The Browning Drake Four Valve Receiver

Radio Record, Volume I, Issue 30, 10 February 1928, Page 12

The Browning Drake Four Valve Receiver

A useful resume of constructional details

(By

Megohm

[THE construction by readera of the \&. Browning- iwrake four-valve receiver as described in this column some time back, has been generally attended by good results, and many spontaneous letters cf appreciation have been réceived from satisfied constructors. As the numpers of the "Record" containing this article have been out of print for some time, it is evident from inquiries continually received, that many readers are still desirous of constructing this popular and efficient receiver.’ A resume of the descriptive article is here given in altered, and, in some parts, abridged form. Illustrations iticlude the theoretical circuit -diaeram, construction of the aerial tuning coil with strips of celluloid, the radio-frequency transformer with piimary and tickier coils, construction of the primary from cardboard discs, and the neutralising condenser, ° THE AERIAL TUNING COIL. _ This is wound on a 8in. cardboard the 20’s s w.g. tinned, or enamelied wire being spaced with 21’s enamelled wire, which is then wound off, leaving :the correct spacing between turns. Celluloid cement is then applied to hold:.the wires to the celluloid strips

_ underneath, then duplicate strips are pressed over the cement on the outside, and the whole left to set for a few hours, when the former can be ent through and tiken out, leaving the 65 turns supported by the celluloid strips alone. A tep is soldered to the Sth turn from the earth or lower end. The coil is screwed to a wood base by screws through the celluloid strips. The celluloid cement is made by dissolving chips of celluloid in liquid acetone obtained at the shemist’s The mix- -~

ture must be made up in a very small bottle, loz. or Icss, Kept well corked, and applied with a stick, Jt sets in two to three hours, VARIABLE CONDENSERS. Two are required, .0005 mfd. being most suited for the aerial tuner. it a condenser of less capacity is used, more turns are required on the coil. kor the R.F, secondary tuner any of the following may be used: Vernier dials are necessary, : "Condenser .0005, fin. coil, 56 turns. -Condenser .00035, 3in. coil, 68 turns. Condenser .00025, 8in. coil, 78 turns. | AERIAL SERIES CONDENSER. | -Bither a variable midget or a fixed .000L. ‘he smaller this value, the greater the selectivity, but too small a capacity reduces volume unduly, NEUTRALISING CONDENSER. ‘With space-wound coils as specified, the self-capacity of the circuit is kept to a minimum, with the consequence that many factory-made neutralising condensers have ‘co great a minimum capacity, and therefore some of the moving plates must be removed. The only alternative is for the constructor

to make a suitable one, which is not a difficult matter. The diagram :llustrates the construction of such a neutraliser. A piece of ebonite 2: by Jzin. forms the panel, supported by a slip of wood, 1} by } by in. Only two plates should be required, made of %2’s brass sheet the size shown. If the capacity is found to be too small, another fixed plate may not essential, it is handy to have a be added, spaced with a nut. Though

pointer and scale on the front, the pointer of brass soldered to the head, fe) t. . At the top are three 1-8-inch. brass bolts, the two outside ones for connections, the centre one, B on side view, has the head outside the ebonite, inside the connecting strip (A), a nut, then the fixed plate, kept in place by another nut. At C is the bolt supporting the moving plate, the head outside. the ebonite, then the pointer soldered on, then a washer,, the ebonite, connecting strip A, two nuts, the moving plate, and a securing nut. The nuts must be adjusted so that the plate may be smoothly turned by a screw-driver inserted in the slot in head of bolt. A metal screw-driver introduces too much capacity to be any use to turn the neutraliser, so a tapered stick 10 inches long should be sharpened like a screw-driver, or a piece of celluloid cemented into a, slot at the end (D)-metal should not be used. AA are strips of 30’¥ brass or copper to connect the corner bolts to the fixed and moving plates as shown. THE RADIO-FREQUENCY ~ TRANSFORMER. In three parts, the primary, a coil of few, turns, inserted inside the first

turn of the secondary coils, which is wound on a 8-in. former m the same way as the aerial coil, with celluloid trip supports, Note how celluloid strips are screwed to the sides of the wood base, the coil strips being comented to the top edge of these. At the opposite end io ‘the primary coil, is a rotatable tickler, 2 inches in diameter, on which are wound unspaced, ten turns on one side and four or more extra turns on the other side as required. ‘The wire to use is 30’s enamel or cotton-covered. ‘rhe wire for the secondary coil is 22’s tinned or enamelled, turns as given under "Variable Condensers.’? Care must be taken that the three coils of the R.I. transiormer are all wound in the same direction and so placed in position. The primary coil may be wound in a 1-8-in. slot" formed by two discs of cardboard cut as shown to just fit in the end of secondary coil, the 1-8-in. , )

space between being made by discs t-in. less in diameter glued together to make 1-8 thickness, the two larger discs being glued outside. ‘The begin: ning of the wire comes through the hole A and winding is carried out in the direction of the arrow, finishing through B. A connects to plate of R.F. valve and B to positive of R.I*. valve. ‘Lhe primary coil is to be wound as irregularly as possible in the slot, and is sometimey constructed without a slotted former by winding the wire closely jumbled on a former or bottle to make the coil a fit for the inside of secondary, then it is bound with thin thread and tied in position by threads over the ends of the celluloid strips of secondary coil, ‘The secondary coil is tapped at the fifteenth turn from the primary or filament end, this tap connecting to one side of the neutralising condenser, i

THE WIRING DIAGRAM. This is drawn to scale, and the position of all components on the radiofrequency side should be carefully, not: ed and followed, as no crowding is perniissible here. British valve holders are shown in the diagram, but constructors are advised to adopt American UX holders, as British + alyes are now all stocked with American bases. A wire connects the filament positive of each -valve, as shown, running under the. baseboard. An output filter is shown incorporating a choke coil, as described in another column, but this is only necessary if a power valve is used in the last audio stage, with more than 90 volts on the plate. If the filter is not to be included, connect output jack to plate of last valye, and to ‘positive B power," which if. the same B voltage is to be used on both audio valves, ,

will be connected to ‘positive B andio" with a short piece of wire, or ‘‘jumper.’’ Tf the filter is not fitted, a fixed condenser of .001 or more, best determined by trial, must be placed across the output, and in any case this may be found to effect a great improvement in tone of the speaker. A fixed condenser of not less than 00L mnfd is shown across the primary of the first audio transformer, and those constructing a two-valve set should not omit this across ’phones, otherwise regeneration may be difficult to obtain. Sometimes a grid-leak cf ‘- megohins will be found necessary to produce smooth oscillation. This depends to some extent upon the valve used. Two transformers are included in the audio circuit, any good make of 8 or 5 to 1 ratio in the first stage, and a Lerranti A.I’, 8 in the second. In place ‘of the first transformer, especially if a cone speaker is employed, a Philips resistance-coupling unit will give extremely good results. ‘his unit, costing 25s., is provided with four market terminals, and connects up in place of the transformer without any alteration in the wiring. The valye preceding the Terranti transformer should not require a plate current exceeding 4 milliamps at the voltage employed on the plate.

% HOW TO NEUTRALISE.’ *‘’ ‘When all 1s ‘complete, the operation of neutralising is carried out as -iollows: Tune in a loud station , near

8YA, maximum volume being " cafe. fully obtained without oscillation, irecentre of broadcast waveband, 1YA for ~ i

action being turned well down. Next turn out the filament of the R.I. valye, then with the neutralising: stick turu neutralising condenser until signals:are inaudible or at minimum — strength, then turn on Glament of -R.1I. -valve, and the station should :come- through well. Now see that the variable condensers are both at maximum, tuning; if not, they must be altered to get. the

The Browning Drake Receiver---Continued ©

best tuning, aud the process of neutralising is gone through again: ‘The second .attempt’ is vractically certain to be correct. If neutralisation cannot be obtained, reverse the connections to the R.I. primary niarked A and B, and tty ’-again. If it cannot be obtained either way, it may be an indication that the capacity f the neutraliser is top great or too :mall, and in either case ‘the correct valye must be substituted.

SUITABLE VALVES. It is important that a valve of only ‘medium impedance be used in the ‘R:F. stage. This saeans an impedance of 12,000 to 16,000 olims, the UX201A (15) having the former figure, and the PM3 (16) the latter. The figures in parentheses are the requisite number of primary turns, approximately, as a turn or two more or less may be found to give greater volume. A useful list of valves and their characteristics was fublished on January 18, and every constructor should file this for reference. From this list a valye may be chosen for any purpssc¢ with a knowledge of its exact suitability. Primary turns increase as impedance increases, aud it is advantageous to keep primary turns to the minimum that will give good results. The following aré approximate turns to suit

varying impedances: 20,000, 22; 18,000, 20; 16,000, 16; 14,000, 15; 12,000, 14; 8,000, 12. The 201A valve has been used in all four stages of this receiver, but ee eee ee eT ee ee eet, ee

such an arrangement is not always the best, and would not suit dry-cell A batteries. As detector the 201A, PM3, or BM8A, the latter especially suited where a first stage of resistance coupla

ing is included. To precede a resistance un*t the PM8A (4 volts), or Philips A239", A425, or A860 valves are suitable, requiring 2, 4, and 6 volts on the filament respectively.

The matter of audio valves was treate ed last week in a two-valve amplifier article, and perusal of this will prove useful to intending constructors.