THE SHORT-WAVE ADAPTER

Radio Record, Volume I, Issue 9, 16 September 1927, Page 14

THE SHORT-WAVE ADAPTER

FURTHER DETAILS FOR CONSTRUCTORS

The short wave adapter described in No. 5 of the "Record" has aroused. the interest of a large number of: readers, several of whom have writ-. ten asking that the theoretical circuit diagram, already published, be supplemented by an illustration of how the adapter looks when made up. A somewhat indistinct illustration has been supplied by the original contributor of the article, and from this 2 drawing has been made on which the connections have been distinctly marked. "‘Megohm" has written up the additional hints that follow. As described in the original article, the coils may be arranged in different ways to suit the constructor and to give the necessary range of wave-length, The three-coils are shown at the right of the illustration, the small near one is the primary, for which five turns are specified. The next coil is the secondary of ten turns, and close behind is the tickier of six turns, The method of fixing the two latter coils to the baseboard is not shown, vut the constructor will soon plan a suitable way with celluloid strips and cement, keeping the lower portion of the coils clear of the baseboard by about half an inch. This arrangement of coils is said to tune up to Schnectady’s 89 metre transmission, but there is no statement as to the lowest wave-length that can be tuned in, but this will prebably be about 45 metres, A small inset drawing is given of a suggestion for winding the coils. }

upon a short tube three inches diameter, of celluloid or bakelite. The tickler and secondary -of six and ten turns respectively are wound with 26’s double cotton-covered wire as already specified, a space of oneeighth inch being left between the two coils. At one and a quarter inches from the secondavy coil a hote is drilled in each side of the tube to take a quarter inch ebonite rod, tv which the primary coil of five turns is attached, the ends being brought out to connect to aerial and earth. If the set will not tune as low as is required, the primary turns may be reduced to three. The ebonite rod is passed through a hole in the front panel clear of the moving part. of the condenser, and is rotated by means of a knob. The wiring is shown as clearly as possible, but does not indicate the exact position of the wires as regards height from baseboard. Leads 4 and 5 from the condenser are shown conected to the top of the secondary coil for clearness, but they could actually be put in this position to keep clear of the leads on the baseboard. Taking the connecting wires in turn, No. 1 comes from one end of the resistance and to it is soldered the wire from the top of the radio choke, then a soldered branch to the .0001

mtd. fixed condenser, situated behind the valve. The lead marked 1 then along the. baseboard to one end of the tickler coil, The flexible cable contains three wires, and reference to the theoretical diagram wili show where each is to be connected according to the corresponding valve pin to which it is attached on the plug. One of these flexible wires goes to the lower end of the choke, and the other two each to a filament connection on valve base. To show the lead (4) to fixed condenser, grid leak, grid condenser, secondary coil and tuning condenser, the left-hand filament terminal has been made negative, This No. 4 lead comes from the fixed condenser to each point as mentioned and finishes at the fixed plate terminal of the secondary condenser. Lead No. 2 goes direct from arm of resistance to plate terminal of socket. No. 3 connects the other end of the resistance to the remaining end of tickler. It is possi.le that the connections on tickler will have to be reversed when the set is first being tested. No. 5 connects the remaining end of secondary coil to the moving plates of secondary con- denser, and No. 6, a very short lead, . connects grid of valve to grid condenser and leak. The aerial and earth terminals are connected to respective ends of the primary coil. The choke .is shown wound on a piece of dowel-stick fitted with flanges of celluloid or other insulating material. A sewing-cotton reel may be, used, but sho~]d have a centre of

small diameter. The choke must be kept away as far as possible from the tuning coils. As .00025 is a rather large capacity condenser to use for short wave work, a high ratio vernier dial should be procured, and a type should be chosen that gives absolutely correct movement, free from any backlash, looseness or uncertain action of any kind, for any such shortcomings will be very apparent when tuning-in short wave. Friction-drive is very suitable for noiseless working. Some types of geared condensers will give a grating sound when moved, and this makes tuning-in more difficult, so any condenser of this class should be tested before final adoption. The parts required for construztion of the adapter are as follow, approximate price being shown.

All the coils, including the choke, could be made by the constructor, but the tuning coils can be bought if desired. A set of three Hammar-lund-Roberts short-wave coils, unmounted, for waves of 9 to 90 metres, is listed at 20/-. Short-wave coils are not much in evidence in catalogues to hand. Hammarlund short‘wave receiving coils are wound in long lengths with 16’s insulated wire, ten turns to the inch, and any length can be cut as ordered, thus if 15 turns are required, 1% inches would be supplied. Price 2/3 per. inch. Connections or plug-in arrangements would have to be added to these by the constructor. The price given for the variable condenser is only an average one. There are cheaper makes, but in such a vital component, quality is more important than price. For short-wave reception the best results are often obtained by discard(Continued on page 16),

ing the earth connection, and from | the ‘end of the secondary coil that | is directly connected to the negative | A filament terminal a wire is run to the earth terminal, to which no other connection will then be made. ‘he potentiometer control of the tickler as shown is quite a recognised method that is in use in some circuits for broadcast reception. A certain amount of careful adiustment will be required to get the adapter working at its highest efficiency. Constructors are advised to adopt the alternative tuning method as shown, enlarging the baseboard suitably. For reception around 30 metres, at present perhaps the most important, the turns recommended are primary 3, secondary 4, reaction 4 to 6. A more usuai way of tuning a short wave set would be to cut out the resistance on potentiometer, leaving tickler connected to top of choke. Plate of yalve wonld then be connected direct to tickler instead of to potentiometer, and a small variable condenser added to the panel would be connected up in place of the .0001 by-pass condenser.

. ; s. d. Panel, ebonite, about 10 x 7 5 9 Baseboard, about 10 x8.. . Variable condenser with vernier, .00025 oe 20 0 Tapped resistauce 10 6 Grid Leak 1/6, grid condenser 2/3 «.» 8&8 9 Fixed condenser, 0001. . 2 38 Coils, or wire to make them 0 4 Valve socket . oe oe 2 Terminals, each _