Radio Broadcast (May 1929-Apr 1930)

STRAYS from THE LABORATORY """"""""" Whether or not the Trends in majority of receivers 1929 Receiver so\d during 1929 will Design be engineered for screen ' "in ' I""" grid tubes is a question that only time can answer. The attitude among some manufacturers is as prevailed at the time the a.c. tubes were first announced, "Let George do it." When several manufacturers merchandise a screen-grid receiver that stays sold, the others will no doubt scramble to get their own screen-grid sets out of the laboratory and on to the dealers' shelves. It is certain that 1929 will see the general adoption of the 245-type power tube, and indications are that most up-to-date receivers will use two of them in push-pull. Many receivers seem to be including some sort of band-pass tuning, some as preselectors and some between tubes as coupling circuits. Many receivers will have but one stage of audio, and of those which use screen-grid tubes, the majority will require but a very small antenna for loud speaker operation from distant stations. The average sensitivity of receivers built in 1928 was of the order of 50 microvolts per meter; those built in 1929 will probably be ten times as sensitive, 5 mv/ m. The advantage of uniform sensitivity over the broadcast band is a talking point (and a good one, we believe) of several manufacturers. Whether or not this uniform sensitivity means uniform selectivity, we do not know. This would be more of an advantage than uniform sensitivity, in our opinion. Some receiver manufacturers feel that the problem of getting a.c. screen-grid tubes in sufficient quantities and of sufficient uniformity will militate against the widespread use of this new addition to the tube line. Tube manufacturers, on the other hand, see no great difficulties in the way of building these more complicated structures. They feel that the experience gained in the production of the heatertype tube will cut down the time of experiment on the newer tube, and, that when manufacturers are ready for the tubes with the additional grid, they will be ready. Some manufacturers have discovered that they will require the same number of tubes when the screen-grid type are used, as in 1928, and, therefore, that the advantage in making a screengrid-tube set is only one of sales appeal. On the other hand, at least one manufacturer is ready to advertise that his receivers will not be screen-grid -tube equipped, and others have discovered that the same performance can be secured from a four-tube set using the a.c. screen-grid tubes as is now possible from six tubes. Such a receiver will have two screen-grid r.f. amplifiers, a 227-type power detector, or a screengrid detector, and one stage of a.f. amplification. Will such a receiver have sufficient selectivity? That is the question everyone asks. With an equal number of tubes it ought to be possible to design a receiver that, with a small antenna or loop, would bring in as much program enjoyment as an older set with a large outside antenna. We hope The following arc among the subjects discussed in ''Strays" this month: Trends in 1929 Receiver Design Data on Electrostatic Speakers How much is an Engineer Worth? Power Requirements in England Humps in Audio Transformers Short-Wave Schedules Resourcefulness of an Engineer Dr. Goldsmith Resigns as Editor the advertising departments will not offer the screen-grid receivers as being more free from static and other unwanted racket. If they are loop operated, considerable discrimination against unwanted signals, broadcasting, or static may be secured. A blanket statement, however, that the screen-grid tube will make a set freer from undesired noise is too good to be true. '"' ' """"" As IN THE CASE WITH Gossip About the a.c. screen-grid tube, Electrostatic some manufacturers will Loud Speakers and others will not use |'» » i i i the newest type of loud speaker, the electrostatic type. Some observers claim it is more sensitive; others say it is "down." It is generally agreed that its space requirements cannot be made much less than a dynamic from an equal performance criterion, and that the chief cause for worry is how long it will stand up in service. The fact that rubber, or some other insulating material, must be subjected to atmospheric changes, to the continuous static field across it, etc., means that it is difficult to get up accelerated life tests in order to find out how long the device will stand up. If the condenser-type loud speaker were cheaper, or more sensitive, or better looking, or more durable, or if it had a better frequency characteristic, the technical part of the trade would be interested. If the condenser-type loud speaker proves to be merely a good sales argument, the technical people will pass it by. It is difficult to believe, however, that some good will not come from the work that has gone into its development. We have the highest regard for the engineers and physicists at the Riverbank Laboratories. Hoiv Much is a Chief Engineer Worth? How much is a chief engineer worth? Let us suppose a manufacturer pays his chief engineer $10,000 a year and that he makes 100,000 receivers. This engineer is responsible, more or less, for the expenditure of money for the raw materials, for the design, and for the production of these sets. Thus, he gets about ten cents per set for his work. Now, if the receiver is made under a license agreement, it pays not less than 7i per cent. ($7.50 on each $100 set), and, if it goes into a big cabinet which brings the price to $200, the licensor gets $15.00 although the chief engineer still receives only ten cents. What is the moral of this story? We don't know. Power Required by English Loud Speakers View of an automatic tube-testing apparatus used by Westinghouse. At the right is shown the control cabinet panel and at the left the automatic feeding system AN EXTRACT FROM A letter from C. L. Lyons, of Claude Lyons, Ltd., Liverpool, radio "" ' ■ and electrical distributors, casts some light on the power requirements for various types of loud speakers used in England at the present time. According toMr. Lyons, who has conducted many tests to determine what his clients desire in the way of volume: 1. Dynamic loud speakers require about 750 milliwatts, 2. Large cone loud speakers, 15 inches in diameter, require 350 milliwatts, 3. Diaphragm-driven horn loud speakers, more or less obsolete, 250 milliwatts, 4. Small cone loud speakers, 7 to 15 inches in diameter, need about 250 milliwatts. Mr. Lyons expressed surprise at the fact that many American technical articles mention power outputs as • JUNE • 1929 • • 101