Simple, Powerful, Cheap! The "Differential One"

Radio Record, Volume IV, Issue 24, 26 December 1930, Page 16

Simple, Powerful, Cheap!

The "Differential One"

EFORE commencing on the description of this receiver, there are two or three little points which should be brought prominently to the notice of the prospective constructor. The first is that the Single-valve receiver now to be described is intended to form the nucleus of a larger receiver. The additions to the set will be described one at a time in future articles, so that the opportunity is offered the comparatively unskilled constructor of eventually constructing a receiver which would be beyond his capabili-

ties if tackled all at once. It will be seen how easy is the construction of the "differential" one, and. we promise the reader that. the future additions will be equally easily. made. That is not all, however. If the programme is carried‘ Sut in its entirety, the constructor. will be possessed of a first-class four-valve receiver. He can equally well stop when he has brought it to the three-valve stage, or the twovalve stage, or, hé.can’ even rest content with the one-valve set as here described; there is no better one-valver. The only thing to remember is that, it is as well to decide once and for all to just what stage the receiver is ‘to be carried, as for a three-valve version the panel and baseboard can be .considerably shortened, the space left for the addition of a high-frequency stage no longer being necessary, while. ‘it only the one-valver is contemplated a still further shortening is .permissible. We publish a photograph showing the layout of a one-valve set. The receiver is to be "all-wave." In this present article only the. broadcast coil is described,. but the short-wave coils will be described in due course. The difficulty of adjusting the tuning ‘ capacity. to long and short bands has been, surmounted by arranging a plug with alternative sockets, one of which places a fixed capacity of .0001 mfd. in series with the tuning condenser, so reducing the tuning capacity to something suitable for short-wave work. If a word of advice is permissible, we are inclined to recommend that if the receiver is to be used primarily for shortwave reception, the constructor should not trouble to add the high-frequency

stage; if, on the other hand, broadcast reception is the primary consideration then the addition of the fourth valve is well worth while. We do not intend to infer that the four-valye version is unsuitable for short-wave reception-far from it; the only thing is that the extra valve hardly "carries il4 weight" at the higher frequencies. P Special Reaction Employed. HE feature which is principally responsible for lifting this receiver out of the ruck is the special form of reaction control employed. To appreciate fully the advantages of this control it is necessary to delve into ancient history a little. Most of us remember the days of the old "swinging-coil’ reaction control; this was capable of providing good signal strength apart from the difficulty of control, as this type of circuit was very sensitive indeed. Its great disadvantages. were the effect the reaction control had on the tun-inge-a station could be tuned in or

out by this-means-and the exceeding coarseness of the. control. We were then provided with the Reinartz and similar capacity-control-led reaction circuits. Once the initial difficulty of hahd .capacity had been overcome these were a great , improvement on the older circuit, was a certain loss in sensitivity as compared with the older circuit, however, and the tuning effect of the reaction control, though reduced sufficiently for broadcast reception, was still very pronounced in short-wave reception. Both these drawbacks have been very largely eliminated in the present design, It has been found that fhe reduced sensitivity of capacity-¢ trolled réaction as compared with swinging-coil control is principally due to the lack of an adequate by-pass for high-frequency currents in the plate circuit of the detector. The first effort at combating this condition comprised an additional by-pass condenser which had to be of just such a size as to provide sufficient by-passing additional to the reaction condenser, yet

not large enough to rob the reaction coil of too much of its current. This was quite effective as a compromise, but nevertheless left something to be desired-as regards simplicity, while the . reaction. control still had some effect on the tuning. With the introduction of the differential type of condenser, the much better. scheme employed in the "Differential One" became practicable. If re-. ference is made to the circuit diagram in Fig. 1 it would be seen that with the reaction condenser at its minimum set-

ting (that is, with the moving vanes fully interleaved with that set of fixed ‘vanes marked F'1), the effect is merely to provide a by-pass from -plate:to filament. . As the setting of the reaction condenser. ig increased ‘so as to .bring the moving. vanes into engagement. with the other set of fixed vanes marked F2, more and more of the current. passes through the reaction coil instead of being by-passed via F2..- : The reaction effect is thus obtained in more or less normal fashion, but we have secured two notable . improyements. Firstly, there is present at all times .an- adequate by-pass for highfrequency currents in the plate circuit of the detector valve; and, secondly, theycapacity between plate and filament is constant, so that the tuningMeffect of the reaction control is reduced to. quite negligible proportions even when reception is on the higher frequencies, It will also be noticed that, as distinct from the older system of differential reaction control, the moving plates of the reaction condenser are at earth potential, so that hand eapacity effects are entirely absent. The Coils. "THE construction of the receiver is : so simple as to render any very detailed description quite unnecessary. One little point that deserves a word of is the plug and socket arrangement for bringing a fixed condenser in series with the tuning condenser for short-wave work. In the practical wiring diagram (Wig. 2) the little "distributor board" can be seen

between .condenser C4 and the ~ coil socket. It actually consists of a scrap of ebonite.on which two. valve sockets are mounted, and Should be supported above the baseboard so that the valve sockets are clear of the wood; alternatively, the baseboard may ‘have a couple of good-sized holes drilled under the sockets so as to clear them. A valve-pin is used to make contact ‘With one or other of the sockets, and this is attached by means: of a short length of good flexor thin wire to the grid terminal on the five-pin valve

u...uer, Which does duty as a coil socket; thence a wire goes to one end of the grid condenser and leak. The only other point which may require a little thought on the part of the constructor is the making of the plug-in coil. The reader will have guessed from the use of a five-pin valve-holder as a coil socket that the coil is to be wound on the base of. a disused valve of the fivé-prong 227 or 224 types. As the coil former will have to be nearly four inches long to accommod.ite the windings, some extension will have to be arranged to fit over the valve base. ‘ Se After the bulb of the defective valve has been loosened by methylated spirits or brute force and the wires leading from bulb to pins removed, a couple of thicknesses ‘of ‘thin’ celluloid may be wound over’ the base and cemented together arid into position with the usual Celluloid cement. "Cathode" is bound to confess that his own junkbox did not run to celluloid so that a couple of thicknesses of horn fibre well shellaced and baked were made to suffice without any ap- parent ill result. If this alternative is used, however, be sure that the former is well dried, as if wet shellac is allowed to soak into the windings and fill the interstices between turns, up goes distributed capacity and down goes efficiency. Coil Windings. ONLY the windings for the broadcast coil are.being given this week,

The windings comprise three in number. The aerial coil Li occupies the topmost: position on the former, and consists of 35 turns of No. 30 D.S.C. The next winding down in the tuning coil varies’ according to -the capacity of the tuning condenser used. If a condenser having.a maximum capacity of .00085 mfd. is used, this coil should comprise: 120 turns "of No. 30 D.S.C.; if the condenser has a maximum capacity of .0005 mfd., then 104 turns of No. 28 D.S.C. should be wound on. It will be noted that the bottom ends of these two windings are connected to the same pin on the valve base, this being the only pinto which two wires are connected. "The last coil, L8, that at the bottom, is the reaction coil and comprises 40 turns of No. 30 D.S.C. or smaller wire;*the preciy gauge is not important in this instane The arrangement of the windings i. shown more clearly in Fig. 8, which iso shows the connections to the valve ase, Other Considerations. "THERE are just .one or two other little details which must be mentioned in connection with the construction. It will be noticed that in the practical wiring diagram C+ is shown connected to A- and B-.. This. may seem a very useless proceeding seeing that no © battery will be used for the present, but the connection will have to. be made eventually, so may as well be made now. Similarly the whole twelve terminals are shown mounted, although some of them will be unused until the receiver grows up. The fixed’ condenser sometimes inserted in series with the reaction condenser has been omitted, as most modern condensers are too well aligned for there to be any possibility of the plates touching. If the constructor feels doubtful about. the point, however, there is no reason why a fixed condenser should not. be inserted in the lead between plate terminal and reaction coil: this should be of .001 mfd., or larger. So soon as the wiring has been completed, following the wiring diagram

given in Fig. 2, the receiver may be. tested and adjusted. . As this process will consist of putting the receiver in and out of oscillation, adjusting the B voltage until the control is as smooth as‘it can be had, be careful that the receiver is not tuned in to any broadcast station or interference may be caused ‘to other listeners. ° Once the (Concluded on page 80.)

The "Differential" One r (Continued. from page 17.) control is nicely adjusted, the local station may be tuned in, being careful to keep the reaction control well away from the oscillation point. The valve used should preferably be of the H.F. or R.C. type: (A.C. resistance of 20,000 to 30,000 ohms), and may be of any filament voltage. Nowadays two-volters are practically as efficient as four and six-volters, so they need not be avoided for fear of impairing the performance of the set. The B voltage to be applied to the plate of the detector valve is best ascertained by experiment as it varies somew with the valve employed. Some val will be found to work really well with no more than 20 volts or so, while others are not at their best until 70 or 80 volts are applied. Generally speaking, though, a voltage of 40 to & will be found to give the best results. | The next article in this series will describe the addition of an audio stage to the receiver. In addition, the construction of short-wave coils and of those for the broadcast band only will be fully described. Complete One-Valve Set. HOSE who have no intention of increasing the number of valves and who want broadcast reception only should make certain adaptations in the set. Two photographs in this article illustrate the neat little receiver that, can be made up. The base board measures 8in. x 64in. with a Gin. panel. The placement of the components can be seen very clearly from the photographs. In explanation one point may be stressed, and that is that the battery cable wires do not run to terminals, but go directly to the proper points in the set: thus A- is connected with the switch, A-+- and Bdirectly with the valve holder, and B+ is taken by a cable to speaker +- terminal, As will be seen from the photographs, there are only four terminals on the back, one each for the aerial and earth, on the right-hand side, and two for the speaker on the left. One of these, of course, is connected with B+, as was mentioned above. The other connects directly with the top of the choke. It will be noted that the choke is mounted horizontally. This is a splendid idea to keep the leads short. Yt will be found quite simple if.a bracket is used. The front panel needs possibly a little explanation. It will be noticed that there are three controls. In the centre is a tuning dial, a .00035 condenser, while to the right is a differential reaction condenser. Now, on the left is a rheostat, but it will be noticed also that there is an amperite used in the set. The rheostat has beeu placed in this position merely that those who have a rheostat on hand or wish to use one in preference to the amperite may have some idea of the, layout. Of course, if a rheostat is, used, the amperite is not used; that is to say, A- connects to one side of the rheostat, the other side of which connetts with the switch. It will be remembered that the other terminal on the switch is connected directly to cable t0 A-.