Constructing a 25 Henry Choke

Radio Record, Volume II, Issue 45, 24 May 1929, Page 34

Constructing a 25 Henry Choke

W. (Alexandra) writes: I intend making an output choke that can later be used in place of an output transformer in the push-pull stage, tap-

— ping it so that it can be used to step up or down as per diagram 1. _ As I suppose that each half would require to be about 25 henries I propose to use a 1} x 1} core with 12,000 turns. This, with 36 wire, should give the value easily. Now, it appears to me that, to get the centre tap in the electrical centre, and to facilitate equal proportions in the intermediate tappings, that it would be desirable to wind the coil in two*halves, on a spool as per diagram 2. Of course I realise that the widths "A" would have to be exactly equal

and the pair of windings equal in all finished dimensions. This is preferable to one continuous winding on one spool with a centre tap taken out at half the number of turns. I would wish to start the windings at the outside faces of the /aouble spool, and I come up against the elementary difficulty that I cannot decide if both coils should turn the same way. [The diagram shows the correct method which may be applied to eliminator windings.] This choke will be used as an ordinary output choke in the meantime; using one-half or both in series or parallel according to 25, 50, or 124 henries, seems to give the best results and is oceasioned by the commercial choke I am using being quite inadequate for its work. My present idea is to work up to two screen-grid stages, an R.O. valve as anode bend detector, with resistance coupling to first audio, then the AFS and a B408, later, converting the last stage to push-pull! by halving the AFS secondary externally by means of two grid leaks,