RADIO AND ITS RECEIVERS

Sun (Auckland), Volume III, Issue 741, 14 August 1929, Page 16

RADIO AND ITS RECEIVERS

Alodern Radio Receiver Out Of A Battery Model

E HOW' TO CONVERT AN OLD MACHINE

Reception F ree F ram Distortion

1113 last few months have seen a great change in the quality T of the radio receivers offered to the public. Batteryoperated receivers are, except in isolated positions, almost a thing of the past, their place having been taken by the light~ socket type, while power amplifiers worthy of the name, and dynamic speakers have killed distortion td a, very great extent.

It is probable that most readers who have heard a modern all-electric set will feel far from satisfied with their old machines, and will welcome the following article by Mr C. R. Russell, M.Sc., who shows how an old—type receiver can be brought up—to-date and made to reproduce programmes without distortion. The question is often asked: “Can a battery-operated receiver be made to operate from the light—socket?” Before this question can be answered we must be certain that we know the definition of a light-socket receiver. The modern factory-built machine uses alternatin current to supply the heat for the inges, which are generally of a special type known as heater or indirect heating tubes, the exception being the power \‘alvcs, which are of

the usual direct-current type supplied ,by unfiltered alternating current. There are. however, a few manufacturers who supply machines fitted with the ordinary direct-current tubes, the filament supply being obtained from an A battery eliminator, and a B and C battery eliminator, the whole then operating from the light sotket. Iteceivers of the latter type are truly light-socket receivers, and have the only disadvantage that they cost more to manufacture. ‘ The two types of receivers can then be summarised as followsz— \ (1) Receivers employing alternating cub‘ rent to heat the filaments. employing heater tubes in the RF. detector and first audio frequency stages. and standard direct current tubes in the power stage (operated by alternating current). (2) Receivers employing standard direct current tubes throughout, the filaments being supplied by direct current from an A battery eliminator. In both the above cases the plate or anode supply and grid bias are by means of a rectifier and filter commonly known as a B and C eliminator. Conversion to Light-socket Sets.

It is not possible, without costly alterations, to convert battery-operated receivers to type ‘l, but such receivers can easily be converted to type 2 at a moderate price. Many radio dealers supply A eliminators capable of oper—ating six to eight valves of either the four or six-volt type. With an A eliminator and a good B and C eliminator, and a switch ar—

ranged to turn off the power from both instruments at one. operation, the user at once has a true light-socket receiver, from the engineering point of View quite the equal to the alternating current type. There is a cheaper method of securing freedom from battery worries although the resulting combination cannot he said to be a true light-socket set. This entails the use of a storage battery and a trickle charger with a “power switch,” which is so arranged that when the filament switch on the receiver is turned off the eliminator is switched off and the trickle charger connected to battery. It is then only a question of filling the battery with water every few weeks. Should the reader decide to utilise this method he is advised, when his

battery has reached the end of its life, to replace it with an A eliminator costing only a few pounds more than a new battery. Power Amplifiers and Distortion. With the coming of the dynamic 'speaker listeners thought that their troubles were at an end, and that once the speaker was installed programmes would be reproduced in a natural manner. Many, however, were disappointed to find that the tonal qualities were ;little better than when the old speaker ‘was used. The trouble is not 'with the dynamic speaker but with the last or power tube, which is often incapable of handling even moderate volume without serious distortion. In a lesser degree the first‘ audio freqfi’ency tube, and the detector‘ are often responsible for distortion. 3 Figure 1 shows a two-stage amplifier‘ of the orthodox type, with a detector employing grid rectification. In most old—type factory-built sets the grid bias wires (11 and C2 are connected together and the same is often the case with the B battery wires B 1 and 82. If the transformers are of heavy construction it will be necessary only to bring out separate B and C wires for each stage, as shown in the diagram; but if the transformers are of the usual cheap variety then they should be replaced by a first-class article, and the connections brought out as shown in dia—gram. t

u Tit; detector circuit in many cases does not require disturbing. It will he

Vnotcd that a condenser is shown across: the first AF transformer; this is not required when throttle regeneration con-. trols are employed, as in the RBI). as; rthe chokes effectively prevent all radio: frequency currents from passing into the audio frequency circuits. ‘ The grid leak is responsible for as certain amount of distortion, and par-E ticularly in cases where two or moreé stages of radio frequency amplification‘ precede the detector. Under these circumstances it is best to remove the leak and condenser. and employ C battery or plate rectification as shown in diagram 2. ‘ The writer employs the latter circuit‘ ‘on all receivers, and has noted that the ‘modern factory-built super receivers ihave discarded the leak and condenser.: Elimination of Distortion. No matter how well an amplifier of type 1 is constructed. there will still be some distortion present due to even, harmonics of the signal voltage. These,; however, may be easily eliminated by the use of the “push-pull” amplifier shown in figure 2. This merely requires two special pushvpull transformers (taking the place of the second trans—;l‘ormer in figure 1) and two tubes. This sarrangement will handle considerably Imore powar than tyne l, and is ideal for any type of speaker. . It should be noted that the second push—pull ‘rans‘former contains an out—put transformer for the speaker, and it is essential. when usins: any power tube larger than [the UXII2. that an output transformer or an output filter (choke capacity) be used, otherwise the speaker may be seriously damaged. An output filter is shown in figure 1. The Choice of Valve-s. ‘ The choice of valves will depend to a great extent on the amplification of the radio frequency amplifier preceding the detector. . For the purposes of this article, 92ceivers will be divided into two classes: (1) Receivers employing one stage of radio frequency amplification of efficient design; or two stages of RF of mediocre quality. such as employed by some of the manufacturers of cheap five-valve re— CBIVCI‘S. (2) Receivers employing two or more stages of radio frequency amplification. This will include most superheterodyne rcCEIVEI'S. Class l—Good detector tubes for this class are the 615, 415, PM4D, 201 A, PMS, 409, 609, and, in fact, any general pur—

pose tube. For a first stage amplifier tube any of the above will do. For the last stage (two tubes if amplifier is of the push-pull type) UXII2, 8406, 405, 605, PMG. If great volume is required from the local station then it may be advisable to use 171, 603, 403, PM254, 1331256, UXI2O. Class 2—Suitable detectors for this class are UXII2, PMS, PMS, 8406, 615. For the first audio frequency stage 615, UXII2, 609, 409. For the last stage UXI7I, 603, 403. For all class 2 detectors it is advisable to use plate rec-' tification as shown in figure 2. It is essential that the grid bias or C battery voltage recommended by the makers be strictly adhered to. and this applies to the C battery required for the detector tube if plate rectification is used. 1: fact, a greater voltage will be required for the detector than specified by the ;makers. the value .being found by ex- ‘ periment, the purity of reproduction beiing the criterion. An output filter or output transformer must be used with the power valves specified for class 2. It will be seen that it is a very easy matter to bring the audio frequency amplifier up-to—date. If this is done and the correct valves are used—particularl 7 in the power or last stage—with the correct grid bias, then the reproduction secured will satisfy the most fastidious listener, particularly if a dynamic speaker is used. It should be borne in mind that a poor speaker will spoil the best amplifier. and that a poor amplifier will spoil the best ofspeakers.