Questions and Answers

Radio Record, Volume II, Issue 44, 17 May 1929, Page 12

Questions and Answers

Coil Length. R F.B. (Wellington): Yes, your coi! *should tune to the band mentioned. Transformer Ratio. "K A.D." (Kelburn) asks whether ¢ 34-1 is the correct ratio for an inter-valve transformer.-Yes. . Microphonie Detector. ~ "Tr ICENSH NUMBER 129,280" has been troubled with a microphonic detector (A415). He is using a threevalve short-wave set with two A4l5’s and one B406. A.: The amplification is too great, and feed-back is the result. Try A409 in place of 415. Impedances Not Matched. "TX W.S." (Auckland) finds that by us- * ing two 605’s in push-pull he cannot prevent blasting, A.: It appears that the impedance of the valves is not matching that of the speaker (a magnetic cone). .Try two 608’s with greater grid bias. Amplifier Circuit Wanted. bd J.H." (Napier) asks for details of * amplifier and wave-trap. A.: A good amplifier was described by "Pentode" in our. issue of November 2, and a combined wave-trap, detector, and erystal set described in November 9. Choke Specifications. WoovrD a tapped tickler serve the purpose of a choke in a short-wave set? writes "J.H." (Wellington). : No; very many more turns would be required. A: tickler has usually about 25, a choke nothing less than 100.

The circuit recommendations are a 6005 condenser for tickler control. Would a .00035 be suitable? A.: Yes; though .0005 would be slightly better. Transformer Particulars, * A Vv." (Christchurch) wishes to ob- * tain particulars of a transformer input 220 to 250 volts, output 22, to 435 volts, providing a filament winding of 1.5 volts, 2.25, and 74 volts, and © -voltage. He asks would a transformer for 250 volts coupled to an American. transformer of 110 volts be practicable? A.: The correspondent will find that the question of transformers has. been dealt with very fully in an article in the "Radio Listener’s Guide," entitled "Small Power Transformers, from A-to Z," and "Building a B Battery Bliminator." As to the latter part of the question, No.. The same correspondent asks where stalloy may be obtained. A.: It can be obtained ready cut from almost any dealer, but Johns, Ltd., Auckland, carry extensive stocks. The Question of Wireless Aerials. CORRESPONDENT has written, asking if his aerial passing over electric mains, is permissible. A.: Regulation 5HA (1) states: "Wireless aerials shall not, without the consent of the licensee for the supply of electricity concerned, be erected above or below wires used for the supply of electricity, or sufficiently near to such

wires to permit of contact with them should either class of wire break or become ‘detached from its support, or its support fall." So it does contravene the regulations. Biasing R.F. Valves. "CB (Nelson) inquires. whether /* there is any advantage in biasing the R.F. valves of a 2-R.F. BrowningDrake? A.: Other than reducing the "B" current, there is little advantage, unless it makes the: receiver slightly more selective. . The correspondent suggests: "I have a cireuit diagram showing the connections thus: The frame and moving plates of the condensers are’ insulated from the shield and the ground end of the ‘coil comes.to the moving plate. Grid bias is applied between. the. moving plates and "A-,". with the negative: to. the moving plate and the "C plus" to. the "A" ‘There is a 1 mfd, condenser from the moving plates to the ground or "A. Would this affect the tuning range of the coils and condensers?"No. 2ZF, Palmerston North. "J A.B." (North Auckland) wishes to * know the nights 2ZF is on. the air. He states that when tuning in last Wednesday night he accidentally logged him: and wondered who it. was until the announcement was made. He heard one or two items very clearly .on the speaker, but then they faded,. Sunday. night he could hear them faintly on the headphones. * A.: Station 2ZF, Palmerston North, operated by the Manawatu Radio Club, power 20 watts, wavelength 285 metres, broadcasts Sundays and’ Wednesday nights. Centre Tapped Primary. AN inquirer from Roseneath, Wellington, who is contemplating eonstruct-

ing a transformer described .in the "‘Radio _ Listeners’: Guide," ‘has to work from the 110° volts. power.: -He wishes to know the adaptations’ to be. made. A.: In our Questions.and Answers page in. Volume 2,’ Number. 40, this problem was answered for a correspondent, It may be remarked in addition. that great care must. be exereised in :thoroughly insulating each half of-the primary winding fromthe other half,- particularly the finish of the. first and the commencement.of the second. These are carrying the full voltage potential, and if not thoroughly insulated one from the other, a short circuit will result. Trouble with Morse. "QUBSCRIBHR" (Central Otago) comPlains that he is troubled with morse on practically the whole of the broadcast band. . . A.: The disturbance is evidently caused by either a ship or an amateur. In the case of the former the trouble will cease when the ship is far enough away. If the correspondent can read morse or has a friend who can read morse, he should identify the station and lodge his complaint. to the district telegraph engineer. Hetrodyning 3AR, "GOULD you tell me the station which causes a whistling with 3AR Melbourne," writes "Satisfied" (Westport). . A.: In all probability it is WDAE Florida, operating on the same wavelength, viz., 484 metres. , _ The same correspondent inquires: "Would. a short aerial give as much volume and less. static than a long aerial?" A.: It would collect less static, but the signal strength would be much reeed. — . .

Elementary. Points. . "L W." (Lower Hutt) writes: ‘"Hear- * ing the anouncement over the air that advice would be given to beginners in the matters.of care of sets, ete., 1 am asking the following questions :- "1. What would be the necessary height of an aerial in this district to bring in the Australian stations?" — A.: Logging outside stations is a com. bination of three factors, -The set and the operator’s- skill, the aerial and the earth, and atmospheric conditions. ‘The correspondent ‘does not state the type of set he is using, but a three-valve set with a good aerial should bring in these stations on the loudspeaker. Aerials were fully dealt with-in our issue of December 14, and again in the "Guide." "2. Would an ordinary piece of water pipe form a good earth, and what is the required length?" A: Yes, it would be quite satisfactory’ if from 4ft. to 6ft. in length, but see also our special issue December 14, for fur ther particulars, 3. Would a 3 or 4-valve regeneratiy set be.a good proposition in the. atter | of price, economy, and purity of tone?’ A.: Yes, an excellent proposition, A set such as this is capable, usually, of bringing in Australian or even Japanese stations, Judging by our advertisements, 3 and 4-valve receivers will be very popular this season:; Watch. our. "Laboratory Jottings" for reports, . Can you refer me to any Wellington firms who make a speciality of supplying reliable kit sets? A.: Yes, glance through our advertisers. Several are displayed in shop windows. 5. Are dynamic speakers better than the horn type, and can'they be worked from an ordinary set? , A.: Yes, they are, superior, but are more costly, both initially and in operating, though if a good amplifier is used with the set the cost amply.repays in quality. ; 6. Are the new power operated sets. better in purity of. tone than the battery . operated ones? . A.: Generally speaking, yes, aa greater voltage can be applied to the power valves.

The 2-R.F. Browning-Drake.. "«N §.B." (Christchurch): has not had * a great deal of success with the 2-R.F, Browning-Drake, the difficulty being with the neutralisation. He proposes altering it according to the specifications in the "Radio Listeners’ Guide," and asks the following questions +- 1, Using two 609 valves in the R.F., how many turns should be wound on the slotted primaries used with a secondary with a 2tin. former?-A.: About 23. 2. What method of reaction control do you recomme:d, and. will you give instructions for fitting it. I have not room to fit a controlling condenser." A.: The moving coil can be used as wes described in connection with the four-valve Browning-Drake, described on, pages %1 to 7% of the "Guide." \ Is the R.F. choke between thetitickad ler and the first audio transformer of the’ same proportions as the other two, and if I decide to purchase the R.F. chokes should I specify the sizes? A.: All R.F. chokes are of practically the same specifications, and. so it is not necessary to specify more than "R.F. choke" when ordering. 4, Whieh would you recommend-the 2R.F. Browning-Drake or the adapted sereen grid Browning-Drake? A.: If simplicity of operation rather than selectivity is desired the ‘screen grid is preferable. Selectivity combined with distance getting is the. outstanding feature of the 2-R.F. B.D., but it is rather more difficult to tune than the sereen’ grid set. See our article in this issue. ; Note: Constructors who contemplate constructing the two stage R.F, Brown-° ing-Drake as described in the "Radio Listeners’ Giuide" should note that the filament end of the secondary, of the first radio frequency transformer * should

be attached to "A-" of the second R.F. valye, not "A-+-" as shown, The lay-out diagram is correct. Concerning a One-valve Set. "J P.B." (Petone) has constructed a * one-valve all wave set, and although he can get several stations off the ‘phones is unable to get Wellington off the loudspeaker,. though, he states, the original. specifications claimed Australinn stations on the loudspeaker. _A.: A one-valve set rarely brings a local station. on to a loudspeaker unless it .is a very sensitive one, and this is no doubt what was used when the original test was made, The additional audio stage should give good reuits.y. If the local station only is required at good speaker strength a crys_tal detector with two valyes as amplifiers will give the best results.