ANSWERS TO CORRESPONDENTS.

Radio Record, Volume I, Issue 21, 9 December 1927, Page 12

ANSWERS TO CORRESPONDENTS.

(By

Megohm

Browning-Drake, R.J. (Wellington) had a B.-D. with close-wound coils, and was unable to tune out 2YA. He has wound lowloss coils and made improvements according to specifications, aud can now tune in either IYA or 3YA_ while -Q@YA is on the air, without a trace of the latter, and without a wave-trap. This indicates that good workmanship has been put iu, and also shows clearly how owners of unselective sets can greatly improve them. ‘The writer wishes to: give the article you mention at an early date. A South Canterbury reader, noticing that a northern reader intends using 199 valves all through his set, states that, in his experience, he finds that there is a marked falling-off if a voltage below 60 is used for the etector. He also finds that a total voltage of about 120 gives best results, This reader also recommends numbering the valves and changing them round, say, once a fortnight, so that each valve gets its turn in the casy positions, thus all retain sensitivity much longer. ° j.T. (South Canterbury).-It is probably high capacity in the R.F. valye that prevents you from tuning below 20 degrees. Yes, some wonderful results were obtained with the high-priced valves of a few years ago. A. (Invercargill) -Your B.-D. should be very selective if made according to specifications. Reduce the small aerial condenser for greater selectivity, and increase height of aerial if possible. Noises may be caused by bad contacts, loose connections, faulty B accumulator, or leak-

ages across same, faulty fixed condensers, or from nearby power mains, or many other causes, but vou do not say enough to enable it to be located. installing a Set. A.M, (Rangiora).-Your proposed aerial would be very good, and’ the full 200 feet would be worth trying, as the height will make up for any smatl loss of selectivity. A strong aerial wire and stays to the poles will be required, No waye-trap should be necessary if your receiver is ‘"‘selective."’ "As you are so far away from a charging centre, get the dealer to supply valves of low current consumption, so that the A battery will not need frequent charging. It is not the purpose of this column to recommend particular makes of receivers. A.good fourvalye set will get all that you require for entertainment, New Zealand and Australian main stations .on londspeaker, or the latter sometimes om headphones. . : Charging With Daniell Cells, Correspondents are asking if the Daniell cell system will keep charged 40 ond &0-amp. hour accumulators. It is clearly stated in the article that the output is limited, and, though it would charge up the small Exide cells, it should not be expected to do more. Then there is the question of consumption, which must be strictly limited to half-amp., which means that valves must be carefully chosen te suit. : Plate Current of Valves, ‘Rongotai? (Wellington) .-Certainly a UX112 power valve will require (Continued Next Page.)

Mainly About Construction (Continued from Frevious Page.}

more plate current at a given voltage than will a 201A. ‘Ihe difference is as follows, correct grid bias being shown in parentheses:-At a _high-tension voltage of 135 (9) the plate current of 201A. is 2.5 milliamps, and at 90 (4%), 2 milliamps. With a voltage of 135 (9) the UX112 takes 6 milliamps, and at 90 (6) the plate current is 2.5 milliamps, It is thus seen that if the high-tension voltage is kept down the difference is not great, but it would not be worth while to run a power valve at less than 90 volts, because it would not be running under advantageous conditions. It would pay you to discard dry B_ batteries, as you could either charge an accumulator or run an eliminator from the mains. Then you may put 150 volts on the plate of UX112, with a grid bias of 10 volts. Crystal Queries. J.B. (Kilbirnie).-The Record" erystal set is described in the issue of August 19. W.H.P.-It is not feasible to use more than two carborundum detectors in one set, and then only by a special dual arrangement. | H.M.J. (Main ‘Trunk).-The grid and plate of each PM4 are connected together for rectifying purposes in the B eliminator.