Questions and Answers

Radio Record, Volume IV, Issue 45, 22 May 1931, Page 13

Questions and Answers

L B.H. (Taradale): What kind of * valves should I use in the Sparrow Hawk One? A.: One of the 221 or 415 type. The new 230 valve would also be suitable, Sut it is more expensive. ISTANCH (Wellington): What type of short-wave adapter should I build for the set mentioned? A.: We suggest the differential adapter from the "Radio Guide." 2. What results should I get? A.: This is a good adapter, and with your set should give excellent results. 3. About what would the cost of such an adapter be? . A.: You would get it probably for about £3/10/-. H B. (Woodville): I am building the ¢ Diff. Four and a little at sea about winding the coils. Is this correct for a .00035 condenser: 65 turns of 22 d.s.c. wire on a 2in. former? If so, how many must I use for the aerial? A.: 20 for aerial, 78 for the secondary. 2. Using a .00035 condenser for the deX tector stage, would 9 turns on the 2in. ‘former be correct? If so how many should I use for the tickler with a .0002 differential condenser? A.: You require 87 turns for a .00035 condenser, with 25 turns on the reaction *tinding.. The tickler is usually the top winding on the detector coil. 3. In winding the five-prong coil socket for a detector coil should not the wire from the cap of the s.g. valve go to "G" instead of "F," as in the layout diagram in the ‘Radio Record" of April "AL: No; the plate terminal is taken to the top of the primary, which is "I."

The tickler is connected one side to each set of plates. 4, Have you any suggestions for improvement? A.: For broadcast use a special coil wound on a 2in, former. You will find it better. Your valves are quite in order.

REGULAR READER (Auckland): My a.c. set splutters a great deal between 850 and 1500 k.c. It is minimised but not. done away with by removing the aerial. A.: What happens when you remove the aerial altogether? It seems to be outside interference and this you can do nothing to overcome other than communicating with the district radio inspector. Your aerial and earth appear to be quite good though it would be better if you could get the aerial a little higher. L A.D. Colac Bay).-Is there any differ- * ence in using an electric set off a range from using the set off a light socket? A.: Not unless there are separate met,res, as in all probability there are. You cannot do any damage no matter from what point you take the electricity for your wireless set. but if you have separate meters it will be cheapest to draw the current from the range. The next best will be ironing point, and lastly the lighting socket. G ©.0. (Nelson).-I have constructed * the double push-pull amplifier degeribed in the "Guide," and it is working wonderfully. I now wish to‘use a.c. on the filament of the valves. What ones do you recommend in the first and second stages? They must be Wnglish. A.: Wither the American type Osram valves or Mullard’s PM104V in the first push-pull stage, and PM164V in the preceeding stage. WwW B.A. (Dunedin).-What are the best pare values of condensers to cut out agit : Use those specified for 2YA in the ui e,? 2. The condenser to cut out the local "B" station? A.: If you wish to eliminate both stations, you_had better use fonmodensers as decribed. 83. Would the wavetrap as described by "Megohm" in last week’s "Record" be more suitable?-No,

K N. (Auckland): I believe manufac- * turers are this year marketing superheterodyne receivers. Can they be used on an aerial? A.: Yes, new models incorporate the improved circuit which is allowed on the aerial. 2. Will you publish a circuit of a super het. that can be used from an aerial? A.: While not making any promises, we shall see what can be done in this regard. ° LA™"2 DOG (Petone)-I have constructed the knife-edge rejector from the 1931 "Guide," but not being able to obtain the specified condenser used the .0021 compensation type, and although I shielded the whole outfit I could not get the wavetrap to work.

A.: If the .002 condenser will adjust to a .001 capacity, as it probably will, we cannot help you. Hundreds of constructors have built the rejector, and been perfectly satisfied, but it is impossible to do anything other than tinker around with it until it works. Are you quite certain that because of the shielding you are not shortcircuiting the wavetrap? 2. About what would a filament transformer cost? 7 A.: We should imagine about £3. Cc H.W. (Invercargill) : I have-added an * yf. amplifier to my set, but it will act only as a wavetrap. A.: Your circuit is quite satisfactory, but it would be better if your transformer-

ORRESPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Box 1082, Wellington. Limit three questions, unias letter is accompanied by 1/ee, . Name of set ..... Tee eee seers eeeee Number of valves .cccsccscecceccves Name Address Seeeeeeesesestaceeeeeeesaee Se HSH RTE SEHEHEESSES ESS EHEEEHEEEERES ed ee ee ee ee el Nom de Plume cevecccosevevececes To be kept in subsequent inquiries. Date PYTITITITITR TTI TT TTT Please Note:(1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side of the paper. (3) We do not design circuits, but accept suggestions for feature articles. Solving trouble, as different from advice, is difficult by correspondence and while letters are given every consideration, answers are not necessarily correct--they are only our opinion based on. the matter, supplied, which may be quite inadequate, Intricate and involved specifications cannot be supplied without a specialist’s fee.

Notice to Correspondents. OME correspondents are not clear about our fee. We make a charge of 1/- for any series of questions more than three in number, but do not thereby bind ourselves to reply by post as several correspondents are intending us to do. Postal inquiries must be limited to three questions. It must be noted that the service is, as before, intended to be a free one. We regret having to impose a fee, but this is only because a few people have been sending in a large number of ‘queries which take much more time than we can afford on the service,

coupled the first stage to the next one. The transformer will consist of a separate primary of about 20 turns on a slightly smaller former placed inside the low potential end. One side would go to the plate of the valve and the other to B+. This would be by-passed with the condenser shown. 2. By neutralising the s.g. valve would all carrier waves, ete., be eliminated? _ A.: No, it is not worth while neutralising the s.g. valve. It is perfectly stable without any extra aids. N D.B.. (Tologa Bay): I have built the Ye Qutspan Five and now want a shortwave set to use with it. I am inclined toward a d.c. version of the super _ het. adapter. Would A409 or A415 -do as detector with A409 as oscillator? _ A.: Hither A409 or A415 will be ‘suitable for the first detector with A409 for oscillator. . 2, Is C8 fixed or variable? It is ealled a tuning condenser. A.: C8 is a .0005 mfd. fixed condenser. 8. Are L5.and C4 ‘required when the Outspan follows. A.: L5 and C4 are not required when the adapter is used in a set having a tuned "input stage, such as the Outspan. Do-not short the output of the adapter right out, as you have done in the diagram. The inclusion of L5 and C4 will always improve the selectivity of the set, but if they are to be omitted the connec tions will be as is shown in the acconpanying diagram. ‘ . Is the negative return in both cases best, and would the ordinary grid leak work as well with a plus or potentiometer return?

A.: A’ grid detector may be used if desired instead of a plate bend detector. The grid return should then be to A+. or, better still, to the slider area of a connected across the "A" battery. 5. The two stages of s.g. do not appear to give the lift they For efficient working with the s.g. valve should there not be more turns on the primary of the following coil? A.: You could experiment with a large number of turns. ‘The numbers we gave were as high as we found practicable.

Beyond that the set. begins to become unstable. Have the s.g. valve tested and we presume you have tried varying the sereen voltage. , 6. Is it safe to go up as high as 180 yolts on the plate?. . You give it in the "Guide," but the directions with the valve give 150.maximum. . A.: Valves are rapidly improved and we haye given the specifications of the latest valyes imported by Philips. Yours is probably a little older. ON (Christchurch): I have obtained improyed results with the Loftin Four by paying. particular attention to the output stage using radio frequency choke between the primary of the output transformer and the plate of the 245 yalve and the B plus supply. I have takén the leads to the speaker in metal armoured flex, but I cannot adjust the trimmer to the optimum position without squealing If the earth connections to the shields on transformer’ and loudspeaker is broken the set begins to'’squeal. I have not tried a choke in the plate lead of the detector or rf. yalve. I have reduced the primary turns without, avail. . The sensitivity ofthe ‘set is quite equal-to a well-known a.c. set using the same number of s.g. valves. ° A.: Try, as you suggest, chokes in the plate leads of the r.f. valve and a separate earth for the shielding and the aerial coil itself. Run the plate and grid leads of the amplifier in earthed armour ed flex. Have you adhered to the layout carefully, as if. this is not followed ‘instability must result? From the rough sketch you have sent us it appears that you have the 245 ‘valve and the power pack ‘too close together. The radio portion must, as shown in our diagram, be kept well away from the amplifier" | | -, . 2, In the meantime I am constructing the super het, adapter ‘described in the "Guide’? -I ‘do not see any, mention of the value of the tuning condensers C1. C2. As I want.to use .0025 condensers. to make .0001 condenser, is it safe to take the number of turns for the detector. coil from the list in the "R.R."? _A.: The tuning condensers specified for the super het. are .0001 and .00014. You can quite well use .00025 tuning condensers without any change in the.coil, but you will find that best-results will be obtained by: using only the lower part of the tuning range of the condenser. The tuning, particularly of the oscillator, wil] be very sharp indeed, and you will receive the same station on two, different sets of coils. If you make the adjustment in: the number of:turns referred. to this difficulty should be overcome, , ' ; -. 3. Would.a ‘25 mfd. condenser be suitable for the values C3 and C5, and is -the tuning circuit L5Cd necessary in connecting the adapter to the Loftin Four? A.: C8 should be a-.0005mfad. fixed. condenser, C5 a fixed condenser .0001 ‘mfds. ‘or smaller, ‘or a"midget Variable may be

used. The tuned circuit L5C4 is not aby: solutely essential with the set employing a tuned input such as the Loftin Four, although’ it is of advantage in improving selectivity. The Loftin Four. has insufficient r.f, amplification to give really satisfactory results with a super het. adapter. Furthermore, you may have trouble with instability. W A.R. (Martinborough) : I have con- * structed the super het. adapter; or rather a battery version of it, taking the grid return to."A-." Could you give the necessary modification for battery operation? A.: You appear to be working along the right lines in returning the grid to the potentiometer in the d.c. In the absence of the potentiometer the: return should be made to A+. The 1875 k.e, coil is designed, to cover the 160 metres band where there is very little to’ be heard. 2. The parts.are not named -in the "Guide." What is C3? A,: They have since been published auld appear in the second. edition. of the "Guide." C3 is a .0005 nifd. fixed. condenser. 3. Is the choke a shortwaye or broadcast? ; * . A.: It should be shortwave, but a good broadcast’ one will usually operate satisfactorily. ; ; 4,’ Should a super het. howl when brought into resonance? If not, why does it do this and how can be be suppressed? A.: A super heterodyne should never howl unless the receiver is oscillating. This trouble can almost invariably be cured by the adjustment of. the B-. yolt-: age on the oscillator and the detector. Reduce these ‘progressively unti] the howl ceases. Where a potentiometer is: used for the grid return adjustment. of this will often result in a cure; 5. Could the windings L2, IL3;. and L4 be made more clear? c A.: Since the 80 metre coil is operating so well copy the connections exactly

coupling coi] 12 sometimes requires ya ation’ when ‘an’ oscillator valve having | ususually high or low, output is used: "yp y varying the of turns on this, M.° G.H (Gisborne) ; Using 26 d.s.e. for’ the’ secondaries’ and" 32 for the primaries, what would. be the coil. specifications for the Outspan using 2in. formers with .0005° condensers? A.: This information has been published in the "Radio Guide." You will require 72 turns on the secondary with about 25 on the primary and 85 regeneration, 2. Would it be better to use .00085 conar Ws to-control the detector coil? : It is unnecessary. You could use anit three condensers the same. 8. Would .0002 reaction -condenser be suitable for .0005 condensers ?-Yes. 4, By mounting a detector coil in the valve base could I use a detector and two saat for shortwave? | Yes,’ providing you used a highgrade broadcast ‘choke. 5. My first audio valve gets fairly hot. I am using M4 in both first and second audio stages, ; for the coils for the other band, If n A

Data. for Moving Coil ‘Speaker Voice Coil *

: A.: You should not use power valves n both audio stages. By not suitably biasing PM4. you are placing a heavy drain upon it, and this causes it to heat. 6. What ampere load will a Columbia 4486 battery stand? |: , A.: About 20 mas. . ONSPEC (C. Otago): I have adapted my receiver to use pushpull in the last stage, but it now goes into oscillation with a loud grunting noise when previously. it slipped in and out quite easuy. A.: Try. reversing the connections to the primary of the audio transformer, and experiment with voltages on the detector. You might try double biasing the , power valves. A Bw. (Timaru): Will an earth wire * connected to a water main cause the set to pick up interference from other sets which are also connected to the iain ? /A.: It is possible that such will happen. ‘2, As an earth I have an electric power cable connected’ to a buried moitorcar radiator. The earth wire passes close to a water main and could ‘easily be tapped to same. Would it be advantageous to do so? . A.: The experiment would be worth trying. Normally it would be better to do so, but if you are likely to be troubled with interference, no. R478 (Hastings): Could you give me ’ the number of turns and gauge of. wire for. the voice coil of the dynamic speaker ? A.: Published this week is a complete table that should assist you. H F.B. (Wainoni) : I am troubled with * interference, apparently from leaky , tramlines. What can I do? \ _ A.:. You must consult the District Radio Inspector. If more than you are woubled have them sign a petition, If any difficulty is encountered, communieate with us again. (CURIOUS (Te Aroha),-Would another make of valve give me better results than the ones in my set? 'They are the same as the make of the set. A.: Probably not. It is always wise to use the valves specified by the manufacng The ones you are using are first grade.

| N=w READER (Newtown).-Is_ the above circuit of a Trinadyne correct? It gives me rather too much volume on 66P speaker. A.: Yes, it appears to be correct. If you are getting such good results, why wonder if there is anything wrong? MI-DLL (Dunedin).-I have constructed a crystal and two-valve amplifier in the 1931 "Guide," but cannot get satisfactory results. There is a very loud erackling in the phones. A.: This no doubt is a defective component. Wither a transformer or your phones. It may possibly be a resistance. Have these components tested. — F.B. (Roslyn).-I propose to install a ° pentode valve in the last socket of a two-valve all-wave combination, ‘Will it be satisfactory? A,: It should be, but read the comments made by Mr. Dawson in our paper a few weeks ago. 2; Is an output filter a necessity?

A.: It is not essential but preferable. 8. Can raw a.c. current be used upon the filaments of the pemtode?-Yes. F{OPELESS (Wellington) -My horn speaker has developed a loud crackling noise when loaded with a strong signal. : A.: This is due probably to your speaker being about done. 2, Which is the better earth, a waterpipe din. in diameter, 8ft. long, or one Lin. wide and 6ft. long?- , A.: There is very little difference, certainly none worth bothering about, although theoretically, the latter is the beter. 3. How many distant stations have been received in Wellington city on a threevalve set? A.: We don’t know. Perhaps some ardent DXer seeing this will take the matter to heart and let you know.

Table of Wire for Field Coil of M.C. Speakers

~ Turns for 25-1 Valveor Turnsfor — Step-down Impedance. H.R. Coil. Transformer 2500 ohms 1000 50 2750 . 1300 . BS 3500 1650 65 5000 2100 105

Volts (d.c.) 6 12 120 230 S.w.g. Gauge. 18 d.c.e. 20 d.c.c. 34 enamelled’ 38 enamelled Lbs. 43 4a 33 3 Layers. 15 20 67 92 Turns, 1,000 1,700 19,000 | 37,000 Resistance Ohms, 2.9 8.4 1500. 6500. Current. 2.1 amps 1.4 amps 67. ma. 30. m.2a.