The All-wave "Round the World" Two

Radio Record, Volume III, Issue 33, 28 February 1930, Page 38

The All-wave "Round the World" Two

A Revision of One of Our Most Popular Receivers

Just about twelve months ago "Pentode" described the "Round the World" Two, and he claimed that it was reliable and did not suffer from the usual drawbacks associated with short-wave receivers. Results have shown this to be a very modest claim, for the set has been constructed by hundreds of radio enthusiasts and we have received numerous congratulatory letters. Only twice has there been something serious to hamper the constructor, and of these one did not follow the instructions. So many have called upon our stocks of the original number of the "Radio Record" in which the receiver was described, that our supplies have long since been, exhausted. The numerous demands for a redescription have prompted us to redescribe the set and incorporate a few improvements, including broadcast coils and an extra stage. .

1HEN analysed, most shortwave 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. Dither they have been unstable in operation and difficult to tune, or inefficient in general performance. Referring to the theoretical diagram, it can be seen to consist of a single detector with a single stage of audio amplification, to which another stage can be easily added. The audio-fre-quency 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 6in. 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 front edge of the baseboard, using two brackets if no eabinet 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. Without this positive connection, the chances

are that many unexplained noises will accompany every movement of the 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 sutaible, but if of .1 amp. consumption (which

we recommend) 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 allow of a smaller tuning condenser, making for ease of tuning. Data for the Coils. PEOCURE 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 2 spacing medium, wind on the two together. This string should be at least the diameter of the wire, but preferably slightly larger. One-eighth inch spacing between wires results in an ex-~ tremely 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 us to paint on, along each of the cellultid

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 coils, each having the following nume ber of turns, 5, 6, 7, 9 and 18 turns.

Mounting the Coils. Fror the base a number of valve pins and sockets will be needed. l'our, sockets will fulfil this side of our re-' quirement, but each coil having two contacts will necessitate the use of 10 valve pins. Choose ones having about din. of tapped metal at one end, and if no shoulder is provided, two nuts to each pin will be required.

Components for "Round-the-W orld" Two

1 Variable comueraen: (low loss .0002 m.f.d.). 1 Variable condenser, .00035 m.f.d. 2 Fixed condensers, .0002 with leak, 7 meg. and .0003 m.f.d. condenser. 2 Anti-microphonie valve sockets. 1 15-30 ohm Rheostat. 1 Audio transformer (1-5 ratio).

2 Vernier dials. 1. Midget neutralising condenser. Panel and baseboard. 1 doz. Terminals. 18 gauge enamelled or tinned copper wire. 1 Amperite. 44 Valve pins and sockets. Ebonite strips, ete.

For the coil mounts, cut out of oneeighth inch ebonite, ten pieces, each Btasuring 24in. long by din. wide. With jcentres a quarter of an inch from each end, drill two holes in each piece to take the tapped portion of each pin. The holes will be 2in. apart. Reference to diagram 2 will show how each coil is mounted by clamping two pieces of ebonite, one on either side. of the wire, and tighténing up by the pins at each end. Two small soldering

lugs fixed undernéath each nut provides a means of connecting the coil to the pins. Mount each of the five coils in the samé way and then turn to the base. A‘stiip of 8-16in. ebonite measuring 6}in. by in., has to be drilled with six holes. Tio, half an inch from each end, aré used to fix the base to the wooden baseboard. The two in the centre are fin, apart. with the remaining two, 2in. on either side. In the four inside holes screw the valve sockets, with soldering lugs to provide contact. This com-

pletes the coils and mounting and is thgymost 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 helpfui. 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, so that when the boiling tube is wound it can be ®laced over the cork. Wind with 100 turns of No, 82 D.C.C. 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 éould be fastened to the front panel with no alteration to the existing wiring beyond an extension of the connections to the switch. If antimicrophonic yalve 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 the other four terminals. Wiring Up. N wiring up run all grid and plate wires direct to their destination, disregarding all thoughts of the finished appearance. Right angle bends haye been indicated for the sake of clearness. The small condenser behind the coil 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 fer a yariable 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 tiring up, and by referring to the theoretical and lay-out diagram the beginner will gec an idea of the meaning of these theoretical diagrams, , ‘When completed, connect up batteries and, using the ‘phones, see if oscillation is smooth and under good control. The reaction condenser on the right should have the same readings fox 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 coil. 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 re-

ceirer 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 the same time making final adjustments on the tuning condenser, The coil used for reaction will be found by experiment, depending upon one or two factors, and no set rules can be laid down for any particular coil. Dead Spots. VERY often it will be found that there are certain sections on the tuning 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. « Unsere the neutralising condénser slightly, and the reaction will be found to work normally. Do not expect to tune in to Holland right away, as it takés quite a while to get used to,a short-wave set, and when a station is received the builder will be surprised to see how finely the stations tune on the dials. A many as ten stations ean come in on each degree, and one or two commercially made sets use a small magnifying glass in order to take more accyrate dial readings. Next week the broadcast coils, suitable valves, and the method of adding an extra stage will be fully described.

Look for Next Week’s Special Feature, | ERECTING AN EFFICIENT AERIAL The of a good aerial cannot be too greatly stréssed and this timely article will be a guide to those who are contemplating JK radio installation or who wish to improve their present installation.