The All-wave "Round-the-World" Three

Radio Record, Volume III, Issue 34, 7 March 1930, Page 29

The All-wave "Round-the-World" Three

Broadcast Coils and Adding an Extra Stage

HY broadcast coils must be constructed in the same general manner as was the short-wave coils, but with this important . difference: that there will be one instead of two coils for each band, and the wire will be left on the former instead of removing it. It could be removed, of course, and célluloid strips used to support the wire, but as this will be necessarily y¥ very fine, it is not practicable to do so. I Two coils will be necessary; one to cover the band from 160 te 300 metres,

and the other from 280 metres to 500. This is necessary because of the small capacity of the tuning condenser. Secure two pieces of ebonite or cardboard ormer, each a little over four inches long. Upon the one that is to be used for the higher wave-band, mark out 3 inches, leaving the most spare at one end, and wind full of 18 DCO wire unspaced, or 20 gauge enamelled wire spaced by half its own width. Secure the ends temporarily, and then at one end where there is the most space, wind 20 turns of 24 gauge DCO wire for the tickler. Separate the two coils by half an inch. Obtain four strips of ebonite each 43 inches long and 1-8-inch wide, and upon two of them mount four valve prongs so that they will coincide with the ~ grips on the valve sockets on the exjsting short-wave base. ; The valve prongs will be secured by a nut or nuts as was described last week, but they will be securéd to a single strip and not passed through two. Near each end of these four

strips drill a small hole which will latterly take a nyt.and bolt. If solder lugs are handy, attach them to the valve pins as was done for the shortwave bands. Bring the wires that were temporarily secured to the former on to the lug, securing the secondary top and bottom to the lugs which connect with the grid and A+ respectively. The tickler coil will be connected to the socket grip which ‘eonnects with the .0005 condenser, and the plate of the detector valve. Now take one of the free strips and put it inside the coil]

that is now attached to the strip with the prongs. It will be necessary now to bolt these two strips together, but as they must not rest upon one another tightly, slip a spare battery terminal on the screw to separate them. Bolt the upper and lower strips together, and it will be found that a neat and durable plng-in coil has beén made. Prepare the next coil for the higher wave-band in the same manner, This time use 24 DCC wire wound on for three inches, leave a space of 4-inch and wind on 25 turns of 26 gauge DCC wire. For the secondary, 24 gauge enamel wire may be employed if spaced by half its own width. This coil is now prepared as the smaller one was. The serial must be brought into taps on the secondary. Clean the wire at the grid end, the centre, and about the 18th from the bottom. Try different tappings, until the best is found. It will be found that the broadcast band can now be covéred fairly efficiently, although the same

degree of efficiency cannot be expected as with a 2-valve set specially built for the short-waves. A constructor may expect to tune in his local station at loudspeaker strength and the other Australian stations on the ‘phones. If he wants better reception he must add another stage. Adding Another Stage. ¥ adding another stage greatly improved results can be expected. However, for short-wave work the extra stage introduces further noises, but is worth while if speaker work is contemplated. The audio valve in the original Round-the-World Two is unbiased, for this is unnecessary in *phone work, but when a third stage

is added, the constructor looks forward to speaker reproduction from several stations, and he wants a certain amount of quality. This can be provided by 4 guitably biased final stage with a well-selected valve. ° The extra requirements consist of a transformer 33/1 ratio, a valve socket, a valve, another terminal, a smaller grid leak (2 megs.), and the writer suggests another jack to incorporate automatic filament breaking when the plug is removed. This does away with the necessity of an extra switch and facilitates operation. It will be necessary to do a certain amount of rearrauging. In the first place, for best results, the 34/1 ratio transformer should be used immediate. ly following the detector, leaving the higher ratio until the last. That is easily done. When this part of the work is complete, the layout can be followed and is self-explanatory. The remarks concerning layout and wiring, that were given last week are equally true here, and no difficulty should be experienced in following the diagram given herewith. A point worth noting is to keep the two transformers with

their cores at right angles to. one another. Otherwise, reception may be marred by audio oseillation. More of this has been dealt with ib our article, "Oscillation: Its Cause and Cure." Suitable Valves. (THE selection of valves in a set such as this is of paramount importance, and the accompanying table shows the six-volt valves we have tried in this circuit with good results. The appropriate grid-bius can be obtained from the instructions. accome panying the valve. Generally speake ing, a 9-volt "O" of the Lissen type will be most suitable, as the appropriate bias can easily be found by mere-

ly adjusting the plug. Use the highest bias compatible with good recep tion. Four-volt valves of the types indicated are able to be used quite’well,

Valve. Mullard Cossor Osram Radiotron 660 Detector PM6D 610HF DBEH610 201A HF610 First audio PM5BX 610LF DBEL610 201A. LF610 in a SE ws Second audi« PM6 610LF DEL610 201A LF610

Valves for the All-Wave "Round the world" Three