A Corner for Beginners

Radio Record, Volume II, Issue 42, 3 May 1929, Page 13

A Corner for Beginners

Information for Readers, \OF late certain requests for tables and special information have been. coming to hand, and. are being ,dealt with through the "Beginner Corner." : Readers should submit requests for any particular topic they would like -discussed to the Technical Editor, who’.is always ready to assist readers, whether it is in their individual problems or in wider fields, as in the "Beginner Corner." Microphonie Valves. EXT to. valves whose emission have fallen off, microphonic ‘valves give the most trouble. The detector valve is normally the worst offender, because the audio-frequency variation of plate current set up by mechanical vibration of the valye elements is amplified through all of the audio. system. While that same valve may not appear to me microphonic to the degree that it produces a howl, when placed in the first audio socket, it is not wise to do so, becuuse the condition usually grows worse and within a short time the amplitude of vibration can become sufliciently great to produce sound from the loudspeaker when amplified by only’ the second transformer and last valve. If the microphonie condition in a set is to be remedied by shifting valves, it should, therefore. never be done by simply exchanging the detector and first audio frequency valves, but always by selecting a quiet valve from one of the high frequency sockets. Sometimes proximity of the loudspeaker causes 2 degree of additional: vibration. In these instances moving it farther away removes the microphonic. condition which existed. In cases where.a separate loudspeaker is used, that remedy is always worth trying. In some cases, when neither moving the loudspeaker nor shifting and replacing valves will effect a cure, the placing. of lead weightsmanufactured for that purpese-on top jof the detector and first audio frequency valves may be effective. _* . If none of these remedies .cures the trouble, the only remaining one is to remount the socket. on sponge rubber, or other shock-absorbing material; and make flexible leads. In normal ecases,: when it is simply a case.of valves::themselves, no valve should be left in. either of the two critical sockets if the ring caused by tap-

ping it sharply with the forefinger is sustained for more than two seconds.-~: "Radio Broadeast." -Matching Impedances. | HOME constructors, and others who are concerned with good reproduction,. should pay great attention to the matching of impedances, especially the relation of the primary of the radio frequency transformers to the impedance of the preceding valve. Not only will correctly ‘matched impedances give better results, but the neutralisation problems will be reduced to a minimum. With the B.-D. the following table of primary turns applies, though its application is not restricted to that receiver. With valves, the impedances of which range round 15,000 (UX. 199, Ce-Co BX, PM3, Cossor 410 HF, Philips 630), the optimum number is 25 turns, The lower impedance valves require a correspond‘ingly lower number of turns. Those of, the order’ of 10,000 ohms (201 A, Cossor 410 LIF, and Philips 209) require 17 turns. Capacities of Condensers. E*PERIMENTERS who wish: to ob4 tain special capacities of fixed condensers by combining two of standard capacity will find the following table useful. Capacities are all expressed in miecrofarads. The first two columns contain the values of the two condensers to be combined, and the other’ two columns give the resultant capacity — ofthe combination. nes Separate Capacities Combination. ~

Numbers of Plates. ORRESVPONDENTS have asked for ‘the number of plates in the different values of condensers. Here they are--the approximate capacities only can be given, as size of plates vary: It is to be understood that the spacing between

plates is with one-eighth washers: 85 . plates, .0015; 57 plates, 001; 43 plates, 00075; 29 plates, .0005; 19 plates, ~ .0003; 13 plates, .0002; 7 plates, .0001. When a condenser has ‘only five plates it is generally termed "Vernier," and is used in conjunction with some other variable condenser. Fixed condensers run as follow, as-* ' suming the dielectric to be of mica: .002- « inches thickness; 7 foils, each 2 inches’ + x 1 inch, .001 mfd.; 14 foils, same size: : 002 mfds.; 21 foils, same size, .003 mfds.; 2 foils, half-inch by three-quarter. inch, .001 mfds.; 5 foils, half by three-, = quarter inches, .0002 mfds.; 8 ‘foils, 2".

.0001 0001 .0001 .0001. .0001 .0001. 00025 00025 .00025 .00025 .00025 .0005 .0005 and and and and and and and. and and and and and 00025 -0005 .002 005 0005 001 005 006 002 Series .000071 .0000838 .000093 .000095 .000098 .000098 .00016 0002 00022 .00023 00024 00038 0004 Parallel. .00085 .0006 ° 0011 .0021 0051 | 0061 .000T5 .00125 £00225 .00325 .00625 0015 0025. :

.0005 _ 0005 _ 001 001 002 .002 . 005 and and and and and and and and 005 006 002 005 006 005 006 .006