Combining Crystal and Valve

Radio Record, Volume IV, Issue 30, 6 February 1931, Page 16

Combining Crystal and Valve

"Cathode’ describes some Ingenious Circuits

EADERS will remember that the last article in this series closed with a somewhat Scanty ‘introduction +6 the "Sharman" circuit. One. feature of this circuit "which renders it. valuable in some circumstances is that it does not follow strictly the square law which operates in the case of the erystal ‘rectifier more normally’ connected. That is to say, when input is: halved, {the signal'strength: is not reduced to ‘ene quarter as with the normal crystal

detector, but falls more in accord with "the input. Thus the: circuit is par‘ticularly useful when signals are on the: weak side. It is fairly’ well-known ‘that, with very weak. signals, it is not ‘much ‘use adding audio amplification after \a. crystal detector. With the Sharman system. of: connection, : however, audio amplification. is very effeetive even. with weak: signals, so that it, is actually possible, to. increase the range of the receiver PY edding:.¢ ‘audio amplification. An Ingenious Cireuit. Ts is only half the story, however. -The simple Sharman circuit previously published has been redrawn in Fig. 1 in a rather more elaborate form, and if reference is made to this it will +

be seen that there is no obstacle to radio-frequency currents between the output-of the crystal rectifier and the grid of the first audio amplifying valve. Thus, any radio frequency currents present in the output of the crystal rectifier will necessarily be applied to. the grid of the following valve. ' Now this is a very important point. It can be taken for.granted that radio. frequency currents will be quite strong in the output of the crystal, as no rec‘tifier is perfect and there is only a minute by-pass available for such cur-

rents.’ So the first valve. will obvious-. ly amplify the radio-frequency currents aswell ‘as those at audio frequencies. The. amplified r.f, currerits will be in phase ‘with those in the tuning coil and it readily occurs to one that it would be a good idea to feed back these amplified currents by means of a coil variably’ coupled to the tuning coil so as to boost up the incoming signals.. Thus we have a kind of reaction circuit. Reaction is very valuable:in any. receiver aS a means of reducing the effect of damping. And as damping is very pronounced in any circuit employing a crystal rectifier, reaction is correspondingly more valuable in eliminating its effect. Just try this Sharman circuit on long-distance work

and see what you think about it yourself, Winding the Coil. HP® exact arrangement of the re action circuit is largely a matter of individual preference. Anyone wio possesses one of the old three-coil tuners with vafiable coupling betweea the tuning and reaction. coils will be able to try out this circuit very readily, using the coupling between coils as a reaction control. Alternatively, the reaction coil could be wound on a re volving former housed within the tuning coil after the fashion of the old ‘type of Browning-Drake regenaformer. ‘ The circuit shown in fig. 1 is actu ally a modified Schnell circuit, employing condenser control for the reaction, Which has the advantage of being very ‘smooth and easy to handle. A special coil assembly is used and this may conveniently be wound on a former: haying an outside .diameter. of three inches, with a length of about -five inches. The tuning coil, comprising 50 turns of 24 d.ae¢,. will. occupy about two inches in the centre of-the former, . while the reaction and aerial coils may each be of about 80 turns of the same or finer wire wound one on each side of the tuning winding and spaced froia it about one-eighth of an inch. The, tuning winding mentioned (50 turns) is intended for use with a .0005 mfd. tuning condenser, but a .0003 or .00035 condenser may be used if the number of turns is increased to 60. The reaction condenser should be of .00025 mfd. or any larger size.

The small condenser preceding the crystal detector is of great importance in this circuit. What is required is a very small fixed condenser. Unfortunately, the smallest fixed condenser obtainable commercially is of .0001 mfd. and, although this is usable, a smalier condenser is desirable. One way out of the difficulty ‘is to use a midget variable condenser such as is used for neutralising. Another solution is. to make a minute fixed condenser, this being readily done by overlapping two lengths of insulated wire an inch or so and binding or twisting them tightly together. The remainder of the lengths of wire can, of course, be usefully em-

ployed in making the.necessary conneetions. Choosing a Crystal. "THE remainder of the cireuit scarcely needs any explanation. An output’ transformer is shown in the diagram as, in its modest way, this little receiver is essentially a "quality’}, receiver and the output transform¢r, ¥ by keeping’ the plate current ‘out of* the speaker windings, assists in maintaining an‘absence of distortion. However, this component can, quite well be omitted if its inclusion is inconvenient, — The crystal detector can be of any recognised "type, although the permanent and semi-permanent detectors are a considerable advantage in any valvecrystal combination. | The carborundum detector is a very happy choice in this particular circuit, in which it is free from its chief disadvantage, namely, the necessity of providing a 0.7 volt biasing potential to realise proper sensitivity. It is difficult to see exactly why the biasing potential should be unnecessary in this circuit, but there is no doubt about this being the case. The "Trinadyne" THE next circuit to which reference must be made is known as. the "Trinadyne," in which some. readers will recognise a doughty performer of other. days. It is a fact often overlooked that, given the aid of modern and efficient components, these long-known circuits a

are capable of giving quite surprising results. When the Trinadyne first saw the light of day, the valve having a suitable impedance (6000 ohms or thereabouts) were hampered by amplification factors of about 3 or 4. The difference which a modern valve of similar impedance but having an. amplification factor of 15 or 16 will make can readily be imagined. Theoretical Discussion, * ITN the "Trinadyne" the application of . rf. currents to the grid of the ‘ valve, which occurs almost automatically in the Sharman circuit, is deliberately brought about by by-passing the

ystal detector and the succeeding audio transformer with a .0001 mfd. fixed condenser, The circuit is shown in outline in Fig. 2. It will be seen that a reaction effect almost precisely similar to that obtaining in the Sharman circuit is obtained. There is-a considerable difference, however, in the part of the circuit prior to the valye. Instead of the audio output of the éryst«t being applied direct to the grid of the valve, it is fed through an audio transformer by means of which a considerable stepup in signal strength is obtained. The by-passed rf. currents are prevented from taking easy path to earth via the transformer (and incidentally interferine’ with this component’s proper operation) by the interposition of an zx.f. choke between the grid of the valve ii\the secondary of the transformer. mturally, this choke does not present any obstacle to the passage of audio currents from transformer secondary to grid. The. two current paths are easily followed. The small part of the r.f. current which is by-passed via the .0001 mfd. fixed condenser is applied to the. grid of the valve, amplified. and fed back to the tuning coil by means of a reaction coil coupled thereto. The other and major part of the r.f. current is rectified by the crystal. the audio output being applied to the primary of the audio transformer. The output from the secondary of this is applied to the grid of the valve, amplified,,and rendered audible by a speaker or telephones in the plate eircuit of the valve. Most of the notes made in connection with the Sharman circuit apply here also, so that it is unnecessary to © over them again. It may be noted. owever, that because of the low imPedance of the crystal detector, the use of a high-ratio transformer wil! not result in impaired tone. A ratio as high as 7 or 8 to 1 may be employed if such a transformer is available, Comparisons. BADERS will no doubt wish to know how the ‘"‘Trinadyne"’ compares with the Sharman. Perhaps the best advice is to try them both. However, it will be some guide to say that where signal strength is reasonably good, the "pyinadyne" will give

an appreciably greater output-on the local station good speaker strength may be obtained with one valve with this circuit-but that on very weak signals the. Sharman is likely to be more responsive. We have now gone far enotigh to enable readers to try out both of these extremely interesting circuits. In the first part of our next article in this series further information will be given regarding the "Trinadyne."