future industrial, commercial and social development,” he added. “At the present time we are but beginning to appreciate the wonders which it will bring to our firesides within the next few years.” W.E.A. TALKS PROGRAMME FOR SEASON Talks will be broadcast weekly by the Workers’ Educational Association from IYA as follow: Thursday, July 25.—Mr. N. M. Richmond, 8.A., director W.E.A. classes, “The Modern Age: JSfo. 1, as H. G. Wells Sees It.” Wednesday, July 31.—Dr. H. Belshaw Professor of Economics, Auckland University College and tutor W.E.A. Advanced Economics, “Waste in Industry.” Thursday, August S.—Mr. N. M. Richmond, “The Modern Age: No. 2, as the Socialist Sees It.” Tuesday, August 13.—Mr. H. Hollinrake, Mus.Bac., tutor W.E.A. music class, “Melody.” Thursday, August 22.—Mr. Richmond, “The Modern Age: No. 3, as John Galsworthy Sees It.” Tuesday, August 27.—Rev. W. G. Monckton, M.A., tutor W.E.A. International Relations class, “New Zealand and the League of Nations.” Thursday, September s.—Mr. Richmond, “The Modern Age: No. 4, as the Liberal Sees It.” Tuesday, September 10.—Mr. Gilbert Archey, M.A., Curator of Auckland Museum and tutor W.E.A. Anthropology class, “The Beginnings of Speech.” Thursday. September 19. —Mr. Richmond, “The Modern Age: No. 5, as the Modern Woman Sees It.” Tuesday, September 24.—Dr. A. B. Fitt, Professor of Education, Auckland University, “Why and How We Dream.” Thursday. October 3.—Mr. Richmond, “The Modern Age: No. 6, as the Man of Science Sees It.” Tuesday, October B.—Mr. F. N. Bamford, tutor of W.E.A. Art Class, late Director of School of Architecture, Auckland, “Designing a Small House.”