Questions and answers

Radio Record, Volume V, Issue 4, 7 August 1931, Page 13

Questions and answers

GUBSCRIBER (Taupiri) : There are no particular agents for the components about which you inquire, though no doubt if you sent to Johns, Ltd., Auckland, or Fear and Co., Wellington, they would be able to do something for you. In constructing circuits from overseas journals there is always a certain risk involved. unless specifications are complete. you fPPwild not attempt to make them up. why not make the a.c. short-wave receiver described in the 1931 "Radio Guide?" It is almost the same as the one in the July "Radio News," and all specifications are given. Furthermore, this set has been built up and tested in New Zealand, and we know exactly what it will 0. 2. When I put my finger on certain terminals a thud can be heard in the earphones. Is this OK? A.: In all probabilty, yes. Of course, it all depends where you put your fingers -we would not try too many places, Af L.B. (Auckland) : What should be the * size of the core and the number of primary and secondary turns for a transformer to supply 250 volts plus grid bias for two 245 valves in push-pull with a dynamic speaker having a field coil of 8000 ohms in place of one of the chokes? e A.: You will require a 100 watt trans. former having a core l#in. x l#in. A primary of 1140 turns of 24 enam., wire. As there will be a drop of 480 volts through the choke you will need to construct a primary that delivers 800 volts. To do this you must wind 4500 turns on either side of the centre tap, using 32 gauge wire. N C. (Masterton) : How can I work out * kilocyeles, ete.? For instance, a station is listed as 95 m., and my set is a _five-valve battery one, with three dials. How could I tune in such a station? A.: Unless you had special short-wave coils you could not. The ordinary set, of which yours is apparently one, will not tune in below 200 m., without being seriously modified. Your dials are probably

calibrated from 0 to 100, but they may be 0-180, as some are. Now the dial numbers 0-100, or even to 180, have nothing to do with kilocycles or wavelength in metres. They are merely convenient numbers that help you to locate a station. There are two methods of~ working out the whereabouts of a station when the wavelength or frequency is given. The more accurate is by calibration, but we do not have space to tell you how to do this in the "Radio Record." We shall tell you more about it in the "Radio Log." As a matter of fact, an article appeared in the "Radio Record" of June 5 expaining how calibration was done, but we shall go into the subject more fully in the next "Radio Log." The other plan is to listen-in and note down the settings of as many stations as you can. For instance. Auckland may come in on 40, Wellington on 65, 2F'C on 70, 83AR on 82. Now you put those numbers down, and beside them write in their frequency. Now. you look "up your chart and you see that 2BL’s frequency is 855 ke. So you see that it is somewhere between 1YA and 2YA. As the number corresponds more nearly with that of LYA you will presume that it is closer to that station and will look in that direction for it. Once having found it. you mark it down with the dial reading and wavelength or frequency, and so you go on and build up your list of stations until you can tell pretty accurately just where a station will be from its frequency. You could do the same by employing metres, that is. the wavelength. but nowadays, nearly all stations, except short-wave ones, are listed by frequency So you will find this probably the easier. HORT-WAVE (Wellington): Could you give me the details and diagrams for adding a power valve? A.: All you do is to take out your last valve and put in a suitable one-say. B605 type, and then suitably bias it. The chances are that at the nresent time you are using 7 or 9 volts grid bias, Perhaps tot that. If you use the B6085, or its equivalent, the bias will have to be brought up to 12 or 15, depending upon the B voltage, and at the same time it would be as well to increase the B to something like 185. By so doing you would get far better results. This combination would give you reasonable quality from a dynamic cone or an inductor dynamic speaker. ; 2. When winding the moving coil I found it necessary to wind the wire in about three layers, using 36 s.w.g. enamelled wire. Is this correct? A.: Yes, that will be quite satisfactory GPARKS (Tauherenikau) : I eonstructed your "Night Hawk" receiver, but cannot get it to oscillate except when the grid terminal is touched. .? Your eoil seems to be connected correctly, but you could try reversing the connections to the tickler coil, that is, reverse the wires going to "C" and to Fil. 2. If this does not act try a few more turus om the tickler coil. ‘ 2. I am using B406 as detector. Is this satisfactory? A.: Decidedly not. B406 will not oscillate easily. It is a power valve. Use A409 or, better still, A415. Do this before interfering with the coil. 8. Can the "Ranger Two" be used for short-wave. and if so. will you be publishing details? A.: Yes, the "Ranger Two" van be used for short-wave, and if you construct a set of coils such ag used in the "Kes-

trel Three" and couple the aerial such as shown for the s,g, valve, you should get satisfactory results, GUPER SET (Balclutha): Would you please give me particulars regarding the ground antenna used in America for eliminating static, etc.? A.: Yes. A trench 100 feet long ana about 6in. or a foot deep is dug. in it run a hosepipe, an old one will do, with the holes plugged up with tape or some other insulating substance. Into this pipe push a wire, preferably ename!led 7/22, like what would be used for an ordinary aerial. Block up the distant end of the pipe and near the wall of your house leave one or two feet of hosepipe protruding, ‘This will serve as a leain shield for your underground ‘aeria! You will find the aerial will reduce net only static but.also signal strength. How: ever, the Americans seem to think a great deal of it. G G. (Gisborne): I have constructed * a short-wave set with valve base coils described in the "Radio Guide,’ 1931. I cannot get the set out of osciilation. A.: Take a turn or so off the ticklers of the various coils. The tickler coil is usually the cause of oscillation trouble although usually the reverse-the set cannot be made to oscillate. We note that you would appreciate a few articles about looking after your set, and we will keep this in mind, and from time to time supply articles that should meet your needs. If any reader has any suggestions about future articles to help those who are well away from service men. please let us know and we shall do what we can in the way of having them prepared. . A D.R. (Karori): I have made the ° "Rejector Two" erystal set, but unless I reduce the volume I cannot separate 2YA and 2Z4W. I am using a .00035 condenser. Is this satisfactory? A.: Yes, a .00035 condenser is satisfac. tory. We cannot explain why vou cannot separate the two stations. and strangely enough we have found that some constructors have been unsuccessful. while others have been perfectly successful in separating the two stations. We can appreciate this difficultv. beeause we have found that in some places the "Rejecta" will work satisfactorily. while in other it is unsatisfactory. Your best plan is to construct a wave-trap such as that described in the same issue of the "Radio Record." and use this in conjunction with the ‘Rejecta." You will find then that you will be able to cut out 2YA without trouble. F.M. (Wellington): I am desirous of * building a three-valve all-wave electric radio set. Could vou supply me with a diagram? , A.: We suggest your using the circuit of the "Advance" a.c, short-wave receiver described in the "Radio Guide." leaving off the two push-pull stages. AMATEUR (Waiorongomai) : I am experiencing difficulty in getting my storage battery recharged. I have a shearing machine engine. Is there any arrangement I can fix on to it so that I can recharge my own battery? A.: Yes; you could use a motor-car generator which you should he able to

pick up from a garage fairly cheaply. WS: should advise you to write to Messrs. Johns, Ltd., Auckland, as they will pos-: sibly be able to help you in this respect. It will take a fair amount of running: to. charge your battery fully. If you hayea car or a lorry it would be possible to. change your radio and car battery over when run down a little. You would find) we think, that using the car a great de in daylight would keep the battery fully charged, [-D*2HB (Gastings): Could the leads to the individual earths of the Pierce earth system be connected to a copper plate which is attached direct to the earth termizal, and would aerial wire be suitable for the lead? . A.: Yes, you could follow all the wires to a copper distributing plate, and you could use aerial wire for the leads ‘to the individual pipes. Your sketch appears to be quite in order. RED BIRD (Wellington): I have an ac. receiver which has been operating for 12 months. Are my valves losing their emission. A.: We refer you to matter concerne ing valves and their emission which is published elsewhere. No doubt the valves are now well past their pristine glory, and you would probably notice an improvement were they renewed. However, try first renewing the 280 valve,..as this supplies the direct current to /alt the valves and has a big effect upon the efficiency of the set. F 2. What make of valves do you recommend? A.: We regret we cannot recommend specific makes. Two or three makts of valves are recognised as being standard, and we should advise you to use one CORRESPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Box 1032, Wellington. Wimit three questions, unless letter is accompanied by 1/ee, Name of set ........cceccescccoooe Number of valves 0000000000006 08000 Name Address Tee oeCOSOSOODOSCCeOlOSS0O00008 20000000000008808Ce88 60000 292052000000 89000000080800000080888000 Tee OeSoeeeeoZooOsSOOOGHCOOSOOBOORBEG008 Nom de plume eeceeescooseoeceeeose To be kept in subsequent inquiries. Date seccocecses SveE~r 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, pe a

of them. The make you mention is first class. However, see the advertisements in this issue and- you will ge. some idea as to what types of valves are offering. -B, (Auckland): I have a six-valve * electric set, but only local stations come in well. Australians are. very faint. I have a reasonably good aerial and a good earth. wA.: ln all probability the condensers are out of alignment and we do not adyise you to attempt to put them into alignment yourself. Call in a reliable ratlio serviceman and ask him to trim them-up for you. You will then find that you will get very much better results, on A.B. (Hastings): I wish to use a helix with an iron core to attract a .piece of iron to complete a circuit. What number of turns must I employ? The mains are 230 volts d.c. -A.: You will require a d.c. lamp of about 30 watts, and this must be ¢connected in series with the helix. To make the helix, take a piece of soft iron rod about fin. in thickness and 6in long, ov perhaps less, and wind round this one layer of, say, 22 d.s.c. wire. There 4s. no need to put any insulation in between the iron and the wire. You will

-_-2find, we think, that this will operate quite satisfactorily. Do not use this device without the lamp in series. You will find that it will light every bit as brightly as an ordinary lamp. Nothing in the helix is critical’ except perhaps that you have soft iron and not steel. NUMBSEKULL (Levin): What length of aerial should I use with an allwave set? A.: Not more than 100 feet and make it as high as possible. Two poles are preferable to one. 2. Is there any advantage in having an aerial higher than 40 feet? A.: Yes, providing you do not thereby add too much to the length. 8. At present I have a 120ft. aerial S0ft. high, and experience difficulty in separating stations up to 20 on the dial Can I remedy this? A.: Below 20 the stations are very closely packed. However, a shorter, higher aerial would probably be more effective. The use of a .0001 or .0003 condenser in series with the aerial would sharpen up tuning a great deal. L I.B. (Christchurch) : Can you give me * particulars of a short-wave adapter suitable for an electric eight-valve -superheterodyne midget set?

A.: Yes; one of the superheterodyne type ‘of sets would be ‘quite satisfactory, but you must couple it to the grid of the intermediate frequency valve. Before you interfere with your set in this manner we should strongly advise yon to write to the New Zealand agents for the set (Box 462, Auckland). 2. How is the (R1-10) speaker strength of a station being received arrived at? A.: Unfortunately it is all a matter of comparison. You must take RO (R10 is never used) as the strength of your local station and grade between this and audibility. You will find a table with more information on ithe subject in this month’s "Radio Log." 8. I wish to use headphones on the above set. Could you give me particulars of how this will be done? A.: Yes, look in the constructional columns of this week’s ‘Radio Record." ©.G. (Wanganui): I have a set * which satisfies me in all respects but in that I-eannot get the American stations as strongly as [ wish, or as many of them as I would like. I am advised to have a screen-grid valve fitted instead of a 227 detector. Do you think the result would be worth the expense and trouble? A:: No. However, on this point you could obtain more information by writing to the Wellington distributors for your set. If you look at one of their advertisements.you will soon find the address. Also, judging by its performance, there cannot be very much wrong with your type of set. 2. Is there any reason why an efficient man should not be able to fit one of the 104in. speakers in my set? Have heard one of these speakers and consider them the last word as far as tone and volume are concerned. .: He should have no difliculty at all, providing he understands matching it into the circuit. 3. The set distorts when on full volume. A.: We do not know of’any set with three stages of radio amplification that does not. EANUY" (Christchurch): I wish to build a transformer to supply a transmitter employing one UX 245 valve. Could I use the wire, a sample of which I enclose, for the job? The gauge of the wire is 30 d.s.c., and is not particularly suitable for the job you wish to perform. The trouble with this type of wire is that coupling takes up far too much space. It is better to use enamelled wire and layer insulate it. To use the gauge you mention would produce a very bulky transformer. 2, Which transformer in the "Guide" should I construct for a 245 transmitter’ A.: The 100-watt will be quite satisfueory. . 3, Should I double the number of turns for the h.t. and take out the centre tap if I use the "slop" rectifier described in the "Radio Record" article "Breaking Into the Amateur Game" Av: Yes, if you use it in the full-wave rectification circuit. EGINNER" (Auckland): I have an eight-valve s.g. set and the selectivity is not what it should be. My valves are 15 months old. Do you think it would help if I replaced my power valve and detector? A.: Selectivity trouble is rarely due t» defective valves, but it has been traced to this cause. Generally to improve the selectivity of a factory-built set you can do very little other than shortening your aerial, or the use of a .0001 or .0003 condenser in-series with the aerial, which is the same thing as shortening it. If your valyes are 15 months old, then we

‘ certainly think you should replace they, detector, the rectifier and the two push- ' ~ pull power valves. If you replace the ‘whole set you would probably get far beiter results. L F.C. (Christchurch) : Here is an in- * téresting point raised by this correspondent. He says: "My Differentia One" continues to give good results. Over 100 New Zealand amateurs have’ been logged on shortwave. Wor speaker reception I couple the "Differential One" to an a.c. broadcast set through a 34-1 audio transformer, whose secondary leads ge into the gramophone jack. ‘This has proved quite satistactory. "PUSH-PULL" (Mata Mata): What are the resistances required for a power pack for the a.c. Radiogram using one 224, two 227, two 245 valves? ‘The transformer will supply’ 300 rolts; 100 mamps, aud the choke has a drop of 17 volts, . A.: Between your maximum "B" ss }tage, 288 volts, and "B-,’ you. quire a tapped resistance totalling 20,50U ohms. This could well take the form of a 75 watt dissipation resistance, and you could get the clips and place them at the correct. intervals. . ‘hese are the resistances between the taps. Between the maximum and the first tap you will need 5000 ohms. ‘This will deliver 180 volts, Between this tap and the next you will require 8500 ohms to provide 75 volts for the screen. ‘To break this down to 45 volts for the detector ‘you will need 2500 ohms and between this and "B-" 4500 ohms. The bias resistances are all given in the back of the "Radio Guide" which you must be using if you wish to deal with the "Radiogram Five." You will find them on page 169, but, if you have a first edition, note that the grid bias of the 224 is 4000 ohms and not 400 ohms. Note that for pushpull amplification you halve this, thus for the 245’s in pushpull you will require vhms, ; 2. What pushpull output transformer would you advise with the Radiogram, O.P.380 or 'O.P.4C? A.: O.P.8C., ratio of 9-1 15 -1 223-I O.P.4C., ratio 25 -1 40 -1 66 -1 "CRYSTAL" (Auckland). The query was answered last week under the nom-de-plume "G.U.,"’ Auckland. We note your request for a future article on Crystal Sets, and shall see what we-can do for you. "AUTO MECH." (Mt. Albert): There was published some time ago in the "RM." a ecireuit diagram of an a.c. shortwave super heterodyne adapter.. In the list of components in this circuit the detector grid bias resistance was given as a variable resistance having a minimgm of 50,000 ohms, whereas in the issue of the "Guide" the value was Sven as 5000 ohms. Which is correct? A.: The 5000 ohms, as given in last week's "R12." 2.-If unable to procure 60 ohms centre tapped resistor for the filament supply, would a 50 or T5 ohms be satisfactory ?- Yes, quite, 3. What should be the capacity of the condenser C3 when the input stage of a receiver .is of the tuned type? ‘ As: As stated. It does not make much difference if the arrangement is coupled to a tuned r.f, stage. Onee the set is tuned it will be quite satisfactory... *

\ \ « _4 The condenser C3 is shown on the diagram as a fixed condenser, whereas in the components it .is.referred to as a tuning condenser, Which is correct? A.: The word "tuning" was put in-in-adventently. A* JAYEM (Kelburn): What is the * difference between square law and linear , detectors? ; A.: A detector is linear when output at audio frequency is directly proportional to. the radio frequency input. Such detectors produce a small percentage of distortion with fully modulated signals in comparison with the larger percentage of distortion produced by other detectors. Anode bend or power detectors are linear only on a small proportion of their operating characteristies, Square law detectors are the ordihary detectors, that is, the audio frequency output voltage is proportional to the square of the r.f. input voltage. These detectors produce some distortion, especially if the detector is modulated at fairly ie percentageS. se f{XPERIMENTER" (Orari): We regret we cannot supply desigus -of receivers. We advise you to follow very carefully the series of articles that will start next week on the super hetero-. dyne receiver, and note the set to which it leads up. This will probably do away with a large number of your troubles. N Z.D.X. (Auckland): I have a twe- * valve set. What is the best method of adding an audio frequency stage (three electrode) ? A.: We cannot tell you in a few words. You had better wait until the description of the "Ranger Three" comes out. it will be on the way shortly. The same applies to a radio valve. It is very difficult to outline constructional details such as this in the limited space we have available for Questions and Answers. O KAY (Blenheim): Is it possible to * use the "Sparrow Hawk" adapter with an a.e, set? A.: Yes; if you:employ batteries for the "Sparrow Hawk" and use the circuit shown in Figure 2, shown on page 111 of the 1931 "Guide." Why not attempt building the super-heterodyne adapter also described in the "Guide"? It. will really be more suitable for an a.c. set than the "Sparrow Hawk." The "Sparrow Hawk" would give easier tuning, but you would have to put up with batteries. GREGG (Dunedin): 1 have had great results with the "Differential One." It is remarkable that such a small set ean give one such world-wide reception. { have had good reception from England, Holland, France, Russia, Indio-China, Java, America, Austrulia, and some N.Z. amateurs. I am using two four-volt valves, with 45 volts "B" battery. As I wanted the set primarily for shortwave work I bought two .00015 variable connsers. However, hand capacity is a tle bad. Can you tell me: 1! Which are the most suitable condensers to use with the Differential One eoils? A.: For the tuning double space .0001 tuning condenser. Reaction, .0002 or .00025 differential condensers, 2. Will I need to alter the coils? A.: Not if you use these condensers, 8. When I tune in a station’ I have to sit very still to keep the set from squealing. If I touch the ’phone cord, i... ad ee

headband, or ’phones the set squeals or else the signals fall away. A.: This is a variety of hand capacity which shielding the set would stop. 4, I wound a choke of approximately 150 turns 30 d.s.c. wire and put it in ‘he "B" plus lead at the phone terminal but it has not effected any noticeable improvement. On one of the bands oscilla tion is ploppy and it will not. oscillate over the whole dial. A.: To overcome ploppy oscillation the best plan is a potentiometer grid returu such as is shown in the theoretical diagram of the "Kestrel Three." 5. What is the remedy for dead spots’ A.: The use of a midget condenser in the aerial or altering the number of turns on the primary coil, generally taking « turn or so off. ‘ 6. Would a PM4DX be more suitable than a PM8? . A.: Yes, PM4DX is a special detector. 7. Would it not be better with more plate voltage? A.: In all probability not. So long as your set will oscillate, additional voltage would be of no advantage. 8. Would it be advantageous to uSe a "B" battery with the one valve? A.: No, it would be impossible. You would have to use anode bend detection, which would not give you the results that

we lM ™ so the grid leak detection does. 9. I have heard a broadcast from the local stations on the Differential One. Does this indicate a fault? A.: No, you are really tuning them iz on one of their harmonics. We are pleased to hear of your good results and think that if you use the .0001 double spaced tuning condenser for tuning and 0002 or .00025 differential reaction, you will have vastly improved results. The squealing may possibly be eliminated by reducing the detector voltage until the set will just oscillate. Furthermore, the incorporation of the potentiometer grid return will no doubt be beneficial.

REO SIGMA (Ngaio): I have recently completed, according to the somewhat meagre specifications in the 1931 "Guide," the super het. shortwave adapter and it failed. I have had to guess:at some of the components. : A.: we published last week a diagram with full explanations, and now publish a diagram of the layout. If you will follow the instructions given and get the components correct you will get your set to function all right. 2. The broadcast set to which the adapter has to be connected has two stages of tuned r.f. Is this sufficient ?- es. 8. Must the return from L.5 be connected to any point of the set?-No. 4, Would increasing the number of turns on L4 cause the set to oscillate? A.: If correctly constructed it should oscillate with the number of turns given, but increasing the number of turns would certainly help oscillation. RIEPLH (Dunedin): I have constructed a Loftin Four with a band-pass filter and obtain plate and screen voltages direct from the power pack through re sistances. Volume control is effected by varying the bias on the two r.f. valves. 1. Could you tell me the probable causes of the following :-(a) Instability. While

stability seems satisfactory, when ithe volume is turned down a pronounced ripple appears. I have effected an improvement by by-passing the plate resister with a 2 mfd. condenser, but the ripple te appears when the volume is reduced to.@ comfortable degree. A.: Are you drawing the screen volts ages from the same source? If so, prom vide rf. chokes in the lead. to eachs furthermore, there should be" chokes: is the plate lead and these, if mecessaryes should, be by-passed with midget condensers to earth. It is probable, too, that some of the ripple about which you complain igs due to the method of controlling volume. The volume on -the ordinary valve should not be control#d by varying the bias, but the screen voltage should be varied. Your plan «is quite satisfactory for multi-mu valves, and if you were using these, probahly you would find many of your troubles would disappear. om (b) The tone appears to be shrill ‘aud piercing on radio, but O.K. on the pickup. " A.: We think this trouble is due the same cause as_that responsible "fo: the before-mentioned trouble, and if ‘you get to the bottom of that, following ‘the lines we have suggested, the shrillness will disappear. (SONDENSER (Marton): I am troubled with interference from _ power lines running about thirty yards from my aerial. Can I do anything~io eliminate the noise? ;

A.: Keep your aerial as short a8 POs sible and higher than the interfering lines if possible. Make the flat. ,top very short and shield the lead-in by using special armoured cable and ‘ earth .;the armoured portion. Try a separate earth, or rather a different earth from the*one you are using, if possible'in a different location. Arrange your aerial so {hat it is at right-angles to the nosy power line, Communicate with the District Radio Inspector and complain about the "noise. 2, When I touch the fixed plates in a condenser the volume drops right away. The same occurs with two others, but hardly any difference is noticeable on touching the fourth. A.: This is quite in order. It is due' to the potential on some plates being higher than on the others. , QGPARKS (Marlborough) : Would it be possible to build a one-valve shortwave adapter for my commercial set ?- es. 2. Could you suggest one? A.: One of the super-heterodyne tyres such as described in the last "Guide" would be most satisfactory, alticugh a battery adapter such as the "Sparrow awk’? would also work quite well. 8. What would be the approximate cost ‘of the a.c. adapter? (Concluded on page 30.)

Questions and Answers

(Continued from page 15.)

A.: About £6. The Sparrow Hawk about £3/10/- without batteries. B CO. (Waitara): What valve should 1 * use in my set, preferably they should be Marconi? A.: In the first two stages you should use H410; in the next three L410, and in the last stage P410. These remarks hold good for other valves. 2. I live in a valley between hills a quarter of a mile apart and 600 feet high, I have approximately 60 stations on my valve set. Is this good? A.: We have seen better logs for a set of this description, though your locality no doubt has a tremendous effect upon your reception. Would it not be possible to suspend your aerial between the two hills? You need not employ the whole span as an aerial, but use fencing wire as supports and keep your aerial about 200 feet, including the lead-in. CuRmux (Napier) :* Does corrosior of a bare copper aerial actually lessen the aerial’s efficiency with regard to signal strength ?-Yes. 2. My aerial is 40 feet at the far end and 20 feet at the lead-in end, and the main span is about 30 feet. Is it efficient for shortwave reception? A.: Yes, but it would be better if you could get it a little higher at the leadin end; say get on another 10 feet. Ww M.B. (Miramar): Could you supply * me with particulars of a closelycoupled primary coil to match a valve with an impedance of 200,000 ohms? A.: Wind approximately half the number of turns on the secondary on to a former which will just fit inside the twoinch former. Use 86 gauge wirc. AMATEUR (Petone).-My set weakens then comes back to normal, and within an hour falls off. The "B" eliminator seems satisfactory because it keeps up a steady hum, and the accumulator is fully charged. The complaint has commenced since putting in a new "CO" battery. Reverting to the old one is no improvement. A.: It is difficult to tell you where to look for the trouble. It probably lies in

the transformer, one of the valves, the speaker, or a condenser. It may possibly be a loose connection. Have your valves tested and take your eliminator along and that he knows to be all right. ask your dealer if he will put it on a set 2. Recently I picked up JOHK. Is this good? I also pick up 6WF, Perth. A.: Yes, very good indeed. 3. I have constructed the Sparrow Hawk differential adapter, but cannot get anything out of it. The wiring seems K. I have reversed the filament leads without making any apparent difference. The third lead from the adapter goes to the anode in the detector valve socket er fhe grid is left blank. Is this correc A.: Yes. There is undoubtedly something wrong with the adapter. Try connecting a pair of phones between the pin that: goes to the anode and a suitable "B44" tapping. Connect the filament to the two filament prongs of the detector. We hope you have noticed that your sockets are Hnglish type with the pins spaced out like the Southern Cross. The anode is the one by itself, the grid and the two filaments are grouped together. The filaments are the two outside ones. TTELME (Auckland): What valves are used in the "‘Sellens’ a.c. Short-ware Receiver" A.; A442, A615, A609, B605. Any valves of the types enumerated will be satisfactory. Note that A442 is a fouvolt valve, and in order to break the six-volt current down to four volts a rheostat has been placed in the filament ot the first valve. 2. What are all the earths on the bypass condensers, etc., attached to ? A.: They are connected with the metal panelling which is connected directly to earth. 3. What would be the total cost of the "Sellens’ Short-wave Set," including nol batteries, valves, etc.?-A.: £10 or GRD (Hastings): I have a four-valve s.g. set which was damaged in the earthquake. I wish to remodel it. What in your opinion is the most efficient fivevalve set within the capabilities of the amateur builder? A.: Undoubtedly the Outspan Five, which was published in the "Radio Record" dated February 20, 1931. You will find all data there. You can use the Colvern coil formers without making any alterations in the number of turns.

Stalloy Cutting Device

A CONSTRUCTOR has shown us 2 very useful device for cutting stalloy to the correct measurement. Many constructors know. that cutting

a large pile of stalloy strips, each one exactly the same'length, is not easy _ unless some means of accurately gauging the length of each is found. He has made a small device of brass, and the illustration- shows -it very clearly. The slot width is the same as that desired for the strip, while the

length is somewhat longer than the longest strip. The stalloy will just fit in between the grooves shown in the end and side elevations until they reach the nut. They are then sheered off along the thickened end shown on the right of the side elevation. The position of the nut, of course, will determine the length of each strip. It is a useful little device, and well worth Inocking up by. anyone who contemplates cutting a lot of stalloy. Even for those who cannot get the brass, and have not the means of turning out the nicely-finished device depicted in the plan, there is the possibility that they will be able to adapt the specifications for some little device of their own. be