The Tramper's Two

Radio Record, Volume III, Issue 20, 29 November 1929, Page 28

The Tramper's Two

For Picnic, Beach and Tramping Parties

(By the Technical Editor)

=" ENGLAND and the Continent, two-valve portable receivers are by no, means ‘uncommon, for they find theix way into almost every picnic party in which there is an ardent listener; but in this country the case is different. «Due to our isolation and to the scarcity of powerful stations, our receivers aré necessaril, bigger. One and two-valve sets are uncommon, especially as portables. There is, however. a demand for 4 small and compact portable receiver that will. be economical to assemble, relatively economical to run, and easy to construct. This receiver must be capable of receiving the New Zealand stations on the phones, and, where ‘close enough, to provide loud-speaker reception from the local station. A receiver of this type need not necessarily use the loop antenna; in fact, its use would be most unsatis- © factory, for it would limit consider: ‘ably the use of the receiver. It would . be necessary to design this receiver so that the maximum regeneration could be uséd; and this would involve coupling the grid of the detector tightly to the antenna system; this in itself would not be an objection, for oscillation would be limited to a very small area, but the signal ‘strength would even then be most unsatisfactory. The net result would be — that there would exist a strong temptation to couple an aerial to the-grid and so increase the range of the receiver, but at the same time it would convert it to a very strong oscillator, and this. beside contravening the regulations of the P. and Lf. Department, would render the receiver a menace to neighbouring listeners. Further, most New Zealand picnic parties favour the bush for picnics, and camp so that the provision of an oe

aerial is no great difficulty. There are few places where neither a fence nor tree exist that are frequented by holiday makers. The subject of aerials has been discussed in another section of this issue. With all these points in view the writer has designed the Tramper’s Two, a receiver which accompanies him on all his tramps, week-end excursions and picnics; it is a faithful companion and has never yet failed, but, of course, it is well looked after, and batteries are supplied when wanted. The compactness of this receiver is much in its favour, for it can be easily packed away in the rucsac and is little extra weight, It is built into a small attache case, in which the batteries are, and the aerial wire can be secluded. Ample provision has been made for battery current, but if higher voltages and consequent better quality are desired, further space must be provided. The writer has not dwelt on this point, for the addition of extra batteries means extra weight, and for a light receiver this is not desired. The "A" battery presents a problem. The relatively high consumption of the filament will soon exhaust the supply of a small cell, and the provision of the No. 6 cell is undesirable owing to its size. This has been overcome by the special circuit, which allows for two low filament consumption valves being used in series. The net result of this is that the consumption is that of the greatest valve,

‘but the voltage is that of their sum. Single torch cells arranges in series parallel may now be used as an. "A" battery. Two-volt valves supplied by nine torch cells make an excellent combination which will last several weeks on moderate use. These cells are easy to. carry, and a replace stock is always available. A.small difficulty exists in connecting the cells, but those who prefer can use "COC" batteries, two in parallel. As the design of the set calls for particular care, the write? has specifically stated. certain sizes and these must not -be ex-

ceeded. Some variations can be made. and these have been mentioned. The constructor should not expect too much from the set, for a casual examination will reveal this is not a powerful set, its main attributes being that it is a set that will go and will not take up much room. On the local station it will work a loudspeaker and to meet the need for a portable speaker the writer has described one elswhere, The Carrying Case. A SUITABLD carrying case may be built from light timber, say threeply, or an attache case may be used. This latter is by far to be preferred, and a good pressed composition one is

excellent for the purpose. The case used by the writer was one of the standard i4in. sizes frequently used by school children. The inside dimensions should be at least 134in. long by 3%in. deep by 9in. wide. This allows for the provision of a 60-volt Lissen "B" battery or a 50-volt Hellensen, two grid bias batteries to be used in parallel, or nine small 1jin.-volt torch cells, series parallel, for an "A" battery. If higher anode voltage is required for the last valve, three Hellensen batteries should be used. Under such circumstances the’ case should be a depth of 16in. This will allow two number 6 cells to be used-as "A" battery. ~ It is necessary to partition off "the ease, making a battery container of the lower half and a component container for the other half. Cut a partition of three-ply to fit the case, as shown in the diagram. . If the 1-+in,

carrying case is used the dimensions will be 184in. by 83in. ‘This partition should be drilled before being mounted. The plan of the partition shows these points indicated by small circles. Mount the two valve sockets as shown, placing. the detector socket 8iin. from the end. .These may now be wired with glazite, using 20-gauge double cotton-covered wire between | battery and valve socket. Leave a, fairly long piece of this to connec? A- of the: detector socket to A-.

Making the Coil. (OBTAIN about 3in, of 2in. diameter coil former, and measure on this a distartce of 2tin. Double perforate each extremity thus measured. In the diagram the extremities are shown separated-either: position might . be used. Thread one énd of the 24: double silk-covered wire through one of these holes, and double it hack through the other hole alongside. The wire is thus held tightly. Wind on as many turns as will go into the measured distance, and finish off by threading through the second hole, as was done in the first. This is the grid coil (L2, schimatie diagram). Over the last few turns wind a piece of insulated tapé or other insulating material half an ir:h wide. On the opposite side from the. performations for the grid coil make another pair of holes and secure another piece of 24 D.S.C. wire, Wind now in the same direction as for the grid coil:as many turns as will go on the overlaid pieces of insulating material.. ' When this is finished performate the former by making a slit with a pocketknife between two of the turns of the grid coil. This slit should be above and to the side of the beginning of the primary winding and on the opposite side from the ends of the grid coil (see diagram). This wire is now brought out by means of another slit. Altogether there should be _ about. 20 turns wound on ‘this overlaid wiring

and this is the primary which connects with aerial and earth (1). It remains now to construct the tickler (L138). Procure a piece of former that will rotate within the former of the grid coil. Drill one side to allow the end of a long bolt to pass through. Better than a bolt is a threaded bar such as that used for the tickler coils for the Browning Drake. This may be obtainéd from local dealers, or failing these, Fear and Co., Wellington. A hole is then made in the main former at the opposite end to the aerial coil, and on the same side as the beginning and end of the primary coil. The hole will just clear the winding (see diagram). On the piece of small former wind with 30 s.w.g. wire, either enamelled or double silk, 25 or 30 turns, half on either side of the former. A little seccotine will keep these in place. Seeure the ends through slits in the former, and take out to lugs or slits on the grid coil former, It is better to make the connection from tickler to main former by flexible wire rather than by the thin wire which comprises the tickler. A soldered connection could be made on the

tickler former. The lugs of this coil should be on the same side as the heginning and end of the grid coil. To make a neat job finish off all grid and tickler wires with lugs, and label P and T (tickler), G (upper grid), and F+- (lower grid). A regenaformer of a Browning-Drake may be used here, but the coil should be twa inches diameter. The primary is disregarded and another primary, the aerial coil, overwound as previously described. Mount brackets such as those for a Meccano set on the coil, secure it to the partition. This completes the coil, and it may now be secured to the partition. Its position is indicated in the lay-out sketch. The short threaded bar attached to the tickler should point to the outside of the case. Fitting the Components, FEW more connections remain-be-fore the partition is fitted into the ease. Mount the grid leak and condenser in the position indicated in the diagram; join the lower end of the grid coil (F+-) to the lower end of the aerial coil (E) to A+ on the valve socket; connect the’ top of the grid coil (G) to one side of the grid leak and condenser, and the other side to G of the valve. Join P of the valve to one of the tickler lugs. Connect by passing a wire under the partition and up the side of the eoil Attach another wire to G of the coil to make a

connection with the fixed plates of the condenser when the partition is in position. A long connection is left to. join A+ to the moving plates of the condenser. Returning to the earrying case, drill a hole half-way between the points joining the handle with the case. This should be a distance of 63in. from the end of the case. Afiix a variable condenser. .00035 mfd. capacity. Move the pertition in place, first by measuring the position of the tickler shaft and making a hole where necessary. The low compartment should be 34in. deep. Thrust the shaft through the hole and then screw the partition in place. Manipulate the audio transformer so that it will come somewhat in the position indicated, clear of the condenser and clear of the valve. In the original model this was 7iin. from the coil end. The partition is now in place ready for the wiring, and at this point the article will be suspended until next week, when the final constructional details and operating hints will be given.