The "Tramper's Two"

Radio Record, Volume III, Issue 21, 6 December 1929, Page 31

The "Tramper's Two"

| Final Instalment

RILL two holes 3tin. from the top edge of the case and underneath the shaft. This will accommodate the aerial and the earth terminals, It will be found easier to attach ‘the terminals and then bring out the wires from the set rather than attempting to solder them from the inside. The remaining connections shown in the. layout diagram should now. be made, by joining. the long wire attached. to the grid coil to the fixed plates of the variable condenser and the wire attached to A+ to the moving vanes. Top of the tickler is connected with P of the audio transformer, which is connected with one side of a .0005 fixed condenser by a piece of glazite wire. The other side of this condenser is connected with B+ of the transformer, to which connect a fairly long piece of double cotton-covered wire and passed through the hole for this purpose; this is B+ detector. G of the audio transformer joins with G of the audio valve socket. GB is to be attached to the grid bias battery, the position of which will be described later. P of the valve socket being one of the output terminals passes to a phone jack. To mount this drill a hole about 3in. on the side of the case opposite the tickler and mount a filament-break jack. This is not shown in the theoretical diagram, but it is essential. to disconnect the batteries when the set is not in use. ‘The second jack limb connects with . B+. From the A+ of the valve socket take a wire to one of tlie filament contacts on the jack. The ‘other filament passes through a hole in the partition to A+ The Batteries. HERE is little to choose from among these, for in order to fit into the small space they must comply with the measurements given. .The "B" block, the Lissen, has been found very satisfaetory by the writer. It has the advantage of having very many tappings, so that the voltage for the detector may be accurately obtained. The most efficient "A" battery is built pp from torch cells. Nine of these are wired in seties parallel in the following manner :-Divide them up into groups of three and solder a strip of brass, tin, or other convenient ¢con- ductor round- each group to bind them tightly. Along the brass caps solder another strip. Place a piece of insulating material, eg. . Waxed cardboard, between each’ group of three, and fasten all by some non-insulating sub‘stance, string or cord, into a compact group of nine.. ‘There will now be six projecting strips, the ends of three negatives and’ ; the: three positives. Leave the ‘first: "negative strip free. Connect the fitst positive strip to the second negative strip-the second positive strip to the third negative strip and the third positive strip is free. This is A-+- and the other free terminal A-. This compact block should fit in to. the space.at the side of tlie "B" battery, which-is placed in the case with the wandti plugs outwards.

AVING fitted the two battery blocks in position, make- the connections indicated in the preceding text, that is, A-+- eonnects with the filament break jack, and A- with a filament termina! on the detector socket. ‘This must connect with the grid coil in order that & positive bias is applied to the detector. From the audio transformer a wire passes through the partition to B+ 40 or thereabouts. This value will have to be- altered, but the minimum should be chosen at. which the set will operate. B- connects to A+ and the maximum "B" voltage to the body of the output jack, The "©" battery comprises an ordinary 4} bias battery laid over the "A" coils, The + terminal connects with A-, the negative with "G" of the transformer. Everything has now been completed, and it remains to try out the receiver. First use a good aerial and earth as for home ‘conditions, and find out the capabilities of the set. Then try an improvised aerial and earth. Become thoroughly acquainted with the operation of the receiver before taking it abroad. It is advisable to build up a spare "A" battery in the method described so that it may be replaced when the other one fails. Those who do not Wish to go to this trouble should ob: tain two 43 volt "C" batteries and parallel them by joining the two 43-+ and the two ‘negatives. This is A+ and A- 23% Choosing Components. [z is necessary in selecting the components to do so with, the utinost care. EXverything is designed so that there will be a minimum of waste space and if anything of the components are too large the whole design will be thrown out. Particular care must be taken with the valve sockets. Use the small unsprung UX type, making certain that these are not more than fin, in height. If this is exceeded great difficulty will be encountered in

fitting the valves in their allotted space. ort ' The transformer and the variable condenser can be of a standard size though smaller ones will make the set both lighter and cheaper. The coil must not be greater than 2" in diameter, Use low filament consumption valves so that the greatest consumption will only be 2 amp. The valves used by the writer are PM1LF and P.M.2. This is an excellent combination where loudspeaker reproduction is required. For ‘phone work use either two PMILF’s or PM2DX and PMILF. PM2DX is a_ special detector requiring .2 of an amp. It. was found, however,. that the extra consumption was compensated by the better results obtained. Suitable combinations may be obtained in Osram valves, using two DEL 210’s. or DEH 210 as detector, and DED 210 as audio, In Radiotron use UX 199 fii both positions, but notice that the filament voltage is three requiring 6 volts "A" battery (two three-volt valves in series). Another good combination may be obtained from 660 valves, using 210 H.F. as detector and 210 LF as audio.