Improved Browning-Drake Receiver.

Radio Record, Volume I, Issue 7, 2 September 1927, Unnumbered Page

Improved Browning-Drake Receiver.

A Popular Circuit For Home Construction.

**By

Megohm

The four-valve Browning-Drake ciruit at present enjoys a very widepread popularity, chiefly on account of ts hiigh efficiency, low up-keep cost and ase of handling. Another point much mn its favour is the fact that the radiorequeucy valve is neutralised in order @ prevent interference radiating from he aerial to the detriment of reception mm mearby sets. The Browning-Drake is a good dis-ance-getter on account of the employnent of reaction, rendering it specially uuitable for searching oyt distant staions, an operation which*can be more ‘aSily carried out than upon a neutrojyne and some types of super-het. Chis four-valve circuit is often mention--d as being equal to many circuits emMloyiig five valves; tliis ‘contention is "ertainly true and may easily be proved

>y deinonstration. The carefully constructed" ‘set will bring in New Zealand and Australian stations at good loud-speaker volume, and subject to the carpices of the ether and atmospheric conditions, may bring in stations at ‘phone strength up to seven or eight thousand miles, extreme range being regulated to some extent by the operator’s patience, skill and vigilance. For the cotvenience of those who nay wish to commence with a twowalve set, the radio and detector portion of the circuit is shown in the diapram in such a way that it may easily be separated from the amplifier portion. ¥he two-valve portion will give good *phone strength of New Zealand and Anstralian stations.

The theoretical circuit is given this greek, together with the construction of the aerial coil. Low-loss’ methods are gsed for the inductances, as careful tests liave shown that by their use there is a gain in both volume and selectivity. Some constructors mav choose to wind the coils without spacing the wires, but it must be clearly understood that this method will introduce a certain amount vf loss, especially if cotton-covered wire’ is used, as it is less efficient than enamelled. If the instructions are folfowed, the low-loss plan will present no difficulty in construction. As an inStance of the selectivity of this set it may be stated that without any waveErap' or shielding,’at a distance of two tiles from 2YA, that station can be rut out and 8YA tuned in, only a faint trace of direct pick-up of 2YA being heard at the intervals. With 1YA there is slightly more background, but of gourse with a suitable wave-trap 2YA is easily cut ont, and such trap will be described at a later date. Altogether, the circuit is an ideal one for New Zealand conditions, and lends itself easily to home construction, and quite as important is the fact that it complies in every respect with the Government regulations.

The Aerial Coil. An illustration of the aerial coil is given, and upon reference to this it will be seen that it is a low-loss coil of thorouglily skeleton form. The 20’s enamelled wire used is held together © by eight strips of motor-hood celluloid about j-inch wide, there being a strip inside and one outside at each quarter of the circle. The minimum spacing for low-loss coils is half the diameter of the wire, and in any case the spacing should not be over done, in order to ‘keep the dimensions of the coil within reasonable limits. ‘The spacing used for the coil in question is best obtained bv winding a 24’s enamelled wire alongside the 20’s, afterwards unwinding the thinner wire. This is a much more treliable plan than relvine upon twine for the spacing, and the 65 turns reauired | will occupy just about four ifichies, which is short enough for atty cabinet. There will be 16 turns to the inch, Th

extra turus above those usually specified are to compensate for increasing the gauge of ‘the wire, and to enable 7ZL to be tuned in, as this station transmits on 5385 metres, which is an unusually high wave-length for ordinary broadcasts in this part of the world. These particulars are correct for an aerial and lead-in totalling 90 feet, and allow considerable latitude for’ longer or shorter aerials, A former of three inches outside diameter is required upon which to construct the coil, and as this same former | will be used in the construction of the radio-frequency transformer, the length should be not less than s.x inches, but seven would make handling easier. The former may be purchased ready-made, or constructed upon discs of cardboard as already shown in this column, but if this latter method is adopted the core upon which the former is made must be kept inside during the winding and cement required. A shilling’s worth or case be a continuous strip 20 inches long and six inches wide, rolled round twice and glued down. Whichever kind of former is used, it must now be covered all round with a sheet of good | naner to keep the wires from sticking ~

to the former. The paper must only be stuck to itself and not to the former. Now take a piece of hood light celluloid and cut eight strips six inches long half-inch wide, by scratching lines on the celluloid, which can then be broken on the scratches. Four of these strips' are now held along the former at equal distances and each end is tightly tied around with twine to hold them in place. Winding is the next operation. Fasten the 20’s enamel and the spacing wire through a hole im one end of thet former, allowing a surplus of six inches the end of the wire, leaving sufficient for connections, is now bound to the former by several turns of twine run‘ning right round and firmly tied, The ‘spacing material is now wound off, Jeaving the wire neatly spaced. > Celluloid cement, which must be pre-

of the 20’s. Now with the coil standing on the table, turn it with the left hand, grinding tle wires on with the right, pulling tightly all the time. When the 65 turns have been wound,

pared overnight, is made by cutting / celluloid into small chips and dissolving it in acetone in a small bottle, preferably with a wide mouth. A couple of thimblefuls is about the amount of cement requred. A shillings worth or a fluid ounce of acetone from the chemening the lower end of the baseboard may be used, but a simple way is to take a square of thin wood 23 inches square, saw off the corners and secure a celluloid double strip to each corner, then by means of a central hole in the wood, secure the coil to the baseboard with a brass screw. It might here be mentioned that iron screws should tieyer be used in a radio set, and in fact iron in any form should be avoid. ed except where it has a legitimate pur pose as in LI* transformers, ‘The cel Inloid strips should be left projecting a

ist’s will be more than is required. The ) cement may be used quite thick, but it too stiff, add acetone and stir. With a small stick of wood, work the cemeit between the wires and upon each strip of celluloid on which they rest. A little Iumpiness will not matter, as the extra strips of celluloid are now pressed down on the soft cement, one strip on each, and after adjusting the spacing of one or two wires at the ends of the coil, the whole is now left for a few hours to set. In order to save any possibility of distorting the coil by, attempting to slide it off the former, it is a much safer plan to clear the centre of former, then with pencil and ruler, draw a line inside the former corresponding to the centre of one of the ‘strips of celluloid. Now with a sharp knife, working half from each end, cut through the former along this line, when one side of the former can be raised and the whole will lift out, and then the sheet of paper is carefully rémoved, leaving the skeleton coil securely held by the celluloid strips alone. The strips should now be clipped off to leave about half an ich outside the end turn of wire. Any method of fast-

quarter or half inch at the top of the coil, as they may be required at a later date to support a small wavetrap inductance. Now deciding which is to be the lower end of the coil, count 18 turns up, counting from where the end of the wire comes out, but not counting the end. ‘To this turn a tap is to be soldered, either by flattening the end of the tap wire, curling it round and soldering, or by attaching a strip of copperfoil and soldering the tap wire to that, The enamel must be carefully scraped off both inside and outside the part of turn to be soldered ‘This tap connects to the small serics aerial condenser, The Aerial Series Condenser. This small condenser is intended to increase selectivity, ‘The smaller, this coudenuscr, the greater the selectivity,

but if the reduction is too great reaction may become somewhat unstable. The only method is to experiment and so find the correct value, giving ample volume and suitable selectivity. In order to save providing a number of fixed condensers for experiment, it is therefore a cheaper and easier plan to put in a midget variable condenser and once this is set it need not be altered. ‘The Aerial Tuning Condenser. The maximum capacity of this coudenser is .0005 mfd. In connecting up both this and the reaction condenser the moving plates, or rotor, should be . connected to earth, as by that arrangement hand capacity: which can be so troublesome is practically eliminated. This should be a low-loss straight-line frequency or the new ceztraline style, and a good friction drive vernier dial is recommended, about ten-to-one ratio. There is no backlash in a well constructed friction-drive, but some of the geared verniers are not blameless in the matter of backlash and uncertainty of movetnent. Brass is now replecing various kinds of white metal, hitherto used for the vanes, ‘Tlie centre-line method of modified straight line frequency spreads the stations more evenly over the dials and so makes for easier and yet sharper tuning. R.F. and Detector Valves.

Mullard 4-volt valves are in nse in the set being described, and ce1tain details that are to be given are suited to these valves, but constructors must suit their ,own fancy in the matter of voltage to be used and the particular make of makes of valve to be adopted. ‘Those whose. situtaion makes the use of dry cells imperative will choose one of thie Q-volt dull-emitters that may be obtained. If the constructor already posscsses a 6-volt accumulator, and wishes to use 4-yvolt valves in orde~ to increase the interval between charges, a 80-o%m filament resistance must be used, If 4volt valves are to be run off a 4-volt accumulator, a 10-ohm rheostat must be included in the circuit. For the detector the braderystal in place of a wirewound rheostat makes a smooth control which may often be used to advantage in DX work. ‘The next ins‘alment of this article will include a baseboard and panel wiring diagram showing positions of the valves, and other components, The type of R.F. valve used influences the number of turns required on the primary of the R.F. trausformer. Mullard P.M. 8’s have a suitable impedance of 16,000 olims and an amplification factor of 13.5, the filament current 8.7 volts and 0.1 amp. The suitable H.T. voltage is about 89 The UX201A, a snitable valve for both positions, requires a filament current of 6 volts, .25 amperes, and a H.T. voltage about 90. The question of valve sockets must be decided as to whether the British or American type will be adopted. It should be noted that the Mullard valyes may he obtained with either British or American base. Referring to the diagram, if only a two-valve set is required, the ’phones will be connected to the two’ black spots above and below the ’phone condenser 14, and the connections to the primary of the I,.0'. transformer will be omitted, (To Be Continued.) _--