Mainly about Construction

Radio Record, Volume I, Issue 18, 18 November 1927, Page 14

Mainly about Construction

BY

MEGOHM

A FULL-WAVE BATTERY ELIMINATOR FOR HOME CONSTRUCTION

ADAPTING THE BH RAYTHEON TUBE

HEATING LAST AUDIO FILAMENT FROM ELIMINATOR

THE SMOOTHING CHOKE. Radio chokes are not often included in an eliminator circuit, but they are simple to make, and are so effective that this eliminator will actually work without any other choke, but it is not advisable to attempt to do so, for the provision of a suitable choke "certainly Improves results somewhat, and more important still, ensures good working -under more or less extreme conditions. However, the radio chokes cut out the necessity for two chokes or two windings on the one core ‘Ordinary black iron sheet running in thickness about 48 to the inch is suitable for the core. This same iron could be used for the transformer core it preferred, but the thinner laminations already specified make a very efficient article, though there is more cutting. Abont 35 pieces of iron 5 by 38 inches are required for the core. A sheet ‘ metal worker will supply them cut to Rize for Ys. 6d. Then a cardboard template must be made with which to make the iron for further cutting. The card is cut the exact size of the iron and then dimensions shown are marked of and the "window" cut out of the celitre with a sharp knife, and also two notches as shown, The straight edge of notch A is to be at exactly the half-way point of the full length. The iron is first cleaned by wiping with a rag and benzine, then the template is laid upon each, the window and straight side of the two notches marked by Scratching. In cutting, the cunts at A and EB are made first, continuing to across the "window" about to the small cross shown. ‘Then the two portions can he hent in the centre on dotted line, when it is easy to cut down the inside of the two arms of each. ‘To cut across the end, one arm must be bent down on the diagonal dotted lines in Fig. 2. ‘Then the iro is bent back and flattened by hammering, The next operation is to give the laminations a coat of shellac all over. The shellac is dissolved in methylated spirits and appliel with a brush. THE SPOOL. The spool for the wire need not be very strong so long as the ends are well supported whilst the winding is in progress. Two squares of strong cardboard are cut 2} by 23 inches with a square hole in the centre about 1 3- 16 by 15-16 inch, but the exact size is to be made to suit the former with cov ering. The former for the spool is a piece of wood exactly 2 18-16 long by 1 3-16 by Zinches. A hole is centrally through the length of this as accurately as possible by drilling part from each end, diameter to take a dowel-stick as a spindle. A strip of manilla or strong paper 2 18-16 inches wide is

wrapped two or three times round the former and well, glued to itself, but not to the former. Now the square holes in the ends are cut to fit over the ends of the paper, and are fixed with glue and further strengthened by glueing in small bent strips of strong paper as shown in Fig 4. During winding, the spool ends are best supported by a piece of thin board about the size of the end, drilled to fit the spindie and held fast, pressing against the spool end, by a nail passed through a hole in dowel. Once the winding is complete, it is safe without these supports. WINDING THE CHOKE. The quantity of 30’s enamelled wire required to wind the choke is 1{1b., and if this quantity is put on it will be quite sufficient for ordinary conditions, and will give about 5000 turns. ‘There is actually room for about 7000 turns on the spool. ‘The 30’s wire gives the de--sitable low d.c. resistance. It is wise to solder on a short length of, say, 20’s d.c.c. wire for a lead-out, and this is passed through the hole marked "in." The wire is then wound on in patches from on? end, working towards the other, but not returning far over that already wound. A layer about oneeighth thick is put on in this way, irregularly wound, and then covered with tissue paper cut wider than the spool to allow for puckering. ‘The next layet is then wound. Great care is required at the ends to prevent any turus of wire sinking below their proper level. The end of the wire is led out at the hole provided, and the winding ¢ vered with suitable protection. | ASSEMBLING The coil is then laid on the table | ready for assembling the laminations in a flat position, Supports are proviiled for the portion of these outside the coil, with smal! strips of wood. The central division of the laminations, C.C., is to be outside the coil, and a picce of thin card is te be bent at right angles and stood upon the table in such a way as to be finally held belween the two ends of the laminations, which are both pushed up against this card as they are assembled. This card forms a gap it the laminations, and its purpose is to safeguard the core from magnetic saturation by the direct current flowing through the coil. The laminations are assembled in alternate positions, so that the central portion comes evenly against one or other side of the card, but the joins inside the coil are in alternate positions, that is, brickwise. The laminations should now be tere poratily held at the outside corners by iron screw clamps or other means, whilst the wood clamps are secured,

the card between ends of laminations having been trimmed flush. ‘These clamps are strips of wood 1 by } by 6} inches, drilled }-inch, 5} centres. The strips should be shellaced and are secured in the same way as the transformer with brass plates and fin. brass bolts 2 inches long. ‘The drawing of finished choke shows how brass anglepieces would be provided at one end to proyide for screwing to the partition in an upright position. GENERAL REMARKS. The transformer as described is thoroughly efficient, and ‘‘wattless’? on no load, and consequently does its work with the minimum current consump-

tion. ‘This type of core is known as the ‘shell.’ It pays to have plenty of iron in a transformer core. A transformer with insufficient wire in the primary winding will consume a great deal more current than is necessary, and will thus make the running cost high. In running this eliminator, negative B must always be connected to earth, but as this connection exists already in most sets, no scparate outside earth connection should be made, Care should be taken in connecting up special circuits that there is no chance of shortcircuiting the A battery by having two carth connections. If running filament yalyes, the two filament winding centres from transformer are connected together above the transformer, forming only one wire, Which is B positive. This wire can be run down the transformer side of partition if preferred. The eliminator is quite noiseless in working. THE RAYTHEON TUBE, For some considerable time the Raytheon rectifier tubes have been in use in America for B climinators, The advantage of these tubes is that one tube gives double-wave rectification, without the necessity for a heating filament, and consequent extra consumption of current and expense of replacing valves with burnt-out filaments, These valves are at present

very scarce jin New Zealand, and 1n-f quiry of a number of leading dealers shows that they are not at present’ stocked iu Wellington, though one or two stated that they would have them in stock at a future date. An Auckland firm has a limited number in stock, and ‘‘Megohm’"’ procured one of these for 386s. On account of this apparent scarcity, and many requests for constructional details of an eliminator, it was decided to describe the filamentvalve type, se that constructors wishing to proceed with the work at once could do so, and get running with the two valves (P.M.4} until suclr time as the Raytheon tubes are available for all, The most suitable Raytheon tube for ordinary sets is type BH, delivering 85 imilliamps at 200 volts. A trial of this gas-filled tube proves its suitability for the work, and its adaptation to the present eliminator is an easy matter, The chief difference is that only one valve socket (American) is necessary, the filament winding on the transformer is not required for rectifying, and there are two small fixed condensers of 0.1 infd. capacity to add ,ome across eaclte HT winding. These condensers are both screwed to the partition by the transformer. A wire connects one side of each condenser together, and runs ‘to connect to H’l centre tap. ‘The remaining sides of the condensers are then connected, one to each of the back end of the HT fuses on transformer panel. The radio chokes are still retained in the two plate circuits as before. Filament wiring is disconnected from the valve sockets, and if not required the ends must be insulated separately with adhesive tape. The two filament terminals on the valve socket is use are now connected to the two plates of the Raytheon tube. Fach of these terminals is connected through a radio choke to HYT1 or HYT2 of the transformer. ‘The HT centre tap still forms B uegative. The common clectrode of the valve is the HT output, and its terminal ou the socket is the one marked P, which connects to one end of the filter choke in place of the filament centre tap. Other connections remain as before. If the transformer is constructed without the filament winding, space is available for an extra layer or two for each side of the HT, which will give increased voltage, but results are quite satisfactory with windings as specified. ~

ULtLAoaNG FILAmnENT WINDING... A good method of getting cheap filamicnt current tor the last audio valve is tu arrange to heat this from the filaincut winding on the transformer when liberated by the use of the Raytheon. The filament of a power valve runs quite noiselessly on raw ac. in this way, and ‘‘Megohm" is now using this method, which gives current much more cheaply than by mediuin ofan accumnlator, This is accomplished by taking away the three wires running down through the baseboard from the distribution panel. The bottom bolt cpnnection is not now required. .V’rom the two bottom bolts to which the resistalice wires are attached, is run a twin exible wire or d.c.c. 18’s twisted together, and these are connected across the filament of the Jast valve. The connection between the two terminals on the distribution panel is broken. If four yolts are required for the filament, the terminals are connected to the respective bolts marked 80, and for six volts to 40 or 45. Voltage is then finally regulated by the two resistance wires, testing with a voltmeter. ‘The centre tap of the filament wisiding is connected to B negative terminal, that is, a point past all smcothing condensers. Tt is a feature of the arrangement that the leads from the eliminater to the valve filament should he twisted together, as this eliminates the hum. Grid bias for the audio valves can also he obtained from the eliminator, fut further experiment is needed to fine the best method ef effecting this before describing it. The versatility of this eliminator will now he apparent to the constructor and exnerimenter, and although it entails rather more work in construction than one desiened with a sitwzle purpose only, its advantares more than outweigh the comparatively small amount of extra Jabour. MATERIALS REQUIRED. When this eliminator was constructed no tally was kept of the amount o wire upon the transformer, but by calculation about 1b. of 84’s enamelled will do the two HT windings and the two radio chokes. Nine sheets of tinplate 20 by 28 inches are required, costing 9d..per sheet. ‘The Raytheon tube as already stated costs 36s., and P.M. (Continued on next page.)

4’s cost 18s, €d. each. By-pass con- |: densers, Dubilier, 0.1, 3s. each, ‘‘Royalty’? or other variable resistances 1500to 100,000 ohins, 8s. each; 2 mfd condensers, 78s. each, Wire for smoothing | choke, 14]b. 30’s enameled, 4s, Wire for filament winding and internal connections, llb, 18’s 3s. 6d. Carbornndum fixed resisters, 2s. each. Rheostat (if required), 5s. Cleartron Nectron valves, 12s, 6d. each. "Megohm" is quite willing to answer replies regarding the eliminator provided a stamped addressed envelope is enclosed with each, and all. questions set out on a sheet with space for answer. Questions must be put so that answers may be made as briefly as possible. Questions of general interest will be answered in this page for the benefit of others.