The "Night Hawk

Radio Record, Volume IV, Issue 29, 30 January 1931, Page 16

The "Night

Hawk

-_--_- This is what we have called the complete ‘two-valve

differential receiver

By the

TECHNICAL EDITOR

BLL, here we are with the Differential Receiver on one side, typewriter in front of us, and a pile of papers and diagrams on the other side. We hope to tell you in as short a space as possible how eats to make this wonderful little receiver, We are really very modest about it when we say it is a "wonderful little set," as we consider it by far the best. two-valve set we have ever described. We have looked over quite a number of circuits and "picked the eyes out of them" so to speak, added a few ideas of our own, until we have a circuit which we feel fills a long-felt need. -Jn the first place it does not cost very much to build. For about £6 or so the constructor can be fitted up with everything except batteries and speaker, It is economical, requiring only 90 volts "B" battery and 4 volts "A" You could. even use two volt batteries if you feel inclined.

It is compact, for the size of the whole set, as we have had it photographed, is not. more than 8in.. high by 103in. long, by 84in. wide, and that is including .a very. heavily built. cabinet, wets ee If you do not wish to build the: cabinet or wish to incorporate some ‘ideas of your own, the actual parts can be used upon a base board 8in. x 6in., and there is no crowding either. All the ‘parts fit nicely into position and youty wate ee points of ‘battery supply in the set and. four terminals connect with the aerial and earth and the ’phones or speaker: A little while ago "Cathode" went to some length to explain the theoretical advantages. of this type of. reaction. We will not reiterate them. here, but those who, use. differential, after having used either swinging coil or, or-

dinary . condenser .for .some time will be amazed and delighted at the working of this new type of condenser. You must be careful, of course, not to make = the set oscillate. When you hear it . whistling immediately. slacken reac: tion, as this whistling causes annoy- . ance ‘to your neighbours. The Cabinet. THE photograph, of the complete set will give a good idea of. the cabinet we have used in our labora: _ tory model, The top and bottom made of inch timber, 8#in. x 104im.,. with a bevel cut along one edge. The. uprights are 6in. long, and just under dn inch. square, The second photograph, that is the one looking down on, the, set, shows the novel idea we — have used for the sides and panel. It will be seen that two grooves are cut into each, and into these slide the sides of three-ply. By this arrange ment the wiring can, be made accessibie without: any trouble. It is ever so ‘muth' ‘better to build a set like this than ‘to build it into a box. At the pack there is a strip of ebonite under. . an inch wide for the four terminals. The position of these can be quite easily seen in the photograph, and it will be noticed that there is a hole cut in the centre through which the four wire cable runs to the set. If you do not want to make a. cabinet, you will need a piece of iin. ‘timber Sin, x. 6in., and on to this ‘the panel must be fastened by two screws, otherwise the lay-out is the ~ same, * .." , > The: Lay-out of the Panel.: O much for the cabinet. Let us get ‘to work on the actual construction of the set. First of all we need to get a kit:of: parts, which some firms are selling :complete. ‘We have given a list with all the separate parts enumerated The first task is to drill the panel. and mount the components thereon. Our lay-out: sketch shows very clearly where everything is to go, The tuning condenser, as will be seen is slightly at an angle,‘and is 2in. from the top, and in the’dead centre. The differential condenser and the rheostat are each" 4in, from the top, and isin. from the side. The switch is placed immediate- ‘ly’ under the ‘tuning condenser, and about lin. from’ the bottom. ‘When all’ the’ parts are in position © ‘mount the panél to the baseboard andlay out the rest of the components. The position of each will be seen very clearly’ from the photographs and from ‘the ‘wiring diagram, It is well before strewing anything into position. ' tolay out all your ‘parts,.as the . size may’ slightly differ from those used in our set,-and some slight adjustment may be needed.. Notice that we have kept our grid and plate wires particularly short, and if you wish to make any alteration in the lay-out you Inust do the: same, —

‘ Further, it will be noticed that the old method of running’ stiff wiyes to each point has been done away with. As a matter of fact we use ordinary 24 gauge wire and cover it with spaghetti. Glazite, of course, would be excellent for the »urpose and would need no extra covering. In the wiring diagram stiff angles have been retained, but this is only to show where each wire goes to. If they were jumbled up as they are in the photographs, those who are constructing a set for the first time would have great difficulty in following them. Notice particularly the way the valve sockets are turned round.

indiedie their positions very clearly, but miake certain they are right. before screwing them down, |...» She Tuning Coil. HERE are very many ways of mak‘ing the coil, but some prefer to do it-on celluloid, as our.own was sound, However, . it is much more simple to wind the coil on a cardboard former, and we notice that-a few firms are turning out the required length of former with the valve base already in position, This will save the con-. struetor a good deal of trouble.: The thet p and the sketch will

Wiring. RE ig. Titile. ‘we can say about the wiring other than take every lead: by ttie ‘shortest’ ‘way, ‘and particularly © keep © thé grid ‘and plate ‘wires short. We. have found. it. not:.a bad plan: to take many of ‘the wires underneath the valve bases, as can be seen from the photograph. One point -where our, sketch.and ‘our photographs ‘differ. might be mentioned. We have omitted in our own model,. the condenser, . which can be. switched: in series with the tuning condenser:to re-’ duce the lower values for shortwave work, , Its position is quite clearly shown in: the diagramatic sketch. ‘The wire from the grid terminal of the coilholder is taken to one terminal’ on this condenser, ‘but the other terminal continues on to the fixed plates. A shorting clip must.be.arranged for so that when broadcast-is required this, clip short-circuits the two terminals and cuts out the condenser, It is a good plan to fasten one’ end of the condenser to the fixed plates of the: ‘moving condenser. : The battery. voltages depend very much upon the requirements of the constructor.. If he wants good results he will use 90 volts on B+2 and.45 or 22} volts on B+1, As for an "A" battery, four-volt' valves will be quite sufficient and the’ two-volters are quite good. -The value of the -"C" battery will depend upon the valve:used in the last stage and the voltage applied. If you are not clear on how much bias to use, try nine ‘volts and reduce it: until good, clear reception is obtained. ~ Otherwise’ ask your dealer when you buy the valve. There are quite a number of different valves that.¢an be used for this purpose, and we shall not enumerate them here. We tecommend a spécial detector such as PM4D, A615 or.UX 112A. For the.second stage we recommend &- high-power gain valve, such as B409 or P.M.4.- : The value of, the grid-leak is best found by experiment, remembering that the higher the value the more sensitive the set becomes, the more difficult is oscillation and the harsher is the tone. We found 5 megohms quite a good value for this ‘set, although it could quite well be reduced to two, * Outside the set B- must be connected to either A- or A+, certainly not to both. If reaction is ploppy try another gridleak and reduce the detector voltage to 224. If now the set does not oscillate easily increase the number of turns on the reaction .coil. or use a larger reaction condenser.. -Our . regeneration coil was’ designed'for .00013 condenser, "but .0002 can be employed by using about 10 less turns on the broadcast band and a proportion‘ately smaller number on the 8,W, coils,

The Set Performs Well. WEN we tried out the set it was in the middle of ‘Wellington and early in the evening. ' Unfortunately we lave’ had to hurry the completion of this. set and so have not had adequate time to go DX-ing on it. We leave that for our constructors and can promise them that there are many thrills coming to them when they begin to explore the ether. Our test took

place one evening | at about 7 o *clock, and when we turned it on the noise of 2YA roaring in our ears was almost deafening, so we put it on the speaker. It came through beautifully clear and the tone was all that could be wished for from a set so small. We were glad that it has been. designed.to use a moderate power valve in the last stage. Then we introduced a , wavetra and Wellington disappeared like magic and "we could hear how the set oscillated. It went beautifully in and out of reaction without a sign of ploppiness or of that rasping reaction that characterised the sets of a year or so ago. 1, 3 and 4YA all came in on the phones and we could just:get the whistle of what was probably 2BL, but it was far too early for sueh a station and so attempting to tune it in was not worth ’ while.

The coil is not difficult to make. A piece of former that will fit tightly over . the valve base is all that is required ‘and it should be just’ over 43 inches long. The tubing is then fitted over the coil base. Drill a small hole right through the former into the base so that later a brass brad. may be forced in. Just clear of this hole and’ immediately above the cathode prong, ‘drill another right through into the cavity of the base. From this measure up 7-8in, and above the remaining, adjacent filament pin drill another hole.: Allow a 1-8in. and measure 24in. . Now drill two more holes, one at the lower of these two marks immediately above the plate pin, and thereafter through the upper mark immediately over the grid pin. . Now, allow ,another 1-8in. and drill a further hole immediately over the plate pin-that is the second over -that pin.. The final hole is drilled at the top mark immediately above the free filament _pin. This explanation of the positioning of' the holes can ‘be made more clear by an examination of the diagram ° of the ‘coil reproduced on this. page. © The idea is to bring out each wire . right over :the pin ‘to which itis to go. When the holes are drilled. take off the base and arrange the former on

Fo Parts List for "Night Hawk’? Two. Te .00035 Variable Condenser. Differential Condenser, .00013 or .002 mfds. , 00025 Fixed Condenser. .0001 mfd. Fixed Condenser. Switch. 2 U.X. Valve Holders. i U.Y, Valve Holder, Four Terminals. Panel, Sin, x 6in. Valve Base Coils. Audio Transformer. 1.30-ohm Rheostat. . Five feet of four-wire cable. Two Vaives. .

The ‘’ Night Hawk"

Continued from page 17

whatever winding jig happens to be available. Thread the 28-gauge dsc. wire through the first hole and then wind the entire length of the marked former. Go right over the holes and the separating spaces and finish off through the top hole. Remove from the winder, and after securing the ends, cut the wire between the two lowest coils, L2 and L3, and holding the finger over L383 end wind off enough of L2 until the hole comes into full view. Then pass the wire through. Do the

same for the top of L8, and then repeat the whole performance for L2 and Li. Melt the solder from the pins of the valve base, and having cleared away the insulation from the portions of the coil ends that will touch the pins, pass the wires through and push the valve base into position. When all the wires are drawn tight solder them to the ends and round off with a file. The brass brad can then be driven home. The particulars of the short-wave coils were given last week.