The "Round-the-World" Two

Radio Record, Volume II, Issue 41, 26 April 1929, Page 29

The "Round-the-World" Two

A Receiver that will Justify its Name

By

PENTODE

ONFLICTING opinions seem to prevail as to the value of short wave for the transmission of -entertainment. On one hand there is the man who has never heard good reception, knows nothing about short-wave work, and who will naturally condenmn it, saying it is merely in its experimental stage. s versatile. opponent, even though he also knows nothing of it, is quite open to be convinced on the merits of this type of broadcasting, and, being interested, will perhaps find the description of an efficient short-wave helpful for the time when he decideds to build one. One has only to glance at a list of the various short-wave stations operating all over the world to realise that much is "on the air" that is being missed unless the necessary apparatus is available on which to receive it. Bvery afternoon two or three stations are transmitting from America, later in the evening Russia is to be heard, not to mention the delightful special programmes for Australia and New Zealand transmitted by PCJ on Saturday afternoons. There is no country on the map that owners of short-wave sets are not in touch pith, and one is kept posted with a £ ost of news not available in the Press. The Short-Wave Receiver. ‘WHEN analysed, most short-wave sets will be found to consist of a detector-valve in one of its most simple forms, followed by one or two stages of transformer coupling. Simplicity is the keynote of efficiency, and although stunt circuits have been published, they have not held their own against the very simplest. Hither they have been unstable in operation and difficult to tune, or inefficient in general performance. The one about to be described is one that has not only been built by the writer, but also by numerous amateurs, and in all eases the results have indicated that it is reliable and does not suffer from the numerous drawbacks

usually .associated with these receivers, Referring to the theoretical diagram, it can be seen to consist of a single detector with a single stage of audio amplification. The audio-frequency path from the detector-valve will be seen to be taken from the opposite end of the reaction coil to the usual method, as this was found to give smoother reaction control with less body capacity. The dotted line showing the earth connection was found to be unnecessary in a number of cases,

sufficient capacity to earth being produced by the various battery cables, ete. Mounting the Components. AN ebonite panel, 12in. long by Gin. high, will give ample room for well spacing, and the baseboard measuring at least 9in. from back to front will allow the coils to be placed at a. distance from the metal plates of the condenser. It is small points like this which make all the difference, and an overcrowded receiver suffers from the disadvantage that it is extremely sharp in tuning, often making a station impossible to retain when the hand is taken from the controls.

Screw the panel to the tront eage or the baseboard, using two brackets if no cabinet is to be used. ‘The two condensers are mounted on the front, the one of smallest capacity being on the left. Choose condensers having low loss features, and if no pigtail connection is provided between the moving plates and the terminal, solder on a small length of flexible stranded wire. With- | out this positive connection, the chances are that many unexplained noises will ; accompany every movement of th ' tuning controls. ' In the centre is mounted the detector rheostat, the resistance of which is chosen to suit the particular valve used. If of .25 ampere consumption, a rheostat of 15 ohms will be suitable, but if of .06 or .1 amp. consumption, then a 30 ohm rheostat will give better control. On the right hand .bottom corner is mounted the speaker or phone jack. As the coils play such an important part, their construction will be given in detail. To obtain a complete wavelength range of from 10 to 70 metres, a set of coils will have to be made, consisting of 4, 5, 6, 8 and 12 turns, This may. seem a large number of separate coils, + but they also cover the reaction coils,

and allows of a smaller tuning condenser, making for ease of tuning. Data for the Coils. PROCURE a length of cardboard tubing of 3in. outside diameter, and cut into three longitudinal sections. The tube will need to be about 9 or 10 inches long in order to possess a good grip, while winding the coil. Now fasten the three sections together with adhesive tape or gummed paper, and wrap round two or three turns of brown paper. This makes an ideal

former on which to wind any selfsupporting coil. The wire used is 18 S.W.G. tinned or enamelled copper, and a length of about 32 feet will be necessary. Perhaps the easiest way of making a neat job is to fasten one end to a tree or post, and, taking the free end in the pliers, stretch the wire until it is perfectly straight. Fasten the end of the wire to one end of the former, and using thick string as a spacing medium, wind on the two to- .

gether. This string should beat least the diameter of the wire, but preferably slightly larger. One-eighth inch spacing between wires results in an extremely efficient inductance. It should have been mentioned that before winding on this wire lay. three strips of celluloid along the former, equidistant, winding the wire round the whole. The reason for this will be apparent presently. A full forty turns in one length is required, which is cut up presently. After making the remaining end tight in a similar way to the first, end, proceed to unwind the spacing string. Fairly thick celluloid cement is used to paint on, along each of the celluloid strips, thus holding the wire firmly, and keeping a constant spacing between turns. When thoroughly dry dismantle the cardboard former, taking care not to damage the coil. We now have a self-supporting, low loss coil, which will have to be divided into smaller calls, each having the following number of turns, 5, 6, 7, 9, and 13 turns. Mounting the Coils. OR the bases a number of valve plus and sockets will be needed. Four sockets will fulfil this side of our requirements, but each coil having two contacts will necessitate the use of 10. valve pins. Choose ones having about }in. of tapped metal at one end, and if no shoulder is provided, two nuts to each pin will be required. For the coil mounts, cut out of oneeighth inch ebonite, ten pieces, each measuring 23in. long by jin. wide. With centres a quarter of an inch from each end, drill two holes in each piece to ian

— Samana ake the tapped portion of each pin. Lhe holes will be 2in. apart. Reference to the diagram will show how each coil is mounted by clamping two pieces of ebonite, one on either side of the wire, and tightening up by the Wi

pins at each end. Two small soldering lugs fixed underneath each nut provides a means of -connecting the coil to the D pins. Mount each of the five coils in

------ the same way and then turn to the base. A: strip of 3-16in. ebonite measuring 67in. by. jin., has to be drilled with six holes. Two, half an inch from each ond, are used to fix the base to the wooden PITT TTTTTTT UU TTT TT ITE TOADITATATATAQTUU TEE TTADHVATGLAV HT VUE VE TELAT ATUL EG RE VTA APEG CET AT GT TD IT

baseboard. Te two in the centre are fin. apart with the remaining two, 2in. o* either side. In the four inside holes ’ serew the valve sockets, with soldering

lugs to provide contact. This completes the coils and mounting and is the most difficult part finished. The Lay-Out. (THE lay-out diagram gives the general arrangement and the relative positions of the various components.

One-word on the RF choke. This has to be one designed for short-wave work and if the constructor wishes to make one himself the following will be helpful. A glass test tube, known to chemists as a "boiling tube," can be used.

A tight fitting cork can be inverted and screwed to the baseboard, sothat when,.the boiling tube is wound it can be placed. over the cork. Wind with 100 turns: of No. 32 DCC wire and space with cotton or fine string. After taking off the spacing give a coat of thin celluloid solution. Fasten each end of the wire with cotton until the solution has dried out. It will be seen that a switch has been mounted at the rear of the baseboard. If a cabinet is to be used, this switch could be fastened to the front panel with no alteration to the existing wiring beyond an extension of the connections to the switch.. If antimicrophonic valve sockets are not used on both valves, the detector is the most sensitive and one on this position is advised. The six terminals are mounted on two strips. one containing two and th other four terminals. e 4

Wiring Up. ps wiring up run all grid and plate wires direct to their destination, disregarding all thoughts of the finished appearance. Right angle bends have been indicated for the sake of clearness. The small condenser behind the soil is a series aerial condenser, and can be in the form of one of the many neutralising condensers on the market. The length of flexible wire to one side of this condenser is used for a variable aerial tapping on the-grid coil, and a small projecting tag will have to be soldered to the centre turn of each coil. This is indicated in the diagram. No difficulties should arise in wiring up, and by referring to the theoretical and lay-out diagram the beginner will get an idea of the meaning of these theoretical diagrams. »

Components for "Round-the-W orld" Two

1 Variable condenser (low loss .0002 m.f.d.). 1 Variable condenser, ..01035 m.f.d. 2 Fixed ‘condensers, .0002 with leak, 7 meg. and ,0003 m.f.d. 2 Anti-miecrophonie valve sockets. 1 15-30 ohm Rheostat. 1 Audio transformer (1-5 ratio). 2 Verner dials. 1 Midget neutralising condenser, Panel and baseboard. 1 doz. Terminals. 18 guage enamelled or tinned copper wire, 1 Amperite. 44 Valve pins and sockets. Ebonite strips, ete. PTT TRIER TTT TTT TIT TT TTT TT TT TTTT ITT TT AT TT TTT TT TAT TTIT TTT TT TTT TT TTT TAT TTOT TTT TATA ATCT TTTAT ATEN TTTTUTITUTVIATATITATIOVDTIATAPTTITATIDHR UT EATATATAT AREA ATATEGTO TTT AT TORT T NIG

When completed, connect up batterfies, and using the phones, see if oscillation is smooth and under good control. The reaction condenser on the riht should have the same. readings for the position when the set goes into oscillation and for when it comes out. Any lag usually indicates a wrong grid leak or an inefficient choke soil. Have a number of leaks on hand ranging from 2 to 7 meghoms, and try each until the best value is found. Connect up the antenna and with receiver on the threshold of oscillation, turn the left hand tuning condenser slowly round until a whistle is heard. This indicates a station and immediately slacken off reaction, at theysame time making final adjustments on the tuning condenser, The coil used for reattion will be found by experiment, depernling upon one or two factors, and no set rules cun be laid down for any particular coil. Dead Spots. KRY often it will be found that there are certain sections on the tuhing range that it is impossible to get the receiver to oscillate. These are called "dead spots," and can usually be overcome by adjustments upon the aerial coupling. Unscrew the neutralising condenser slightly, and the reaction will be found to work normally. Do not expect to tune in to Holland right away, as. it takes quite a while to get used to a short-wave set, and when a station is recieved the builder will be surprised. to see how finely the stations tune on the dials. As many as ten stutions can. come in on each degree, and ene or two commercially made sets use , small magnifying glass in order to take more accurate dial readings.