An Output Filter Choke

Radio Record, Volume I, Issue 24, 30 December 1927, Page 12

An Output Filter Choke

DETAILS FOR CONSTRUCTION

Many inquiries have been received regarding the construction of a choke coil for an output filler, and although a number of readers have had an old atidio' transformer on hand and have rewound that, others find it necessary to either purchase or make one, A correspondent has written inquiring if the B eliminator choke coil would answer for filter purposes. ‘Megolim’"’ had already tested that choke for the purpose and found it answer splendidly, but it is rather bulky for inclusion in a cabinet, and the clloke now to be described is equally effective and efficient right through the musical scale, giving excellent reproduction. ‘here is no advantage in unduly increasing the d.c. resistatice of a choke coil, and to that end the gauge of wire is not made too fine, whilst with a closed core with gaps, it is possible to keep down the number of turns to a reasonable limit. This choke coil has been in use by the writer for many inonths, tried against several others, and always been at least slightly better.

' WINDING THE COIL. The spool is made on a wooden former Zin. by jin. by 2 3-16in. long. This is covered with one or two thicknesses of manila paper sud ends of thin fibre or stout millboard fitted on and glued. The winding should be done in a jig and the spool ends supported in some way by cheeks fastened to the winding spitidle, in order to prevent them being forced outwards by the wire during the process of winding. The beginning and end of the winding, which should be of thicker wire, are passed through holes in the spool ends. ‘The spool is to be filled with 36’s enamelled wire, over half a pound being required, which gives about 6000 turns. The wire is run in irregularly in patches, piling up a certain thickness, say 1-16in., and then passing along to another pile until the other end of the spool is reached. ‘hen a strip of tissue paper is put round, care being taken that it closes up to the ends, as although the wire is being put on irregularly, one layer must not be allowed to come in contact with another by wires sinking down at the ends. This easily happens, especially if the spool ends sre not well supported. When the spool has been filled and the lead-out wite soldered on and passed through the hole in the end, a covering of stout paper and then a piece of suitable fabric are put on to protect the winding. THE LAMINATED CORE. ‘he core is built up of laminations of ordinary tin-plate, cut as shown, in full size, eighty pieces being required. The tin has the advasitage of being thinner than black iron can be obtained, and thus serves to more efficiently suppress eddy currents in the core. Stalloy can be used, but it is not always procurable. One sheet of tin 28 by 20 inches will be more than sufficient for the laminations. ‘he tin should be marked out into 80 squares 8} by 2% inches, these squares being cut out, after which a cardboard template is made of a complete lamination in one

piece, to be laid on each piece in turn aud scratched round the two "windows" to mark their position. Now a piece of card is taken the size and shape of half lamination, but without the windows, ‘This is used as a guide to scratch the diagonal line across each. hen eut each square right through on

the diagonal line, afterwards _ cutting out the ‘window’ pieces, bending the metal where necessary, afterward straightening out by stroking on the bench with the back edge of a flatiron, the point being held up in the air. This method of straightening the cut plates’ is easy and effective. |

‘he laminations are now to be shellaced with one coat on both sides, for which purpose shellac is dissolved in methylated spirits and applied with a brush. ‘his coating of shellac is important, as its purpose is to insulate ' each lamination from the next. When the shellac is thoroughly dry, laminations, all the same way round, are packed into one end of the coil, the last few being pushed in under the top one, and having the sharp corner cut off the central piece to prevent the manila being cut. A piece of strong, paper is to intervene between the two ends of laminations where they meet outside the coil, so large enougli pieces are placed there, while the laminations on the second side are packed into place. As many must be got in as possible, so that both lots are free from any looseness. ‘Ihe ends of laminations must both press against the paper, so that they are only separated by jis thickness. ‘This forms a gap in the iron, the object of which is to prevent magnetic saturation of the core by the direct current passing through the coil. FINAL OPERATIONS.

The wooden clamps, four in number, are 44 inches long and 7-16-inch square, drilled at each end to take a 3-8 brass bolt two inches long, about a half-inch of which can be cut off after all is assembled. A washer should be put under the bolt head to protect the wood, but at the other end the nut will be sufficient, A slip of ebonite about 2 1-8 by 1 1-8 inches is now screwed to the clamps and fitted with two terminals, to which the leads are connected underneath. When completed and stood on end the choke accupies a floor Space of alout 3 by 13 inches, and, of course, the ebonite slip may be placed in, any convenient position. A small diagram of the circuit is included. ! he wooden clamps should he finished with the shellac applied with a piece of rag and rubbed. The clamping bolts must not be allowed to touch the ends of the laminations and so connect some of them together. If care and finish are put into the work the coil has quite a neat appearance. _ If desited the outside of the laminations may be finished with black cycle enamel. Experiment may show that a higher value than .001 gives better results for the fixed condenser across the speaker. With no condenser there, tone is inclined to be harsh and thin. Too large a capacity causes woolliness.

A British wireless journal gives the following :-Don’t throw your old and seemingly "dud"? crystals away. They can be utilised by grinding them up into a coarse powder, mixing them with a quarter of their bulk of litharge, and adding a small sprinkling of fine, clean iron filings. Mix the components of the mixture together very thoroughly and then make it into a paste with water containing five drops of pure acetic acid to every ounce. Spread the crystal paste on the surface of small copper or brass discs, and allow them three davs to dry. The crystal discs will afford great rectification if they have been so prepared.