Circuits for the Crystaphile

Radio Record, Volume IV, Issue 23, 19 December 1930, Page 16

Circuits for the Crystaphile

Some New Crystal Hook-Ups

BY

CATHODE

ITH this issue we commence | a short series of three or four articles covering various uses . fi of the humble crystal recti- @| fier. For the most part only 2h circuit diagrams will accom"2a,| pany the notes, but where it ~ ds thought that a particular arrangement is worthy of being made up in permanent form, constructional details will be provided. We propose to commence with the simpler circuits, using a crystal alone, working up gradually through the less involved combinations of crystal and valve, and finishing the series with a mainsoperated crystal-valve combination of considerable range and power. ‘The simple condenser-tuned crystal circuit shown in Fig. 1 will be familiar to every reader. Its capabilities are well known. It gives fine loud signals, and is ridiculously easy to construct. And you can make your coil as rough as you ‘like, because the damping of the crystal is so-heavy that a low-loss coil makes no difference. Very, very nice-so long as we have only one local station. The only trouble is that we are all going to have two or three local stations before very long. When we find 2YA coming in with an obligato by 2ZW, we shall look long and anxiously at our faithful crystal sets. If we are wise and knowledgeable enough-or if we read the ‘Radio Reeord" faithfully each week-we shail know that certain quite minor changes will enable us to overcome that "flat-as-pancake" tuning of our crystal sets and relieve us of the necessity of being so blatantly and indecently greedy as to listen consistently to two programmes at the one time. Direct-Coupled Crystal Set. "THERE are two reasons why a direct-coupled crystal set is un-

selective. One is damping. And the other is damping. On the one hand the aerial system is connected right across the tuned circuit; on the other hand we have the erystal also con-nected-through the ’phones-across the whole of the tuned circuit. .A big improvement can be made by connecting both aerial system and crystal across only part of the tuned circuit. If at first sight it seems that signal strength will be reduced by applying only part of the voltage across the coil to the crystal, remember that the voltage across the coil is likely to be very much greater than was the case previously owing to the reduction in damping. It may even be worth while making a decent coil now it has half a chance do its job. Everyone has his own ideas ubout coil construction, but for ease of winding combined with fair efficiency a, straightforward winding of 70 turns of 22 D.C.C. on a 3-inch former may be recommended for use with a .0003 or .00035 mfd. tuning condenser, This may be tapped at 20, 30, and 40 turns up from the earthed end of che coil for alternative connections for the aerial and crystal, or a single tapping at. 30 turns up will make a good compromise. The connections are shown at Fig. 2. Still Sharper Selectivity ALTHOUGH the arrangement just described will provide adéquate selectivity for most circumstances, we. can go still further if we do not mind

complicating our coil construction a little. A separate untuned coil may. be very tightly coupled: to the tuning coil, and the crystal connected to the untuned coil. The circuit then appears as in Vig. 3. There is only one way to obtain sufficiently tight coupling for the two coils of this circuit, and that is to interwind them.. The small coil for connection to the crystal should be wound with small wire (28 or 30 S.W.G.) and wound turn for turn with the tuning e¢oil, If this task is found unduly. awkward, the tuning

coil may be wound first with the first 40 turns or so from the earthed and slightly spaced; the crystal coil may © then be wound in the spaces. For the 70-turn tuning coil described above, the crystal -winding should consist of about 40 turns. The Aerial Tapping Y now you will have formed the habit of reducing damping so badly that you will be regarding the aerial tapping with a critical and jaundiced eye. Well, you can. loose-couple the ferial now. A third coil of 20 or turns may be added to.the other and the aerial connected: to this instead of to a tapping on the tuning coil, There is no need for such tight © coupling as was employed for the crystal coil, and this aerial. coil may be wound on a 2% inch former inside the tuning coil, or it may be wound over it on paraffined match-sticks or | strips of ebonite spaced round the coil, The winding may be of 28 or 30'S. W.G. and, should be located at the low-poten-tial or earthed. end of the tuning winding. The circuit will now be that shown at ‘Fig. 4, Incidentally, — this method of construction provides about the maximum selectivity that can be obtained from . a erystal receiver Without sacrificing volume, so, following the programme *

, | set forth in the first paragraph, constructional. details are provided in Fig. 5, There is, of course, no objection to using a permanent or semi-per-manent detector instead of the catwhisker type shown, Crystal and Valve. HAVING exhausted the possibilities of the crystal- by itself, we can now undertake the addition of a valve and see what combinations of valve and crystal are worth experimenting with. For the time being, the familiar transformer-coupled amplifier will not be taken note: of, although it will come in for a little attention later on. Most of us: have at some stage in our career tried to use resistance coupling fMlowing a crystal. The outcome is usiially something like Fig. 6. Not having any particular place to connect the other end of: the plate resistance to, we leave it to its own devices. Likewise the grid leak. And. yet the blessed thing works. As a matter of fact, if we remove the plate resistance but leave the coupling condenser and grid leak-using the latter to bias the valve-we have a directcoupled amplifier, and one _ that works very nicely indeed. Nevertheless, we can do better yet, If we eliminate the grid leak altogether, reduce the capacity of the .condenser employed to .0001 mfd.: or less, and change the position of the condenser so that it comes before the crystal instead of after it, quite an appreciable improvement will be found to have resulted. The difference will be particularly noticeable where signals are normally weak. Under these circumstances the direct or transformer coupled amplifier does not show to advantage, the "lift" on a weak signal being nothing like that on a strong one, With this circuit, however, the weak signal recéives just as much amplification as does the strong one, so. that it is particularly useful to. listeners located at some distance from a broadcasting station. It is known, by the way, as the "Sharman" circuit. The most interesting feature of this circuit, however, is that there are present.in the plate circuit of the amplifying: valve impulses at radio frequency, and these are in phase with the impulses. in the tuned circuit. It is therefore possible, by coupling a coil in the plate circuit of the valve to the tuning coil, to obtain a very useful reaction effect. This is particularly valuable in a receiver using a crystal rectifier in. that. it nullifies the serious damping imposed on the tuned cifenit by the detector.. Furthermore, provided it is not carried to excess. the use of reaction does not result in distortion in this type. of ‘circuit, while greatly increasing the range of reception. It will be noticed that no bias is applied to the amplifying valve. This does not, as might be expected, result in grid ‘current distortion, as the grid is -isolated from any source of current... The crystal

is a voltage rectifier as distinct from a current rectifier. This first article has now reached its allotted length. Further notes on the "Sharman" and other circuits and constructional directions will "appear in the next article in this series,