VALVES

Sun (Auckland), Volume IV, Issue 943, 9 April 1930, Page 7

VALVES

How to Select Them DIFFERENCE IN TYPES Purchasers of valve sets, and even those who make their owu receivers, are often puzzled by the apparently large number of types of sets and of circuits from which they are required to choose iu making a selection. As a result of this a set is frequently chosen quite at random, or at the best from the results of a crude comparison with others under the unusual conditions which often apply in the demonstration rooms of dealers. When ■ installed in the purchaser’s home the receiver is often exceedingly disappointing. VARIATIONS IN DETAILS Although the number of types of receiver and circuit now available for broadcast listeners is almost unlimited. practically all are basicly identical, and are distinguished only by variations in detail. Many improvements in technique, of course, have been made recently, but the circuits in use now are fundamentally the same as 20 years ago.

For purposes of reception the valve is used in three general ways, and if these are thoroughly understood listeners will find it relatively easy to select or construct a set for any purpose. The three uses of the valve in receivers are as a radio frequency or high frequency amplifier, as a detector, and as an audio frequency or low frequency amplifier. In order that the meaning of these terms may be grasped it is necessary that the process of picking up a broadcast programme be understood. The receiving aerial picks up the programme from the broadcast station in the form of a series of very high frequency alternating currents upon which the programme is impressed. These surges may be amplified very greatly, but however much they are amplified they cannot be used without further treatment, because neither loudspeaking equipment nor the human ear is capable of responding to them. The function of the detector Is to render the programme carried by the high frequency currents audible. In effect it may be said to filter out the high frequency currents and to pass the low frequency currents which constitute the speech and music iu the programme. As delivered from the detector these

low frequency speech and music currents are not strong enough to work a loud speaker, and they must be amplified further. This is done by the low frequency or audio frequency amplifying valves. x\ peculiarity of the wireless valve is that, although it will operate perfectly as an amplifier of a current, no matter how weak it is, it cannot act as a “detector” unless the current is relatively strong. STRENGTH OF IMPULSE Many circuits intended purely for local reception employ no radio frequency amplifier valves. The impulses from the local broadcasting stations are sufficiently strong to operate the detector valve directly, and are impressd upon it as soon as they are picked up from the aerial. For the reception of distant or weak signals, however, the use of the radio frequency amplifier is essential. By its aid the high frequency signals from a distant station can be amplified until they are powerful enough to work a detector valve. The number of high frequency amplifying valves required will depend mainly on the range desired and on the size ol the aerial. A large aerial delivers more current to the receiver than a small one, and thus makes possible an economy in the number of valves used. On the other hand, a large aerial aggravates the problem of eliminating interference from local stations, so that aerial dimensions* cannot be increased indefinitely. An important advantage of the radio frequency amplifier is that, in addition to giving great range, it also increases materially the selectivity of tuning. In practice it is difficult indefinitely to increase the number of valves used as radio frequency amplifiers, and for ordinary broadcast reception it is seldom wise to use more than two stages. The reason for this is twofold. There is an inherent tendencj r to instability in a radio frequency amplifier. This can be. and in modern .sets is, corrected by certain devices which, however, generally reduce efficiency. Moreover, for greatest efficiency, each stage in a radio frequency amplifier must be tuned. At one time this involved the use of many control dials on the set. Ganged condensers, which enable many circuits to De tuned simultaneously from one dial, have lately eliminated the difficulty of tuning radio frequency amplifiers, but again a loss in efficiency has re- | suited. For these reasons the advanI tage of more than two radio frequency stages, except for special purposes, is more than outweighed by the extra I cost of Installation and maintenance

When used as a detector a valve requires special adjustment, which, however, is usually provided for in the operating directions supplied by the maker with each valve. In practice the principle of regeneration or reaction is usually associated with ?. detector valve. YVhen a valve is used in this way it virtually performs the double purpose of detector and radio frequency amplifier. The range and selectivity of the set are improved by the use of regeneration on the detector, but invariably at the expense of quality of reproduction. Regeneration is usually necessary in circuits, such as the popular Reinartz circuit, in which no radio frequency amplification is employed, but it should foeavoided where a radio frequency amplifier is used. The size of the audio frequency amplifier will depend on the amount of volume which is required, and to some extent on the type of speaker used. Modern speakers, such as the dynamic or moving coil cone types, require more power to produce a given volume of sound than the old horn type speakers, and therefore require a more elaborate low frequency amplifier. In general, however, not more than two stages of low frequency amplification should be used ; after a detector valve. Additional stages, of course, will give greatly increased volume, but unless the set is handled by an expert there is a danger of valve overloading, which results in distortion. Moreover, an elaborate supply of power for the plate circuits of the valves, such \ as a large-sized eliminator, becomes necessary, and the amplifier becomes . instable. This instability is sometimes noticed by a tendency of the set to “howl.” Quite frequently, however, this symptom of instability % not present, and the instability results ‘ in annoying distortion, which is extremely difficult for the novice to trace.