Questions and Answers

Radio Record, Volume III, Issue 9, 13 September 1929, Page 30

Questions and Answers

Condenser Control Reaction. ((QovLp you publish a circuit of "Pentode’s" crystal and valve, using condenser. control reaction? "R.J.H. (Christchurch). : ee A.: Construct the Tetrode crystal and amplifier described in our issue August 9, 1929, but if you prefer to do so, use ‘the three-electrode valve in place of the tetrode. Logging the Japs. HY.cannot we get the Japs. and . ¢he Americans? asks "K.J.S." (Blenheim). . A.: We cannot say! You have given no particulars of the. number of valves you are using, the voltage applied to the valves, the state of your batteries, or the condition of your aerial and earth. Crystal Set Problem. Gourd a erystal set be constructed to receive short-wave? asks ‘C.W.V." (Wellington).-No.

2. On what band do the commercial and ship Morse stations operate? A.: Below 500 kilocycles, or above 600 metres, . List of Stations Required. OULD you give a list of some highpowered American stations? asks "Listener" (Southland). A.: Such a list was published in our issue of January 4, 192 Artificial Static. ‘ "QATISFIED" (Westport) asks the following questions: 1. Will old valves make a receiver howl when tuning in? A.: Very rarely. Only occasionally do old valves develop unusual symptoms. ‘The general sign is all-round weakening. 2. My set is reproducing artificial static but. my "B" and "C" batteries are new. A.: See our article on interference. 38. My "A" battery positive plates are seratched. Would this cause the static?

A.: It will probably denote that the battery is worn out, and this could account for the static. How may I test my transformers? A.: Connect a pair of telephones (or a loudspeaker) to a battery, so that one terminal of each is free. Touch these two terminals, and a loud click will result. Place these on the terminals of the transformer. to be tested, and a click should result when the two primaries or two secondaries are touched. ‘The two primaries are denoted usually by "B plus" and "P." If a click does not result, or if a click results by connecting one primary to one secoridary, the transformer is defective. The Browning-Drake. HICH is the better set to construct, asks "K.D.R." (Kaiwarra). The §.G. Browning-Drake, or the 5-valve shielded Neutrodyne? A.: For tone theNeutrodyne will probably will be better, but for best allround qualities the two R.F. BrowningDrake, for simplicity, the A.C, Browning Drake, which is best. . An Output Filter. a" J.J." Auckland) sends a sketch of * an output filter. He asks which of the leads is plus and which is minus. A.: Once having passed through the output filter it is immaterial which side’ is connected to the speaker. Valves for Testing. HAYS ruined three valves, states "N.D.C." (Auckland), I wish to use the base of one in conjunction with an electric flash-lamp as a safeguard. A.:. As you suggest,-cut away the glass of the detector valve and solder the filament leads to a 1.5 volt torch battery. This would blow out when more than about 2.5 volts were on the filament. Distortion with Fading. I HAVE a good log, writes "J.B." (Murchison), and wish to strengthen’ the reception of weak stations with little cost. I have a good aerial and earth. A.: There is little you can do without adding another radio stage, which would cost you probably £1 10s. 2. When the station fades, distortion occurs. Is this usual? A.: A certain amount of distortion is always caused when a station fades, but it is rarely perceptible to’ any more than the trained ear. However, it appears that you do not appear to have enough grid bias on the last valve. Increase this to about 12 volts. 8: Can you give me any information regarding the Air Line 6? I would like a circuit diagram, A.: We are sorry, but we do not have details of this set. Probably it is a six-valve neutrodyne. A Request. As you are going to publish an allelectric edition, would you kindly give an article on the construction of an "A" eliminator for a two-valve amplifier? writes "G.H." (Wellington). A.: With the greatest of pleasure. Burnt-out Rectifier. "G HJ." (Mangaweka) has found that * his new gaseous rectifier has burnt out in six weeks, whereas the one pre-

vious to this lasted for ten months. He states that he discovered that ‘his ‘"C" battery had been disconnected. A.: Unless the rectifier was built to rectify less than 50 milliamps, per hour, the fact of the "C" battery burning out would haye no effect. The defective,"C" battery means that the power valve¥was drawing its full rating of current, which may, in some power valves, go up to 50 milliamps. However, the usual filamentless rectifiers would deliver over 100 milliamps. It appears rather that the valve was defective. Volume Control. "S 0." (Ngaio) wishes to know how he | ° might control volume without spoiling the quality of his two-valve amplifier. A.: Secure a variable resistance of 0 to 10,000 ohms and place this between the aerial and the earth. ‘Battery Charger Problem. How ean I use a vibrator charger delivering 6 volts to charge a 4-volt or a 2-volt accumulator, when only one tapping is provided,. asks "R.H.R" | (Southbridge). A.: A very heavy resistance of } oh in the case of the 2-volt accumulator, and a 4+ ohm. in the ease of the 4-volt accumulator, must be placed in series with one of the leads to the accumulator. The carrying capacity of this must be at least 8 amps. The charging rate would also be reduced. Linen Diaphragm Speaker. "TT S.E." (Blenheim), asks. if window * blind material will be suitable for the linen diaphragm loudspeaker, but it is rather too heavy. Suitable linen materjal ean be obtained from George and Doughty, Victoria Street, Wellington. 2. Where can I obtain a suitable unit? -tTry Smyth and Co,, Victoria Street, Wellington. ‘ 8. Is the enclosed circuit correct?-~ Yes, from what we can see of it. The Two R.F. Browning-Drake. wit the Two R.F. Browning-Drake ° _work from an electric B eliminator? requests "Newcomer" (Cambridge). A.: Yes, very well, because hifSyoltage can be obtained for the last Palve. If filament winding are provided the power for this valve can be drawn from this source. 2-Is it possible ta construct the radio stages and detector, and pick up the main stations with ’phones. I wish to get this part of the set working before adding the audio stages. . A A good idea. It will work well. 8. Where can I obtain an illustration of the Two R.F. transformers showing the methad of mounting? A.: Tuning coils were fully discussed by "Megohm" in our issue-of July 5, 1929. A diagram igs given on page 30 showing precisely how this is accomplished. } 4, Which of Philips valves are suitabye for the R.F. stages, and what is ‘the number of turns required in the primaries, and what is the value of the rheostat to control them? , A.: 609’s with twelve turns on the primaries, that is providing the tuning condensers are .0005, and the secondary

-_---~ 56 turns. A rheost&t will be unnecessary unless for a volume control. { 5. What ‘B voltage is required for the ; zuitto frequency. valves and the detecor ain \ A.: The voltage varies, usually about 60 for the R.F., and from 45 to 224 for detector, =. — Sereen Grid as Detector, "S W.?’ (Southland) asks :-~ 1. Would a screen grid work as a detector and give approximately maximum amplification ? ; A.: Sereen grid as a detector has been tried out in only a few cases, and it has generally proved less satisfactory than the ordinary triode. 2 Would the screen grid require a tickler? A,: No, it is impossible (so far) to use regeneration. with this valve. Would it. detect on short wave?0. { 4, Would the A.C. impedance be quite suitable for ordinary transformers? YF: No, a resistance of 500,000 ohms in the anode would be required. . 5. Would two stages of resistance coupling work as push-pull? Not. satisfactorily; by doing this resistances are placed in parallel, their value lowered, and. the efficiency lost. Changing Valves. Lf Y five-valve Neutrodyne is equipped with American valves, which seem to have lost their emission, writes "Beginner" (Bunnythorpe). Could you advise me what valves to use to bring my set up to date? A.: You cannot improve on the valve eombination you already havé, unless you wish your set to handle greater volume. In this casé, use the 171A in the last stage, with 20 volts bias at 135 volts anode. ‘The use of this valve, however, will slightly drop the volume level. You should use 9 volts bias on the 112A. qeour aerial and earth are quite O.K.