A Cheap Eliminator for Small Sets

Radio Record, Volume II, Issue 49, 21 June 1929, Page 28

A Cheap Eliminator for

Small Sets

Minimum Cost with Maximum Power

By

PENTODE

a} HHRE are many who, having yi purchased a crystal set, have ‘jlater added a small one or two valve amplifier and are now on the point of looking round for some small, in expensive device to further improve their results. They have heard that an increase of B and © supply will allow of greater undistorted volume, but the idea of buying batteries to give, say, 150 volts, and their added maintenance does not appeal to many. Besides, the average experimenter derives most of his pleasure from the actual making of small devices, and any improvements afterwards is all the more pleasureable. A small one, two or even three valve set, whén correctly designed, should take no more than 8 to 10 milliamperes from the B battery, and and a small eliminator to give this output can be constructed for quite a moderate outlay. The eliminator derives its power from the A.C. mains, and the actual running costs are negligible. As very high voltages are not required, the smoothing condensers need not have a very high test voltage, and the price will be correspondingly ow. One or two valve amplifiers have been mentioned, but the eliminator described is quite suitable for small receivers employing a valve detector. Slightly more smoothing may be necessary, and this will be dealt with later. First of all the principle will be briefly dealt with and the functions of the individual components described. In

this way the contructor will be able to tackle the job with a Petter understanding. The Theory. An eliminator can be divided into four essential parts. Hach of these different operations have to be done properly to ensure a steady and sufficient direct current output. In the first place there is the power supply. In the case under consideration the supply is derived from the A.C, mains and a power transformer is employed to step the 230 volt mains either up or down to suit requirements. After having the eorrect voltage available the current is rectified, and a vaive is employed for this purpose. This direct current is not yet suitable to be applied directly to the amplifying valves, as it is made up of a series of direct current impulses which, if used.in this state, would produce a loud hum in _ the speaker. The next process is the smoothing out of these ripples and is accomplished by a series of iron-cored chokes and large capacity condensers. The direct current should now be quite smooth, but of one voltage only. As

the detector or first audio valye requires less voltage than the final power valve, some form of voltage divider will have to be used. This constitutes the fourth essential in the working of an eliminator. Commercial power transformers are available, but for the amateur who wishes to construct his own we shall spend some time describing the manufacture of one. For the sake of economy the rectifying valve used is one .of the ordinary general purpose valves of which there are two or three makes as low as 2s. 6d. each. A current of 10 m.a. can be drawn quite safely, and the valve can be expected to last quite as long as a set of "B" batteries. The Power Transformer. O begin with a description of the power transformer. Winding will be given for both the 110 and 230-volt supply, or the 280 winding can be divided and used for the 110-volt mains by following the directions given. The core is of stalloy, and this will have to be bought in the stampings cut to size. The diagram shows the sizes and shapes of the different pieces. As stalloy is usually of 1-64in. thickness, 64 pieces of each size will have to be used. The quantities and sizes to be obtained are as in the list of components.

Hach of these stampings will have to be given’ a coating of shellac dissolved in methylated spirits and: allowed to dry. While these are drying the preparation of the spool can he commenced. Obtain a piece of wood about Gin. long and just over lin. square, tapering very slightly towards one end. Over this wind a strip of brown paper 2 inches wide and about 12 inches long, so that three or four thicknesses form the basis of the spool.

The paper should be of a fairly hea¥y type and glue or secotine painted /on between each layer. The spool ends can be made out of }-inch fibre or formica. Hach end is 24 inches square and has a square hole cut in the centre of each, so that it fits tightly over the brown paper spool centre already mentioned, forming the two ends of the spool. Glue or secotine should be used liberally to fasten these two ends tightly, otherwise the ends will be likely to bulge when the wire is wound

List of Components for Eliminator Stalloy stampings, 128 pieces, 3in. x lin.; 128 pieces 1Zin. x lin. ib. 32 s.w.g. enamelled wire. alb. 38 s.w.g. enamelled wire. zlb. 24 s.w.g. enamelled wire. Valve and valve socket. 2 burnt-out audio transformers. 5 x 2 mfd. condensers. Resistor and clip 100,000 ohms (wire wound or carborundum). Panel, terminals, insulated sleaving, ete. 128 pieces 3in. x lin. 128 pieces 13in. x lin,

\o. To prevent any possibility of his small blocks of wood can be nailed to the centre spindle so that the end pieces cannot possibly move. When the whole is dry we are ready to commence winding. Winding the Transformer. FIRst comes the primary, consisting of 1850 turns of 32.s.w.g. enamelled wire. To do this successfully some form of winding jig will be necessary. Details have already been given

\in previous issues for the construction of g, simple winding jig, and the reader is advised to refer to either these, or the current issue of the "New Zealand Radio Listeners’ Guide," a chapter on "Small Power Transformers from A to Z," gives a wealth of information on the subject. It is well worth the slight expense involved. But this is incidental. A short length of heavier flex wire should be soldered to the beginning of this fine wire to pass through a small hole in one end of the spool. Begin by winding on one layer of wire, taking care to not have any adjacent turns cross any of the preceding ones. As this guage wire allows of 87 turns per inch, there ought to be room for approximately 150 turns per layer. Between each layer wind on a turn of thin paper. As there will be about

48 layers in the primary only, this paper will have to be of the thinnest ‘kind yet thick enough to provide in+h sulation between layers. Over this turn of paper continue the winding of the next layer, and so on until the whole eighteen or nineteen layers have been wound on. Care should be taken to see that none of the turns of wire slip down either of the spool ends between the fibre end and the wire already on, as the transformer would

quickly burn out if just one turn became short circuited. Finish off with another short length of flex wire passing through a hole in a convenient end. As an insulator is to be applied while winding, shellac is not to be recommended, as it is liable to dissolve the enamel where adjacent turns touch, and instead of improving destroy the insulation. Molten pitch is the insulator used commer: cially, but, providing the wire has been wound reasonably tight to prevent any turns slipping, there is no need to apply any form of liquid insulation. The melted tops of old dry cells make a good insulator. Primary for 110 Volts. {[t would be just as well to mention here the procedure to adopt if the transformer is to be used for the 110 voltage. Two methods are available. The one in which the transformer is permanently wound for the 110 volt mains and the one in which, by 2 simple alteration, it can be used for either the 110 or 280 voltage. In the first case, half the number of turns will be needed, as specified for the 230 volt winding, namely, 925 turns. As the lower voltage winding takes. more amperage it will have to be wound with a heavier guage wire. Number 288.W.G. enamelled wire is quite suitable and should be wound on as for the higher voltage winding separating each layer by a turn of paper. The second method in which the transformer can be altered makes use of two windings which, when connected in parallel, can be used for the lower voltage supply and connected in series are suitable for the higher voltage mains. To do this, proceed as with the 230 volt winding already described, using the same (328.W.G.) guage wire. When the 925th turn has been put on, cut the wire, bringing the lead out of one end of the spool. Mark this lead number 2. ° Commenec again with a lead (mark it 3) and wind on another 925 turns, marking the final turn No. 4. ‘The first turn to be put on mark as No. 1 connection. And now we have the primary divided into two equal separate halves, all the leads being marked as follow :- Input of first half, No. 1; output of first half, No. 2; Input of second halt, No. 3; output of second half, No. 4. The following connections should be used for either supply voltage. For

the 230 volt main connect 2 and 8 together and feed to 1 and 4. For the 110 volt main connect together 1 and 3, also 2 to 4, and lead the suprly wires to No. 1 and 3. Having completed the primary wind on two or three turns of empire cloth -or brown paper, over which is to be ‘wound the high-voltage secondary. As the fine wire used and also the rectifying valve used has a fairly high resistance, the a.c, output of the trans-

former will have to be slightly higher than that needed at the output. One hundred and eighty to 200 volts proves quite OK, and to obtain this the secondary will need about 1800-turns of number 38 s.w.g. enamelled wire. Wind these cn exactly as for the primary winding, taking just as much care over the insulation. For either main voltage, the secondary turns remain the same; any adjustments being. made are done to the primary only. . Finally there remains the filament voltage for the rectifier valve, and this

winding must be carefully insulated from the previous high ‘voltage winding. If a six-yolt valve is to be used, then 52 turns of 26 s.w.g. enamelled wire will be necessary. This winding should be centre-tapped at the 26th tur, and a lead ‘brought out to one end. If a two or four-volt valve is available for the rectifier, the number of turns should be in proportion, and the winding tapped at the centre turn. All the leads of each winding should be clearly marked to avoid any mistakes,

-_ When finished either. empire cloth or electricians’ tape. should be wound round the whole, and the spool finished off ready for assembling the core. The core is built up by placing a short and long strip. alternately. The next section of stampings should be arranged so that the joints cover those of the previous layer. Four pieces of wood 8% inches long, 3-8-inch thick and 4inch wide will provide means for clamping the whole core together. Bolts, two inches long, are arranged as shown. Having got this far. the most tedious part of the work is finished, and we can proceed with the eliminator itself. Full details of transformer assembly are given in the "Listeners’ Guide," and any constructor in doubt as to arranging the laminations should refer to that publication. Assembling the Unit. For simplicity the whole has been arranged upon a board with a special screening’ box for the power transformer, but there is no reason why the ingenuity of the constructor eculd not be called into play to design @ container, in which the whole is enclosed. Unlike a rado- set, the actual] oe e

positions of the components does not matter greatly so long as the point to point connections are adhered to, When constructing apparatus to work from the a.c, mains, it is essential to include a fuse in the input leads go that in the case of an electrical error or breakdown, no damage will be done. A smull ebonite or fibre strip carrying four terminals answers the purpose quite well, and is to be seen on the daigram fixed close to the power transformers. The Loard on which the whole is fixed needs to be about 12in. x 8in., and the transformer screwed by two small brass brackets close to one corner, The small connecting strip containing tive terminals or soldering lugs, is screwed to the board on the opposite side of the transformer to the fuse block. Slip short lengths of insulated sleeving over the leads from the transformer, and connect to the five terminals (see diagram). A cover of sheet-iron or aluminium to protect the transformer can be made with flanges round the base to fasten to the board. The dotted line shows the position this will take. "U’-shaped pieces will have to be cut in this screen corresponding

to the terminals where the leads are to bé brought out. Chokes and Condensers, THE next components to be considered are the chokes. These can be purchased ready for use, but if the amateur has a couple of old audiofrequency transformers these can be rewound. Dismantle the old transformer and construct « bobbin in which the core can be assembled. Wind this bobbin full of 38 or 40 s.w.g. enamelled wire winding straight on with no further attempts at insulation than that between: the core and the wire. The two ends of the wire can be brought out to two terminals and when the core is assembled the choke is ready for use. Old transformers having the greatest amount of iron should be chosen, and if two similar makes are available the iron from each can be used to make one core. Discard the wire in the old transformer, as besides having a high resistance it would not carry the current. Proceed to arrange the condensers in one block on the board, and screw the various components in place as shown. The resistance used to cut down the "B" supply for the detector valve should preferably be of the wire wound variety and valves of 50,000 to 100,000 ohms should be tried, depending upon the voltage required for the detector. By following the diagram the wiring will be comparatively simple, and is carried out throughout by lengths of copper wire insulated with sleeving. It will be noted that the grid and plate terminals on the valve holder have been connected together. A slightly greater output is obtained when this is.done. Check over carefully before plugging into the light and when in use it is as well to turn on the amplifying valves before the eliminator and to turn off the eliminator before turning off the set. The output taps are as

-_-_--- follow:-B-, B+1 ( detector volte age), B+2 (first audio stage), B+-3 (last stage only). : + anne