More Crystal-Valve Circuits

Radio Record, Volume IV, Issue 37, 27 March 1931, Page 16

More Crystal-Valve

Circuits

Constructional Details of the Trinadyne

(By

CATHODE

"j N the last article of this series ; the theory of the Trinadyne ' receiver was briefly. discussed.and further : details. pro- + mised. Now, the Trinadyne circuit happens to be’ one with " which . the writer has exipertmented very considerably, and in’ the course of his experiments he has formed a very high opinion of its capabilities. Therefore, following the pol- ' iey outlined: earlier in this series, fairly complete constructional data will be given of a typical receiver employing this principle of reception. : A two-valve version has been chosen for description, as, while a one-valve "will give loudspeaker strength within reasonable distance of a broadcasting station, listeners located at a distance from any station will be glad of the additional amplification afforded by the second valve. -Moreover, who is there among-us who does not occasion ally succumb to that major malady of the radio enthusiast, DX fever? A well-made Trinadyne is very good medicine for DX fever, considering its modest two valves, and its capabilities in this direction do not detract in the slightest from its merits as a "quality" receiver for the local station, The Main Features. ‘A GOOD idea of the main features of the receiver can be gained from the circuit diagram given in Fig. 1 The output of the crystal detector is applied to the primary of an audio-frequency transformer, the secondary of which feeds the grid of the first valve. As in any case where a transformer follows a crystal detector, the transformer may have a high step-up ratio (5-1 or more), The detector has a comparatively low a.c, resistance, so that there is no object whatever in following it with 2 transformer having a _ tremendously high primary inductance. The second transformer must be considered rather differently, as its primary is in the plate circuit of a valve. This transformer should have a fairly high primary y inductance, and this is

more likely to be obtained in an instrument, of low ratio than in one of high ratio. In any case, the constructor should, if possible, avoid using two identical transformers in the construction of the set, as this is apt to result in bringing the little resonances to which any transformer is subject into an unpleasant degree of prominence. The Detector. . LTHOUGH the Trinadyne will operate quite satisfactorily with the cat-whisker tyj\e of detector, one’ of the permanent or semi-permanent type will

be found very advantageous. The considerable amplification of the set renders the adjustment of the detector rather a noisy process, so that it is just as well to avoid the need for it as far as possible. Quite apart from this consideration, the semi-permanent detector is more convenient, it is quite cheap, and its life should be at least twelve months. ’ By-Pass Condensers. [Tz will be noticed that the plate circuits of both valves are by-passe:l with .001-mfd. condensers. Although the receiver will usually operate without these, their inclusion is strongly recommended. From its very nature, the Trinadyne is rather more apt than most sets to suffer’ from high-frequency currents feeding through into the low-frequency amplifier. Adequate by-passing will eliminate any possibility of trouble on this score, while the value of the condensers has been so chosen as not to impair the frequency response characteristic of the low-frequencsy amplifier,

The H.F. by-pass condenser. bridging the crystal detestor and the first trdnsformer is shown as having a value of .0001 mfd. It is difficult to obtain a fixed condenser having a less value than this, but if one can be secured it should be used. Within limits, the smaller this condenser is the better, but of course there comes a point where it is too small to provide a proper reaction effect. The two high-frequency chokes .employed should preferably be of different types, as cases have been known where the use of two identical chokes has resulted in oscillation. The Coil Unit. "THE construction of the coil unit will be the first item to occupy the attention of the builder, A 5}-inch length of ebonite tubing 24 inches in diameter will be required. Starting about an inch and a quarter from one end the tuning winding of 90 turns of No. 22 d.c.c. may be wound on, the ends being brought out to soldering tags or small screws and nuts affixed to the former. The aerial and reaction: coils may then be wound one on each side of the tuning windings. and separated from it by an eighth of an inch or so. The aerial coil comprises: 35 turns and the reaction coil 58 turns of No. 82 d.c.e. . Do not worry about the direction of winding, as if the reaction coil is "back to front it is a simple matter to reverse the connections to it. The i1eaction coil may seem unduly large, but it must not be forgotten that we are depending upon reaction to overcome not only the aerial damping, but also that due to the crystal detector. The low-potential ends of the tuning and aerial windings are, of course, connected to the same tag. Mounting the Components. With the coil wound, it and the remaining components may be mounted more or less in accordance with the constructional plan given in fig. 2. The lay-out is not critical and constructors will find that any reasonable changes from that shown may be made quite safely. The coil is mounted in a vertical position by means of two small anglebrackets. A terminal strip has been

dispensed with, the leads for batterigs and speaker’ having been brougnti straight out from appropriate point; on the wiring. f With these remarks, and ith the theoretical and practical wi ‘ing diagrams to assist him, the constructor should. not have any difficulty in completing the set.

Material for the Trinadyne Panel, 14in, x Jin. Baseboard, 10in. wide. 2 Terminals, mounted on serap of ebonite. 1 Semi-permanent. Crystal Detector, 1 .00025 mfd. Tuning Condenser. 1 .00025 mfd. Reaction Condenser. 1 .0001 mfd. Fixed Condenser. 2 .001 mfd. Fixed Condensers. 1 Coil Unit, as described. 2 High-frequency Chokes, 1. Audio Transformer (ratio 5-1). 1 Audio Transformer (ratio 3-1). 2 Valve-holders. Wire, Screws, ete.

A filament switch may be included to avoid the inconvenience of connecting and disconnecting the "A’’ battery. jf this is done, the switch should be a in the A-+ head. The Valves Required. THD first valve should be of the L.F. type, having an impedance of 6000 to 10,000 ohms; the second may be a

small power valve, or, very near a broadcasting station, a super-power valve. A high plate voltage is desirable, and 90 or 100 volts must be regarded as the absolute minimum. The "©" voltage should be that recommended by the valve makers for the particular plate voltage employed. Do not make the mistake of using too high a "OC" voltage with a view to economising in plate current, as it is important that the first valve shall not rectify, and it would be likely to do this if biased too much, . Operation ‘of the Trinadyne. HW operation of the "Trinadyne" is exactly the same as that of other yeceivers, except that the crystal detector must be adjusted. As a matter of fact, the best method of doing this entails putting the receiver into oscillation for a few moments, ‘ The carrier wave of a rather weak station is tuned in with the detector roughly adjusted and the set just oscillating. Then the set is brought just out of oscillation and-the detector adjusted until signals are at their. strongest and clearest. ‘Once adjusted, a detector of the semi-permanent type should maintain its adjustment for a considerable length of time. "Cathode" feels sure that.those readers who construct this little set will be surprised and delighted with’ their ‘re sults. He would particularly like tu hear from readers regarding resulta, ete., as the set is one with which he has' spent many very delightful hours.. ~ The "Hale" Circuit [Ty Fig. 3 is shown in outline a typical "Hale" circuit. Many reader's will remember the immense popularity which this circuit achieved a year or two ago in Hngland, and it is indeed a delightful subject for experiment, being capable ‘of many antarens ing variations. . It must be confessed, though, that it lacks the absolute reliability of the "Trinadyne," since not every audio transformer will consent to function in the rather involved manner demanded of it. Moreover, the crystal detector must necessarily be of the semipermanent type and, even among this type, some units will be found very much better than others.

How It Works. 4 Bete Hale circuit depends for its : operation on the self and mutual eapacities of the windings of the audio transformer, The high-frequency impulses from the tuned circuit are ap plied via the primary of the audio transformer to the crystal rectifier, ° Considered as an inductance, the winding of the transformer would effectively bar ‘thé passage of high-fre-quency currents, but a large winding like this always possesses a considerable self-capacity, and through this unintentional condenser the high-frequen-cy currents readily make their way.

The audio output of the crystal also flows through the transformer primary, of course, since this forms a part of the crystal circuit. BHnergy is transferred to the secondary of the transformer and the resulting E.M.F.' is applied to the grid of the succeeding valve. The valve operates as an audio amplifier and the signals are audible in phones connected in its plate ‘circuit. So far, then, it seems that the arrangement operates similarly to an ordinary crystal transformer-coupled to an audio amplifying valve. Reaction. JN addition to the self-capacity of the primary, however, the audio transformer possesses a mutual capacity between primary and secondary and. via this second unintentional capacity a part of the high-frequency current is applied directly to the grid of. the valve. The valve amplifies the highfrequency currents and by coupling a reaction coil in the plate circuit to the tuning coil a reaction effect may be secured in a very similar manner to that obtaining in the case of the "Pyinadyne," :

It has already been explained how potent is reaction in reducing the damping effect of a crystal detector so that it will be gathered that, onec properly adjusted, the "Hale" is capable of giving the same remarkable results as the "‘Trinadyne" and others of its kin. Its principal objection is that, as mentioned before, there is sometimes a little trouble in finding a transformer which will get the best out of the circuit. A. splendid field for experiment, though, Further notes concerning the "Hale" will appear in the next article of this series. It is hoped also to make a start with the: problem. of powerising receivers of the erystal-valve type. "One-man" Sets THE "New York. Times" quotes the following remarks made by W. R. Bryan, Professor of Mechanical and Machine Design at New York’ University, anent the pressing need in that. country for "one-man" sets. ; Loudspeakers are driving hundreds into the divorce courts annually," said Professor Bryan. "No two members .of a family want to hear the same radio programme, What is music to one ear is merely a neurosis-producing racket to another. In this apartment age -each person should be equipped with a small portable radio set which could be plugged into a light-socket. "Then without annoying each other, mother, with comfortable sponge-rub-ber earphones, could tune in on the opera, dad on the fights, sister on the dance orchestras, and little brother could do his homework, at last, in peace." . .-