The Radio Sets of To-day

Radio Record, Volume III, Issue 35, 14 March 1930, Page 1

The Radio Sets of To-day

An Answer to ihe Questions raised by the-non-technical Listener

| ECIDING what. radio: receiver to buy is harder this year than it ever was. before. And, curiously envugh, the difficulty now is due to to the fact that there are so many really fine sets on the market.

Cabinet designs have become so s‘andardised that dozens'of different ‘lio. sets varying widely in power, ice, and so on look practically alike except for minor details in ‘the finish of the cabinet. Of course the more expensive sets have finer cabinets, but the less, expensive. outfits are housed in cabinets so carefully built that they give the impression of high ‘quality. In many cases only an expert -cabinetmaker can distinguish between them. Some of the important questions our readers are asking about this year’s radio receivers are answered. below. The answers should prove helpful to prospective set buyers. .

HAT are the meanings of all the new terms used to describe tone quality?’ Good tone quality means just one thing-the ability of the radio recéiver to reproduce, as perfectly as ( is scientifically possible, the tone pro‘duced in the broadcast studio. .And that in turn means that the electrical and acoustical characteristics of the receiver are such that every tone frequency is treated in a uniform manner. None should be over-emphasised, none suppressed, and none’ distorted. Many high-sounding but utterly meaningless phrases and words ‘have been coined and applied to radio receivers to indicate that in one: way or another they are better than other makes as far as tone quality is concerned. The basic fact -emains, however, that a receiver is a piece of mechanical and electrical apparatus designed to do just one thing. That is to reproduce as faithfully as possible the air vibrations produced by the artists broadcasting. That function may be ealled by any name imaginable without improving the results. % HY are screen grid valves better? The sereen grid valve, from a th¥oretical stancpoint, is a vemarkably efficient radio-frequency amplifier. If used in a well-designed circuit it produces, in a practical fashion, more radio-frequency amplification than can be obcained by the -.lder type yalve. It must be remembered, however, that the presence or absence of the sereen grid valve in the cirenit has nothing whatever to do with the tone quality that a radio re*eiver will produce. Just because a receiver has screcn grid valves does not guarantee improved tone qua:ity. Thousands of yraato receivers that have no screen grid: valves, are being manufactured, and will be sold this year, and they will give excellent satisfaction to their owners. If the prospective purchaser is interested only in local reception or he is located where a good antenna can be erected, screen grid valves will he of no particular benefit. On the other

hand, if he is located where it is impossible to put up a good antenna, and local reception conditions are very unfavourable, then, a.set using screen grid valves will help him to bring in stations that he. otherwise might not receive. OES power detection ;"*é greater distance and more volume?’ Theoretically, power detection ‘gives

not powér detection should not be considered for or against it, provided the tone quality is. satisfactory: A power detector is not as sensitive ‘to weak signals as is the conventional grid condenser and grid leak method of detection, This, too, is relatively unimportant. In circuits designed to use power detection the radio-frequency amplification is greater than in circuits

better tone quality simply because it eliminates a certain amount of distortion which takes place with ‘the older type of grid condenser and grid leak method of detection. The difference. however, is hardly noticeable, except to the trained ear, and then only when the audio amplifier of the set and the loudspeaker are both of excellent quality. The fact that a set: has or has

not designed to use the new system of detection. In some sets the power dedetector is coupled directly to a single audio amplifier stage using power valves. This arrangement inherently produces less hum than does the circuit using two audio amplifier stages. In some eases, however, the manufacturer has taken advantage of the reduced ham to cut down the filter circuits so

that the net result is little better than it has been in past years. In any case, ‘modern radio sets are practically hum free in operation. N what way are this season’s sets better than last year?" Judging from tests‘ of a number: of ‘different receivers, the radio- sets produced this season are more sensitive, more selective, and | give better tone quality than last season’s sets. ‘The improvement is perhaps more noticeable ‘in the low-priced sets than in the high-priced ones, simply because there was more room ‘for improvement. The increased ‘sensitiveness and selectivity of this’ season’s products is due-in some cases to the use of the screen grid valve and-in-other cases to a. better design of the radio-frequency circuits used with the type 227 valve, In addition, there has. been .a notices able improvement, in factory , production methods, so that the tuned stages are more accurately synchronised with each other. pe the new sets cost more to operate? The cost of operation. of any, radio’ receiver can be divided into depreciation, cost of current per hour, cost for tube replacements, and repairs. | | : cannot be figured by any ordinary method because a modern radio receiver will last for years, The cost of electric current depends on the number and size of valves used in the set. If, for instance, a set uses three screen grid valves, type 224: two heater valves, type 227; and two power yalves, type 171A. it will use just as much current whether the set costs 100 dollars or 300 dollars. In any case the amount of current consumed, as compared with sets of last year of approximately equivalent price; will be only a small fraction greater. If the set uses 245 power valves the current drain. will be somewhat heavier. THAT is the advantage of automatic volume control? Automatic ‘volume control is another improve-. ment for convenience rather than operation, In one form, automatic control is. obtained by the use of a special valve in the circuit, so connected that the strength of the received signal changes the plate current flow, and the change in plate current flow, in turn, changes the grid bias on -the radiofrequency stages, The result is that all local stations sound alike in volume. In addition a hand control is provid- ed to cut the volume below the level to which it is controlled by the automatic arrangement. Wat is meant by uniform sensitivity? A theoretically perfect radio receiver should be equally sensitive on all wavelengths or frequencies. Most radio receivers in the past have shown greater sensitiveness to signals on the lower end of the wayelength band: in other words, on the higher frequencies. A station received on 545 metres or 550 kiloeyeles, for example, usually gave considerably less "volume from the loudspeaker than a

station received on 286 metres or 1170 kilocycles, There are two reasons why this situation existed. First, the radio-fre-quency transformers then used were more efficient on the higher frequencies; and second, the natural tendency of the tubes to regenerate was greater on the higher frequencies. . " These conditions still obtain to some extem, but other factors working in the opposite direction have been introduced into the circuit in some cases. The net result is a reasonably uniform degree of sensitivity on all waveJengths within the band used for broadcasting. yuart is the advantage of push-pull ‘. amplification? Push-pull amplification is featured in a large number of the latest sets. The push-pull circuit definitely reduces distortion and therefore improves tone quality. Briefly, the characteristics of a power valve are such that a small amount of distortion is produced in its opera‘tion. In the push-pull circuit the distortion produced in one valve is balanced out by the distortion produced in the other valve. ‘AB the new speakers any better than those of last year? The high-grade loudspeakers this year produce no better tone quality than the high-grade speakers of last year, but in the less expensive models there has been a vast improvement. There is less difference in tone quality between a low-priced speaker and a high-priced speaker now than there ever has been in the past. D° the new sets need antennas? Since the latest sets are, on the -- =

aoe average, more sensitive than their predecessors, in any given. installation a shorter antenna will produce the same results as the longer antenna of previous years. In an unfavourable location where reception. is poor at best and the building is of steel with wire lath construction, an antenna still is necessary for really satisfactory results. Or course, conditions vary so that it is impossible to give definite rules, . T’ new improvements may be expected next season? It is difficult to see how any radical improvements can be made in the latest radio receivers, Hrom the standpoint of tone quality, for example, it is extremely doubtful whether anyone except a trained musician, could detect the difference between the type of reproduction obtainable from the best of modern sets and an outfit that was theoretically perfect. In other words, with the best sets of this year it ig practically impossible to determine, when listening from another room, whether the music is from a radio set or from actual musical instruments. Future radio receivers may be made even more sensitive and selective than they ate at present, but any great improvement in sensitiveness would be of no use because the static level is, in any case, the limiting factor. Any great increase in selectivity can be accomplished only by sacrificing tone quality. Improvements from now — on are likely to be in the nature of mechanical refinements in construction and operation, and of changes in cabinet design. =