THE "RECORD" B BATTERY ELIMINATOR

Radio Record, Volume I, Issue 30, 10 February 1928, Page 13

THE "RECORD" B BATTERY ELIMINATOR

LETTER FROM A SATISFIED CONSTRUCTOR

A Christchurch reader set out to construct the B eliminator a while ago, and up to completing the transformer was successful, when a hitch occurred, the symptoms were explained to the writer, and a "short" was suspected, as_ subsequently turned out to be the cause of the trouble. Here is the constructor’s letter. "I have got the eliminator going at last, and the results are splendid, quite equal te the accumulator high-teusion battery I have. On two occasions have had continuous runs of approximately six hotirs, and the transformer was as cool at the end of the run as at the ‘start. The trouble with the transformer was caused by one of the "stampings" cutting through the insulation when pushing in the last few to get them as tight as possible, thus ‘shorting’ the filament winding.. Needless to say, this did not occur the second time. ‘Again thanking you for your kind help, and congratulating you on designing such an efficient instrument. "Megohm" congratulates this constructor "on his perseverance in face of bad luck. The construction of an eliminator is no small affair, and calls for care and patience from start to finish. But for the amount of taouble it will save when working, it is well worth while. Careful construction will bring its reward, and no "sling-together" methods should be attempted. J

The care required in putting in the last few laminations was mentioned in the instructions, and a recommendation was given to slope off the lower corners of the last few T’s. It would be a wise additional precaution if constructors would, as soon as the laminations begin to get tight, narrow the long leg ‘of the T’s by an eighth of an inch on each side to prevent any chance of cutting the manilla, which should be double thickness. It is important that the packing should be tight) as any loose-. ness tends to produce mechanical hum. Another point is to put each of the last few laminations in under the outside one, so that friction on both sides is taken by metal instead of by the manilla. If it is found that a short ‘piece of tin can be pushed in at either end to tighten the pile, this should be done. All sharp edges should be carefully rounded off the last few lamina-tions-it pays. This constructor was tunable to procure a Raytheon tube, and is for the time being using two P.M. 4’s as rectifiers. With these valves the total current consumption of the eliminator is about six watts. With a BH Raytheon tube the consumption increases to about ten watts, owing possibly to a small amount of current being by-passed between the electrodes in the tube. It is quite an easy matter to check the amount of current consumed by

reading the household electricity meter during the daytime when no other current is being drawn. The "hundreds" dial only need be watched. Each division on this dial equals ten watts. Take the consumption of the eliminator for one hour, and if it is efficiently constructed it should not show much more than one division (10 watts) on the dial in question. If only a little over half a division is indicated in one hour, | the highest efficiency has been secured. A thousand watts for one hour is one unit of electricity, for which the average price is sixpence. One unit will therefore provide ten watts for one hundred hours, and six watts for 166 hours. Quite a number of readers are wisely "taking their time" in building the eliminator, and the constructor in question is the first to report the finish of his task. WIRING UNDER BASEBOARD. In the issue of Octoher 28 a diagram was given of the wiring under baseboard, with figures for reference. As the instructions progressed, there appeared to he sufficient data to guide constructors without referring to these numbers. A correspondent has just Written asking for references to these figures, and as it took some time to trace out the numerous connections, they are given here as an additional guide to constructors. 1 to one side of all fixed condensers, and joins to 2. 2 to H.T. negative terminal on left of panel, . 8 to H.F. resistance: Other end to ccndenser 5, . 4 to detector resistances: Other end to condenser 4. 5 to L.F. resistance: Other end to condenser 8 6 to three resistance arms: Other end to condensers 1 and 2. 7 to condensers 6, 7, 8. 8, 9, to rheostat. 10 to FI.T. 1 on transformer (fuse). 11 to H.T. centre tap. 12 to H.T. 2 on transformer (fuse). 13, 14, to choke coil. 15 to filament centre tap. 16 to plate terminal valve socket. 17 to transformer panel (marked "rheo.’’), 18 to Fil. 1 on transformer panel. 19 to Vil 2 on transformer panel. 20, 21, to same filament terminal of each socket, 22, 24, to other do. 23 to plate terminal valve socket. S to tin case, Tin partition and condenser holders should le connected to the case by a soldered wire, at Sh, the _ ‘

0 a SUPER-POWER VALVES. When running a super-power valve in the last audio stage, an extra fuse and terminal can be provided below the rheostat, and through this the full high-tension may be applied to the plate of the last value, a 2 mgd. condenser being placed across this output in the eliminator. This method gives increased plate voltage for the last _ valye, and also reduces the work for the L.F. resistance. | A reminder is given that when the filament output is né longer required for rectifier filament heating, it may be quite successfully used to heat the filament of the last power-valve, provided that it has a robust filament to retain the heat between cycles. The P.M. 254 or 256 type work well on this system, but leads from the eliminator to the set must be twisted together to neutralise the a.c. hum, which might otherwise be picked up by the set. Alterations required for power-valye filament heating are described in the article. The eliminator construction articles appeared in seven issues of the ‘"Record," commencing Octoher 14 and ending November 25, 1927.