And now... "The Differential Two"

Radio Record, Volume IV, Issue 28, 23 January 1931, Page 16

And now... "The Differential

Two"

Short Wave Coils for "Differential One" \ i

By

CATHODE

HE intervention of the holi- { days has delayed the continuation of the description of this popular "Differential One" receiver. However, here are the specifications of the short-wave coils :-

"[HESE windings should be made with 26 or '28 D.S.C. wire. Except in the case of the 60-100-metres F B coil,

Yoom can be found on the valve base itself for all the windings if they are not, separated frdm each other more than 1-8-inch or so, The 80-metre coil, however, will probably have to be wound on a former of the type described for the broadcast coil, unless the constructor cares. to experiment with finer wire. The 8-15-metre coil is intended to have its tuning turns (12) spaced: to the extent of 1-8-inch or so between turns ;. all. the others are close wound.

The description of the broadéast coil : was made in such: detail that ib should be unecessary: to say more‘here than’ that’ the short-wave coils are wound in ‘an exactly similar fashion. Reference . should be made to Fig. 3 in the article appearing on December 26 last, where: all the connections are shown diagr am- : matically, ‘ Use- ‘as Short-wave Adapter. yours. have already reached us: as to the. practicability of using _ the "Differential One" as.a short-wave-adapter, This can readily be arranged. Tt’ is probable that an adapter: based on the design of this receiver will be published shortly, ' and this, of : course, Will, be arranged in a properly’ ‘compact manner. In the ‘meantime, however, a ,few notes on the ‘minor . changes necessary to use the "Differential One" as an adapter will no doubt be appreciated. In the first place, of course, the broadcast coil will not be required... Neither will the filament switch, so that a wire may be run direct between. the A+ terminal and the moving plate of the differential condenser instead of taking the same path via the switch. The next step-is to prepare the plug for inserting into the detector valveholder of the broadcast set. A burntout valve haying the same type of base as the detector valve normally used must have the bulb removed, preferably by using methylated spirits as a. solvent for the cement. Three flex, leads should then be soldered to the pins, the three pins used being the plate pins and the two filament pins. It will usually be found that the pins are hollow, and’ thin leads maybe run. down them and soldered at the tips. After the flex has been properly soldered in position, melted paraffin wax may be run into the valve base to lend a certain degree of mechanical strength to the assembly. The piece of flex which makes connection with the plate pin on the yolve base should ®ow be connected to the terminal marked "Phones "Differential One." The _ other two

lengths of flex are to’ be connected With the termihuls marked "A-" ‘and "A-+." Tt will be necessary fo make sure, however, that the pin.’on the valve-base adapter which connects with ‘the "A--" terminal makés contact with the actual A positive socket on the detector valve-holder of -the broadcast set. It is not always safe to trust to the markings: on the vaive-holder in this connection. However, it is some consolation to-reflect that no harm will be done if this connection is wrong except to the performance of the adapter so that it is even possible to achieve the correct connection by a° process of trial and error if the broadcast set is too inaccessible for proper investiga-. tion. : mo In case this description is not abso-. lutely clear, the necessary connections ‘have been shown in theoretical form in Fig. 1. If, now; the.valve-base adapter is plugged in in place of the detector valve of the broadeast set and the aerial and earth changed over from broadcast set to short-wave adapter, there should be no difficulty in tuning in both morse and short-wave broad‘cast. For the reception of morse, the adapter should be just oscillating, and for the reception of music it should be just on the verge of oscillation. The "Differential Two." Tt has often occurred to the writer ~ that the ideal method of construct: ing a multi-stage receiver ts to make a start with the detector alone, subsequently adding the other stages one by one, By this means one is enabled, if perfection is not attained at once, ‘to search for any fault in a strictly localised area. When it is knowi that a fault lies in one of three or four components or their connections, it is not usually a difficult matter to find it. Where, however, one is faced with the problem of finding a fault in a complete four or five-valve receiver, one’s logic is apt to be falsified by a confusion of causes and effects. This has been one of the couisiderations which prompted the description of the "Differential" receiver in the form which it has taken. Those numerous readers who have constructed the "Differential One" have bad little or no difficulty in securing its perfect operation. , If they had had a fourvalvey to wrestle with from the commencement, it is possible-that they would never have had their-detector stage operating at the summit of efliciency which they have now attained. Now, with the detector operating smoothly both on broadeast and on short-wave bands, it is time to consid-

f er the addition of the first audio anplifying stage. For the conversion of the "Differential One" to the "Differential Two" four additional pieces of apparatus are required. These are as follow:1 Audio-frequency transformer (see text). ' 1 Valve-holder. 1 50,000 ohm.. resistance. 1 2 mfd. condenser. The receiver is worth putting a highauality transformer in. It is recom-

mended that this should be of a ratio not greater than 3.5 to 1, although if it is not intended to add the second audio stage later on, it is permissible to use a transformer of higher ratio, say, 5 toil. Incidentally, in case anyone should be tempted to buy tw transformers of the one type in readi ness for the addition of a second audi stage, it is well to mention that it i inadvisable to use two identical trans; formers in @ two-stage amplifier owing to the danger of peaks being apparen

Parts List for ‘Differential Two." 00035 Fixed Condenser, Differential Condenser, .00025 Fixed Condenser. -0001-mfd. Fixed Condenser. Switch, HLF. Choke. 2 meg. Grid Leak, _2 Valve Holders. "Dozen Terminals. Panel, 18in. x Tin, . Valye Base Coils. Audio Transformer.’ One 50,000 ohms, Decoupling Resistance. ’ One 2 mfd. Condenser.

in the output. The two transformrn should differ either in make or e. The filter circuit comprisii a. 50, 000 ohm. plate. feed resistance™with a 2 mfd. by-pass condenser is intended to prevent back coupling between stages. With this included, there is prattically no risk of encountering that distressing audio howl which has been becoming rather common of late. ‘The precaution is perhaps unnecessary, unless a second audio. stage is to be added, but there is no doubt that even with the single audio stage. tone quality is like-. ly to be improved’ by the inclusion of thesé components, which, after all, are not:at all costly. It is not absolutely essential that the 50,000 ohm resistance’ should be wire-wound, although this type -is- recommended. A metallised cartridge type resistor. can be used if cost is very much of a consideration. Avoid carbon resistors, however, as these almost invariably become very noisy after a little use. The 2 mfd. condenser need not have a very high voltage rating, the ordinary Mansbridge or low-voltage foil condensers being quite snitable, Mounting the Components. AVING arrived at the stage of mounting the components, reference should be had to Fig. 2 to aid in the matter of locating them. The previously-mounted valve-holder and high-frequency choke have been drawn in in Fig. 2 as a guide to the situation of the four new components. It may be necessary to bend one or two of the existing wires in order to clear the new apparatus. The only wire which it is necessary to remove, however, is that which runs between the high-fre-quency choke and the phones (negative) terminal. With the components firmly mounted, all that remains to be done is to do the additional wiring. As has already been mentioned, one of the existing wires must be removed, or, alternatively, it may be shortened so as to make connection between the H.F. choke and the transformer terminal marked "P’" instead of between the chokes and the phones-terminal. Altogether there are eight new wires to be added. These can probably be put on most readily from the diagram, but may subsequently be checked from the following list of connections :- (1) From H.F. choke to terminal marked "P" on transformer. (2) From filament positive terminal on detector valve-holder to corresponding terminal on newly-mounted first audio valve-holder. (3) From terminal marked "G’* on transformer to grid terminal « on new valve-holder.

(4) From filament negative terminal on new vyalve-holder to one side of condenser O1, thence to B negative terminal, (5) Brom other side of condenser ©1 to terminal marked "B" on trans- ’ former, thence to one side of resis-° tance R1. (6) From other ‘side. of resistance Ri to B positive.terminal. . , (7) From. plate terminal on new valve-holder to phones negative terminal. (8) From terminal marked "OC" on transformer to C negative terminal. It should be noted that some transformers carry different indicating letters to those shown, The terminal: for connection. to the sueceeding grid is always marked "G" or "Grid." The letter "P," however, is sometimes replaced by the letter "A," the letter "BY" by "H.T.," and the letter "OC" by Before going on to other matters it may. be pointed out, too, that the. wires shown in dotted: lines are’those already exist--‘ing in the "Differential One" itself. Only the heavy black lines represent the new wiring. ~ Comparison with the wiring diagram of the "One" will assist considerably if the constructor is not quite clear on any detalii. The complete circuit diagram of tie iwovalver is also given in Fig. 3 for tkore who yiefer to wire from a theorctical diagram. Gperating the Two-Valver. pan zeceiver should now be in working order again and very much more powerful. The B voltage should now be increased to sométhing in the xegion of 120 volts, although 90 voits or so will give good resulis if the hgher voltage-is not availabie. Proyision has now been made, too, for the inclusion of 2 "C" or grid-bias battery. At present this need only be of 8 or 43 volts, but a 9-volt battery wili come in handy later. on for the second audio valve. A new valve will have to be purchased unless a suitable one is on hand. This should be of the L.F. or small power variety. having an impedance betwee. about

6000 and 9000 ohms snd an amplification factor between 9 and 45. If the constructor has faithfully followed instructions, he will now be in possession of a very nice little shortwave receiver. In ease of handling ‘it will be well-nigh unique, as the filter circuit in the plate lead to the detector valve is of great assistance in reducing bota bats coupling and. our old enemy, "threshold howl." Ampiification, while already considerable, is not so great as to reader it impossible to listen with headphones, ‘Thus, for

long-distance shortwave work, these ean be donned and the very most made of the rather astonishing qualities of this little set. -- Then, for purposes of entertainment, the broadcast coil may be plugged in and the local station put . on the loudspeaker. To get good speaker volume on the outside. stations,. however, means using the reaction control, and, as this is a thing to be ayoided as much as possible, it is reeommended that, on the broadcast band, the receiver be used principally for the local station in the meantime. In two or three weeks -will follow a. description of the addition of a second audio, stage and a sé¢veen-grid amplifying stage ahead. of the detector. Then the constructor will have an excellent long-distance receiver ‘for tise both on the broadeast band and on short waves.

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