An Experienced Constructor

Radio Record, Volume II, Issue 45, 24 May 1929, Page 39

An Experienced Constructor

Made Three "Moving Coils" AM always interested in the constructional articles in the "Radio Record," and always look forward every Friday for it, writes Mr. Schofield (Hamilton). My son and I have done a great deal of constructional work in radio as a hobby, and find a great deal of pleasure in it. We have built up several sets, and like all good radio enthusiasts, we are never satisfied. We are using at present a 5-valve B.-D., and get very good results, We have also a shortwave set, with sereen grid; H.F. det., and one audio, This we plug into 3-valve audio of B.D. last stage is push-pull, giving in all 6 valve on shortwave. The big American shortwave stations come in loud enough to be heard all over the house, in fact KGO, S.W. station, on the Wednesday evenings can be heard over 200 yards away. Our eliminator was made from "Megohm’s" instructions, using B.H. tube, and has been a great success, we never having touched it since it was made. We have also made three moving coil speakers, and they also are a great success. The first one was a junior, by way of experiment, with a lin. core, and about 24$lb. of 20ft. enamel wire, and high resistance coil of 1000 turns. The results were fairly good, but she took over 1 amp. and did not like too much volume. The next we made had a 2in. core, and T7lb. 20ft. wire, with 5in. cast iron pot. This speaker, we thought. was perfect, till we made another one. This last one is similar, only we made the pot and core of steel, and the pot larger, to take 10lb. of 20ft. enamel wire, and we also connected to this speaker a dry rectifier, and made a small stepdown transformer. We are putting through this magnet coi] 15 volts. The moving coil has 1000 ohms The enamel we used for this was taken from a Ford spark coil, and looks about 40 odd. . The moving coil was made on a collapsible former, and every 200 turns during winding we gave a coat of ami: lacitate of celluloid. After leaving to dry for a day the former was taken away, leaving a strong self-support-ing lightweight coil. The cone is about 8in, in diameter, and diaphragm about 3in. wide, made of packanette, or, in cther words, rubberised linen, The gap for moving coil is 8-82in. The speaker is about the last word in reproduction, and still we are not satisfied.

I was greatly interested in your article in the April 12 issue, referring to matching impedances for dynamic cones. We would like to build a new audio amplifier, the best possible, to be worthy of this speaker. What do you say to Ferranti 5 transformer in first audio, followed with Ferranti push-pull, and using 2-250 valves in P.P. with 450 volts on plate, or would you recommend using 2-210 valves? The impedance of the 250 is only 1800, while that of 210 is 5000, and this, of course, in push-pull would be doubled. A.: Use two 210’s, two 250’s would cause complications. Do you recommend the matching of impedance with high resistance, or step down transformer?