Questions and answers

Radio Record, Volume IV, Issue 16, 31 October 1930, Page 12

Questions and answers

POWERLDAK (Trentham): The district is notorious for power ‘leakages. Could I use a frame aerial on my four-valve a.c. set? ; . A.; No, you must have designed set to use a frame. You could use on indoor antenna but this would greatly weaken the signals. Try a ecounterpoise earth, a different earth, a shorter aerial, altering the direction of the aerial or an aerial slung between the. ceiling and the roof. W G.S. (Wanganui): I wish to miake * an all-wave set. from the Bestaways’ description. Would this set be suitable for New. Zealand conditions? ‘What grid bias would be required? A.: We cannot tell you anything about the set until we have seen the circuit. The amount of grid bias will vary according to the valve used. 2, What would be the size of the "B" batteries? ~A,: About 90. volts on the . second ‘valve aud 45 on the detector. . . R H.B. (Auckland): How can I take * grid bias from my eliminator? — A Separate the "A" battery from the *B" ‘and join "C-" of the valve to be provided with bias to "B-". Now take a 8000 ohms variable resistance, attaching one side to "B-," and "C-," and the other to "A-" (if the usual connection is "A-,"’ "B-’; otherwise "A plus’). Sunt a 1 mfd. condenser across the resistance. 2. Could an "A" battery charger supplying + an amp- be used with a set drawing .4 amps?-~Yes, 8. What is the aerial length. equivalent to a Ducon? A.: One cannot say definitely, but it

is slightly better than a good indoor aerial, L S.P. (Gisborne) : Is the enclosed cir. * cuit suitable for a phone transmitter and does it comply with the regulations? os A.: If you are interested in amateur transmitting you should write’ Mr. S. Perkins, Puru Crescent, Lyall Bay, Wellington, and he will tell you all you want to know about getting started in the amateur game, ONDENSER. (Temuka): When testing my condensers I get a series: of clicks in the phones, but when these are substituted by a sensitive meter -there is no reading. Are the condensers all right? . A.: A more satisfactory test should be ‘made. Use, say, 90 volts and see if there is any reading. The .0001 condenser is probably in good condition, but you cannot test the larger one until you apply a higher voltage. ~ 2. Nearby power lines quite spoil my reception. Do you think short-wave reception would be better? A.: Yes, power line interference is worse on the broadcast band than on the short-wave band. ue: (New Plymouth): Music and speech are blurred when my set is called upon to handle any volume. It has been in use about fourteen months, and the valves have not been renewed. The ‘detector is a 227, there are two 201’s and UXi112. A.: We cannot quite understand the valve-combination, As it is a three-valve set one is evidently a rectifier, and -from what you state it appears that a 201A

is used for this purpose. If that is the case another one ‘should be tried in the rectifier socket, as it is probably exhausted. Is the bias on the last valve appro-. priate? If your valves have been in use 14 months they will want renewing. If’ the 112 is used for a rectitier it may have to be renewed. Bone ‘(Waikato):- Would an aerial with: a span of 100 feet and a 40 feet lead-in be improved if the lead were. enclosed in @ small- water pipe? A.: No; the lead-in should be of heavy insulated wire: If the water pipe is brought near the lead-in it is, in effect, reducing the effective height of the aerial without any compensating gain. 2, Could a. long aerial be used with a superheterodyne?. A.: ‘No},a super het. cannot be used with any type of outdoor aerial. $3. Can several filaments be run from the same winding of the transformer? A.: Yes, providing the windings will carry a eurrent to be supplied and that coupling between the stages will not. be the result: of joining several filaments together, 4, Are the direct coupled satisfactory if the r.f. and detector valves are on the same filament ?-Yes, Note--Thank you for your book of diagrams, We shall return it to you shortly, wed "TRICKLE "CHARGHER (Wellington) : Will you supply details of an output filter choke? : ’ A.: In our next issue we are same from the. "Listeners’ Guide." 2. Must: thé construction of a filter choke yary.when used with different output valyes?-No. PUSHPULL (Kelburn): What is the eapacity of a variable condenser with two moving and three fixed vanes? ' A.: If it is a standard sized condenser it will be a..0001, 2, How might the area of the plates be ascertained? A.: Make a sketch of them -the exact size and divide up into small squares of a known area. Count the number of squares and multiply by the area of each, a 8. Where can I obtain a copy of the "Listeners’ Guide’? Why don’t you print a few more, they would sell like ice-creams in "t’other place"? A.: We do not know where you can obtain a "Guide," and it would not be. worth while reprinting; as the metal’ has long since been: destroyed, and the 1931 "Guide" will, however, be out very much earlier than last year, and we can assure everyone that it will be even better than the 1930, : G L. (Inglewood): I have constructed | e a 2-r.£, Browning-Drake and found it remarkably successful. I am interested im your statement that the screen-grid model will -be coming: out.. When ‘will , this be? A.: We cannot state any definite time, | but it will not be very long. N B.B. (Wellington) : Could the adap- * tor, a diagram of which is ‘enclosed, be used with a Six valve a.c, set? — 2 . mma naman nmnnanna

2. How does .the detector function when used in the adaptor? A.: It. cannot function, for one-is a i.e, and the other. a.c; 8. Must the list of parts be adhered. to? . . Pa ® » A.; No, so long as*you use good parts that is all that matters. 4, How could this set be used with phones? : A.: You could use it as a short-wave set by providing ‘separate batteries, QGHORT WAVD (Johnsonville): Can I use the short-wave set, a diagram of which I enclose? A.: Join the primary and the secondary together for stable operation, Use a 10 meg. grid leak and a .0001 grid condenser, increase the. detector voltage to 90 volts, or.changé, the detector valve. A good detector 9n broadcast is not always successful on short-wave, 2, How can I test. the choke and fixed conilensers? , A.: Hither with the phones and cell or voltmeter and cell, described a week or two ago, UUtRA (Woburn): I. enclose a dia-, . gram of a set. Would it. be superior to a circuit using a periodically-coupled’ r.f, amplifier? co, A.: Yes, in that harmonie can readily be eliminated, . 2. Do you approve of the method of coupling between detector and r,f.? The gain is not as great as transformer coupling, but ease in. handling will compensate. Lt. 3. Have I used a surfeit of by-passing? A.: No, it is an excellent practice, and use condensers’ wherever possible. 4. What voltages must be applied (1) to the plate of ‘the s.g, detector valve and (2) to the sereening grid of the same valve? te A.: 135 to the plate, 674.to 90 to the screening grid. 5. What should be the value of. a fixed grid condenser?-.0001. . I propose winding my own coils on iin. diameter tubing ‘to. be fitted

into valye bases, The coils ‘will cover a range of 14 to 100 metres and 250 to 520 metres. Condensers are .00016. A.: Metres. Sey. Coil. Tickler. 16- 30 5-14 5-26 29- 58 12-16 9-26 54-110 22-16 15-26 103-225 45-26 24-26 225-350 115-80 12-34 300-550 195-32 50-36 All turns spaced except tickler, which is close-wound, spaced a jin. from the secondary eoil. ‘The figure after the hyphen denotes the gauge of wire. PAPER (Dunedin): I have a. sevenvalve a.c. set and although I am in an excellént position I cannot receive the American stations. A.: We cannot really help you. If you could get your aerial up another 5 or 10 feet you might: have a_ better ,chance. When your valves are begin- * ning to drop off and you want to renew { them, try standard "Radiotrons"-they i might suit your particular. set better. 2. Is an aerial effective if it has been eut and soldered? A.: Soldering certainly decreases the efficiency, but not greatly. 3. When I tune the local station I get more volume with the control about half-way. Does this‘ signify anything wrong? A.: No. When the volume control is full on the set is overloading and choking.

MATEUR (Port Chalmers): During rain a drowning crackling noise mars reception. A.: Try vemoving the aerial under such conditions and you will see if the noise is in your set or in the aerial. Jf it is in the aerial examine this and the earth very earefully for short circuits, for it is evident that the rain is making a good path to earth. W A.T. (Wellington): I have an a.c. * set, but find the station distorts on all but three stations. .: As it is a commercially-made set, we cannot advise you to do anything other than have the two 250 valves tested, They may be unbalanced. 2. Where could I get particulars of a short-wave a.c. adaptor? A.: Write Mack’s Radio, Kent -Terrace, Wellington. 8. My aerial is 100 feet long, 30 feet high. Would two wires or spreaders be better? A’: No, your aerial is now at maximum efficiency. 4, Can I have my valves tested? A.: Yes, any local dealer will test them for you. oy NXT (Hamilton): I_ have constructed a two-valve’ set, but I get better results from a one-valve. . A.: Probably the audio transformer is at fault. Are-you quite certain you have it connected in the right way? Have it tested with the voltmeter and ‘cell method. Volts (Motueka) : I find my electric plant seriously interferes with radio reception. I have connected two 2 mfd. condensers across the line, but it is still ad. A.: See our article on interference published in the "R.R." of Sept. 13, 1929, H.® = (Wellington): What value» * fixed condenser will be necessary to reduce .0005 to .00016?-.00025. J H.W. (Lower Hutt): I had in mind * eonstructing the Loftin White, but note that unless the resistances are practically correct the results. are indifferent. Will a cirenit employing the screen grid valve transformer coupled to a 245 be better? A.: You need have no worries about the Loftin White. If you get your resistances from a reliable dealer they will be auite OK. The only resistance likely to give trouble is the one in the grid circult, and this can easily be replaced. It ‘will be far better than the cireuit you send along, which would probably distort and world not handle a great deal of volume. Note that the filament voltage on the 245 is 2.5 volts, not 5 as you‘ have marked on your diagram. 2. Would the combination shown be better than 201A used with a 171? A.: Yes. but it would not be particularly brilliant. You need push-pull or Loftin White for satisfactory gramophone amplification. OSBY Parker (Christchurch): We have heen interested in your experiments with the pentode valve, and in a very short time will have a very comprehensive article on the subject which should interest you. AY DEO (Waikato): Concerning your diagram of a circuit using 2 sg. and regenerative detector. Would it be suecessful if the three tuning condensers were ganged? A.: Yes, but connect them on a shaft parallel to the panel and shield each stage. 2, Would it be necessary to have frimmers on two of the condensers? A,: Not unless you want really maximum efficiency, 8. I have logged 60 stations on my present set, but it is unselective. Can I reasonably expect hetter results with two stages of screen grid?-Yes, 4, It appears that the reaction condenser would need to be insulated from the metal panel. Is this correct? . A.: In that circuit, yes. A better

wway of controlling regeneration would be to make‘the tickler coil distinct and connect one end to the plate as usual. Take the other end of the tickler to the tixec plates of the reaction condenser and the moving plates to earth. Another lead. from the plate to the valve goes through an r.f, choke to P of the first mudio transformer, 5. I intend. using ‘a stage of pushpull, .What would be the best combination? _ AS Out of the two mentioned there is very little, if any, difference. It is all a matter of choice. 6. Should I expect better results with this than with the Akarana Band Pass Four recently published?-Yes. 7, Could the coils be enclosed in a cocoa tin to assist in screening? A.: If each stage is shielded there will be no need for the cocoa tins. L J.B. (Otahuhu).-I enclose a sketch * of the property. on which I intend to erect my aerial. As you will see, there are electric lines close handy. Would a length of copper wire earthed screen my aerial from electrical interference?-No. 2. Could the aerial be placed in a better position ? A.: The best position would be in the vacant paddock at the back of your house, at right angles to the power line. The fact that it passes over the 6 foot hedge © will make very little difference. This is the position where there will be minimum pick-up from the power lines. . Would 90 to 100 feet of wire he about right ?-Yes, 4, I have for an earth, copper plates — about 9 inches square, buried about three | feet deep, and kept down by a galvanised pipe.-Good. R20P!I0 (New ‘Plymouth).-In answer to your and many inquiries, we are publishing néxt week details of. an audio choke, Reprint from the 1930 "Guide." H V. (Takapuna).-When can I hear ° the short-wave stations? A.: Usually in the afternoon and evening,. but consult the short-wave page conducted by Mr. Sellens. 2. Should I have the dials of my broadcast receiver at minimum or maximum when using an adapter? oo A.: As they are cut out of the it does not matter. how they are positioned. ONI (Gisborne).-Would a high-im-pedance valve be suitable for the yadio stages of my 5-valve set? A.: It should be satisfactory, though it is generally safer to use general purpose valves in the radio stages, as the yalve to which you refer is specially designed for impedance coupling. 2. Would three "B" batteries coupled in series run down quicker than two?No, HJ. (Mangaweka): In push-pull * does the "C" battery have to be doubled or is it the same as un ordinary stage? A.: The same as an ordinary stage. though it could be doubled. 2. Can two stages of push-pull be added to a set immediately after the detector ?-Yes. 3. Would PM5 be satisfactory for a first stage and PM256’s for the last? A.: Yes, but one PM5 would be just as good, as it could overload the two 256’s without itself causing distortion, 4, Can a transformer be obtained with a eentre-tapped primary and secondary for use between two stages of push-pull? -Yes, as far as we know, 5. I have plenty of "B" supply to spare and I want to use a good amplifier for a hall. A.: Under these circumstances why not use two 245’s in the last stage? You could put a filament winding on to supply the current and you would have plenty of "B" supply. You could then use two stages of push-pull. Better still, why not construct the Loftin-White Amplifier ?

Aka (Hataitai) ; Can phones be con- . ‘ nected to an ae, without having to undo the shielding to get at the output transformer? A.: Yes, you can take leads from the voice coils of the dynamic speaker, which is accessible These would have to. be light flexible wires, and although they would give less signal in the telephones than in the speaker, that would be an ~ advantage. . . : : AUPIt (Auckland): I have recently added an eliminator to my ‘set and I cannot control ‘a whistling noise. A.: Are you sure the voltage is correct, that you are not applying more voltage than specified by. the makers? . You see by applying a higher voltage _ you may be causing oscillation and instability, It would not be a bad iden to have the eliminator tested in case any of the condensers are at fault. EPPUNCH (Christchurch) : I enclose a diagram of a receiver. Is it a good ‘one? A.: Have you really thought, "Pep: punch," how long it would take us to unravel your diagram? You should have sent along a theoretical diagram. However, the set appears to be standard. 2. What heighth, length and gauge of Wire would you recommend for an aerial for the circuit? A.: 70 to 100 feet long and 40 feet high, using 7/22 gauge enamelled wire. See the article devoted to aerials which appeared in our issues March 21 and 28, Is my transformer a good one? A.: Yes, it is one of the best of the lcwer-priced instruments. , Could an extra stage be added? A.: Yes, but we would advise you to: bias the last valve by inserting a "C" battery between "F" on the transformer and "-* of the valve. (OAMERA (Hastings) : I wish to oper4 ate a loudspeaker from a one-valye set. Would: one audio stage be sufficient? A.: It is very doubtful as you are a long way from a broadcast station. Two stages such as that described by "Megohm" in our issue of July 27, 1928, would be better. 2. What "C" battery voltage will I require ? (Continued on page 29.)

Questions and Answers --

(Continued from page 11.) A.: That depends upon the valve and the "B" battery used. You would find the bias: specified on the valve: carton. 8. Where would "C" plus go?-To A), 4, Could the transformer be placed correctiy if it were placed directly leading from the phone jack?-yYes, 5. What complete set of parts would I require if I had to add two audio. stages? A.: Two audio transformers, two valve sockets and a few terminals. 6. Is an earth consisting of a pipe driven 10. feet into the ground and kept filled with water sufficient?-Yes. Note: We congratulate you on the neatness and legibility of your letter. We wish all our correspondents would set out their questions so precisely and neatly. It makes answering them ever so much easier. .