Questions and answers

Radio Record, Volume IV, Issue 42, 1 May 1931, Page 13

Questions and answers

H.R. (Southbridge): Is resistancecoupled amplification suitable when following a regenerative grid-leak detector? A.: Yes. 2. Would PM1A or PMIHF be | the moze suitable as detector?-The latter. ‘ Would cocoa tins make _ suitable shixAls for the "Outspan Five’? . A.: Yes, but they will not be as as aluminium or copper ones. "FINAINUI"’ (Dunedin): I have a commercial a.c. super. het. on which it seems difficult to receive Japanese and American stations. Aerial is 60ft. long, 40ft. high at one end, and 30 at the other. Can you offer any suggestion for improvement? A.; Have you a good earth? for your aerial is quite O.K.. Your situation may be against your reception of distant stations. 2. Does locality affect distant reception ?-Yes, ; ."FLOPELESS" (Wellington) : Is insulation tape satisfactory for insulatjing my lead-in, which comes down_ between some electric light wires and the guttering of the roof? -A.: Your lead-in should be at least three or four feet away from the power lines, with no possibility of swinging across them in the wind. Otherwise, you may possibly seriously damage both the mains and your set. Insulation tape in such an event would be of little use. In any case, if your lead-in is close to power lines you will probably be troubled by interference. Use heavily insulated wire. 2. How many insulators should I use on iny aerial? A.: Three at each end. . Note.-Your log is quite a good one. G S.N. (Wketahuna):.Can the "Sellens _ short-wave set" be used for reception’ on the broadeast band? If not, eould it be adapted for both? A.: Yes. Particulars of suitable coils will appear shortly.

C H.W. (Invercargill): I have two * power valves, a Mullard PM4 and a B.T.H. B4 (6 volts). Could I use these valves in push-pull in the manner shown in the diagram? A.: No. "The characteristics of valves used in push-pull should be fairly closely matched, and hence should, in general, be of similar make. 2. Which additional valve would you advise me to buy? A.: Possibly the PM4, though both are quite suitable. W L.L. (Devonport): I have built the ¢ "Outspan Five," using a stage of push-pull and A and B eliminators. have logged 22 stations, without much exploring, at full loudspeaker strength, but am troubled with what I think is a modulation hum on local stations. This comes in only with the carrier wave of the locals such as 1YA and 1ZR and 1ZB, ete, It is not in evidence on distant ‘stations. A.: You might try one of the following three remedies :-(1) Place a good quality mica fixed condenser of .0001 mfd. in series with the set and earth. (2) Break the connection between the primary and the secondary coil in the first r.f. transformer. (8) Reduce the number of turns on the aerial coil. Also see that the cores of the transformers are grounded. "oO B.S. (Auckland): I have a_three- * yalve commercial battery set. Is it suitable for short-wave reception? A.: It is satisfactory, but not as efficient on short-wave as it might be. Coils may be obtained from the agents.

"AHORTWAVDH" (Marton): Are the windings for the short-wave coils in the "Diff. Two" close or space wound? A.: Close, for space reasons. "NEWCHUM" (Marlborough) : I ‘have a six-valve commercial a.c. "Midget" which vibrates when in operation. What is the cause of the trouble? A.: This may be due to a number of faults such as loose speaker diaphragm, loose lamination in the power. transformer, a defective valve, etc. See the article published in last issue (dated April 24) on "Locating Noise in Receivers." You must remember that midget sets will not handle a great amount of volume. 2. I am in a good locality and have a good aerial and earth. Should this set bring in all the Australians and Dunedin at good loudspeaker strength? A.: Yes. 8. Are my valves of a good make? If not, would it be possible to improve on them? A.: We have not tested this make of valve, and so cannot advise you. Better use one of the recognized makes of firstgrade valves, "(NOAL" (Greymouth) : I am using two 45-volt "B" batteries. with a Tvalve commercial set. Will it improve reception to add another block? -. A.: Yes, you will secure much greater volume, and your set will be-more sensitive on distant stations. Do not, however. ndd a new battery to two already nartially run down, for it will ruin the former. ae 2. What type of loudspeaker would give the best reception on my set? A.: A good moying-coil speaker would

probably be the best, though a cone capable of handling a large output, such as the Bluespot 66R, would be quite suitable. C J.S. (Auckland): I substituted a ¢ .0005 condenser for a .00035 in my set and altered the coils accordingly. The volume and tone are better, but distant stations which were previously quite good are barely audible. I appear to get the carrier waves but cannot bring them in. Signals seem louder when I use a wavetrap. I tried a few more turns on the primary coil, which is three-eighths of an inch from the secondary, without any apparent effect. A.: This is just a little difficult to explain, particularly, when you say volume and tone are better but distance stations are weak. You see, we don’t quite un-

derstand what you mean. ‘The distance between the primary coil and the secondary slould be less than three-eighths-say one-eighth or one-quarter of an inch. Try increasing the number of turns on the reaction coil and just make quite gure that the connections on the new coil are quite right. BAt (Wellington): The three cells in my accumulator show a different reading when I test them. Does this make any difference? A.: This should not be so to any marked degree, though sometimes small variations are apparent and permissible. The best way to overcome this is to give the battery a long, steady charge and leave it on until all cells are fully charged, Another plan would be to put the charger on to the cell that is lowest, of course adjusting the voltage to 2. >. I have had my "A" battery three years and wish to change the electrolyte. How do I go about it? A.: Drain off the old electrolyte after the battery has been fully charged. The cells should then be washed out with. dis-. tilled or rain water that has been collected in an earthenware vessel, Now obtain from the chemist a small quantity of pure sulphuric acid, or, better still, ask him to mix up for you a solution of sulphuric acid having a specific gravity of 1.240. You can find out the quantity you will require by measuring the amount you drain off. 8. I was told the gravity test should be 1.240, but my hydrometer reads 1.250. A.: This is of little concern. It only indicates that your accumulator has slightly more sulphuric acid than normal, but it is so little that it does not matter. QuuALity (Auckland): I am operating an a.c. set with a heavy cone speaker, but I wish to use a pick-up and find that the speaker jars on a heavy recording. Would there be any noticeable falling off of volume if I used a dynamic speaker? A friend of mine made the change and his results were very much inferior as regards sensitivity.

A.: A good dynamic speaker: should be equally as sensitive as yours, The.: one you mention is an excellent: one... HANGHABLE (Te Kauwhata):: My commercial set was designed for 6 valve and I have always used Continental valves, but I have done everything I ean. . to get beyond Australia. Would ‘the’. use of American valves improve sensi-~ tivity? . A.: It is most unlikely. Probably your locality is to blame. 2. Is my set noted for distance work? A.: It is a good one, though, of course, more recent ones using s.g| valves have a tremendous advantage over sets of your type. ELMONT (Lower Hutt): Would it be possible to convert a trickle charger into an eliminator? If so, how? A.: Yes, providing the current required by your set is not greater than the charger will deliver. You must- use a smoothing choke and an electrolytic condenser. jack (Invercargill) : I have a batteryoperated all-wave set. Will you: supply particulars to enable me to cover the band from 500 to 850 metres? ‘A.: To get a coil that will be anythin: like efficient you must use a three-inch former and it will require about 200 turns. The reaction requires about the same number. You should use fairly fine wire, say, 30 gauge d.s.c. FACE (Dunedin): I wish to use a loading coil with my crystal set. Is my cireuit correct and where must a loading coil be used? A.: You must use it in series with the secondary coil. In other words, it

ORRESPONDENTS must attach this coupon to all queries sent to the Technical Editor (Box 1082, Wellington. Limit three questions, unless letter is accompanied by 1/ee. Name of set ......... eeccccccccces Number of valves ..cccccvccesvcsess Name Pe weer esses ser essesaeseseses Address Coe reeecaesesesesassseese® oon cc cncccccccccecscocqccsoqcepsoesce POSS HHH ee ETHSSSSHERSESSHHOSESOHSSSORET Nom de plume eereereveeseesarers To be kept in subsequent inquiries. Date eeccccccccccoce TTT T TTT Please Note:(1) Be specifie 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 eannot be supplied without a specialist’s fee. et eT ee ea

fnot occupy the position "A" in your ak Ki. (Dunedin): Do you know who "are the agents for the "Supertone" receiving set? : Sorry we. do not. Perhaps some reader’ seeing this may be able to advise our of the address of. the agents. G.* T.H. (Christchurch): I wish to use . ‘the Philips equivalent of the 245 in the L.W. What alterations must be made? ,

Al: You need not make any alterations ;.the valyes are so much alike that efficiency will not be impaired if it is just substituted. J UMBO (Otago) : What fixed condensers must I place in series with my .00015 tuning condenser to bring it to yee following "capacity : .0001, .00085, and A,: By using a series capacity you will the total eapacity. Therefore the only capacity you can get by seriesing is the .00O0L and for this you require a .001 condenser. If you want to bring your condenser up to the other

two you must use in parallel a .0002° or a .00035 as the case may be, but by using a condenser in parallel you arc. reducing the minimum capacity and lessenling the tuning range of your set. 2.:1 have two coils which work well on the top wavelength but which give me Morse. only on the lower on, : Probably your set is oscillating continuously when on the lower wave lengths and you must reduce the :tickler turns until it will just oscillate on. the upper wavelength. If this is not the case you will need a little more practice ‘in tuning in broadcast stations, though on’ the lower wavelengths there is more Morse than telephony. QGEARCHER (Kaikohe): I have had a difficulty with the s.g. stage of the Diff. Four. Oscillation is difficult to control. : Use a .001 coupling condenser instead of a .0001 as specified, when: the set was first described. This wil! prob« ably overcome your trouble. The condenser, by the way, is the one between the s.g. stage and the detector. L. D. _(Ohristchureh) + I am building the short-wave set in the 19381 "Guide, ba but I intend to use .0005 variable condensers. What size coils should IT make to match them?

Av: It. ‘is not practicable to use .0005 condensers: in short-wave sets. Use those specified if you want worth-while results. MILLYCYCLE (Pokeno).: Will zine do for shielding an ordinary: set? No; it must be copper or aluminium. 2. Would 30 gauge d.s.c. wire be too heavy for the primary. If not, what alterations must I make in the number of turns? A.: You can use 32 without any departure fromi the specification. 8. Will the incorporation of a 30 henry output choke improve distance-getting ability? A.: N 0.; it will only save your speaker. A. W.2E. (Christchurch) : What voltages do you suggest on the "B" side of. "Radiogram" power supply? 22 "Bt maximum" at least 135 "B+ audio," either the same as "B+ maximum," or. 45 volts less. "B+ detector," 224 to 455, "B+ sereen" half the voltage on the plate. 2. Concerning the 14 former the list of components @ inches and the directions for making up the coils call for a one-inch piece. Which is correct? : Only one short piece is required, say ‘Lin. 3.° Will 3 ‘bypass condensers do instead of .257?-Yes. P. A. (Wellington) : What are the capacities of two. condensers, one with 11 plates and another of 24? A.: Probably .00025 and .0005. 2. How many plates in a. .00015 and -00025 condensers? A.: Seven and eleven respectively. 3. Where could I obtain some information about’ condensers? A.: The 1931 "Radio Guide." TRICKLE CHARGER (Khandallah) : I have an Osram DES, valve, What type is it? ~ , A.: A general purpose 6y. valve. 2. When are you. bringing out an.allelectric version of, the Outspan Five? A.: In the 1931 ‘Guide’ we gave an electric version of a somewhat similat set using one less stage. Before long. however, we will add’ "another stage for the "Radio Record." ‘This will equal the Outspan, Five only it will use push-pull. 3. I haye a Ferranti OP1 and a Piloi 894 output transformer. > Which is the most suitable for the A.: As both are of the same ratio both are equally suitable. PUKERKO | ‘(Hineura) : My house is situated on a knob, and I am forced to run the aerial parallel and yery close to a power line. I can lengthen it without sacrificing much height only by: haying the lead-in off the centre of the aerial. Would this improve matters?’

A.: If the distance from the centre gf } the proposed .aerial to one end is gréat than the distance from the existing leadin to the mast, then it will be an improvement, otherwise. not. If you are near a power. line there is little to -be gained by increasing the length.’ Where possible inerease the height and shorten, 2. Could. you recommend a handy textbook to assist in the location of trouble that might develop in a. set such as. mine? A.: Yés, the 1931 "Guide" will help you, but you would find "Phe Service Manual" (Radio News) gives more detail as far as a.e. sets are concerned, PARALLE (Nelson): Would a 50 ohm. potentiometer be suitable for centre tapping the filament wires of two Osram P625 valves in push-pull?-Yes, ° . Which would. be the correct resistfor biasing two P625’s? A.: 500 ohms seeing they are in‘ pushpull, and double the current will be passing. 3. What value potentiometer do I e quire for shunting across the fila leads of the. detector wtnige 200 ohms, . ME=Gostar (Wellington) : Can eliminator designed for 200-220. volt mains be worked.safely. on 230-volts mains? A.:. Providing your lines are not eubject to great fluctuation, yes. . How do amateurs find out their perof modulation? .: By the use of meters, 3. Would:a 100 ohms potentiometer use any current across a 6-volt accumulator? -60 mils. (Note.-We restrict the numbéer of questions that any. one person may ask without payment of a fee to three.) ‘J.P. ,(Blenheim): I am using two * midget condensers in conjunction with my tuning condensers to: spread the amateur Morse. but I cannot get near the main tuning ‘control for hand capacityt Would adjusting them by bakelite rods and shifting them to the back of the pahel help me? A.: Yes, Receive the station on the main dial, and vary the ‘midget condenser from minimum to maximum. If the station: disappears move the main dial one point and tune‘in again with-the midget, and so on. as the midget tune~ the wave between the two points on the main dial only. When not, in use the midget condensers are left at minimum capacity. In your recent data for short-wave coils you give three coils. Are these for the detector and are they put on in this order from the’ top? ‘Yes, primary on the bottom and tickler on the top.’ The s.g. coils are of the same dimensions. 3. My-set will not oscillate with the r.f. choke between the detector coil and the first transformer. Do I need more reaction turns on the coil? ‘ . ~ o

net we } A.: This may be due to a poor choke. Try increasing’ the number of reaction. ‘turns or a different. choke. . QGUPREMA (Wellington) : I have a four. valve set.and am using an A425: in the first audio,. -I-.find that there is a very objectionable whistle. A.: This whistle is due to your using an ‘unsuitable valve in the first audio. A425 is designed for a different class of work from what you are using it for. Use a different class of valve in its place and your trouble will disappear. PURUEI (Picton) : Owing to an accidental short-circuit I burned out a condenser in my eliminator, but after a short time the choke burned out. Can a suitable one be obtained ready-made that I might. instal myself, or could I make one? It is yery small. . 'A.: Your best plan would be to make a choke such .as deseribed in the 1931 "Radio Guide." You would need to pick oyt one of the standard chokes from the given in the chapter-on chokes that ‘ld fit into the space you.have available. . This, when built, would serve the purpose as well as the one originally in the set. QUISPAN (S. Dunedin) : I have built the "Outspan" circuit and am getting good results, but I believe a certain amount of damping is taking place in the coils. Could you give me particulars ‘for valve base coils? A.: Valve base. coils will really be quite inefficient owing to their shape. However, if you want to try them they will be as follows for .00035 condensers : 32 d.s.c. wire; secondaries, 90 turns; primaries, 40. turns; aerial coil, turns. : : . 2. Could you arrange to publish an article on the design of primary coils? .: Yes, we shall see what we cau do in the matter. .. °

= a 3. Would a stage of push-pull such as described in the "Guide," using ordinary transformer, be suitable for the span"? A.: Yes; it would be better, however to use a push-pull transformer next to the speaker. If you already have the ordinary transformers, you can = use ‘them N. R.B. (Kerikeri). -I hive a factory built wireless set. Could you send me the circuit of an- aerial tuner? : An ordinary wavetrap would suit your purpose, though the commercial Reesonator"’ would be better. . 2. Will you inform me when your 1931 "Guide" will be published? It was published just five weeks ago, a full-page. advertisement has been regularly appearing in our magazine announcing that fact. Just for the time being we are short of copies, having sold out our first edition, and are waiting the wo. which will be ee in a day or wo. H. W. (Waipukurau).-I have a receiver of my Own layout, consisting of Schnell detector and two audio. I was recently fiddling around with. the set full on, and the coil right out. I disconnected the earth from the variable condenser, and then to my amazement heard Morse on the telephone. Reaction control had no effect on. the signal strength or note, and neither had the tuning condenser. Half an hour later the signal was still going strong. Hand capacity had quite an. effect, though the grid filament potentiométer had a very slight de-tuning effect. : Had you broken ‘the lead to the grid potentiometer you would have had no signals, as with this closed the circuit becomes a vacuum-tube yolt méter, and some part of the choke is resonating at a harmonic of the incoming signal. Further, by varying the potentiometer which in-

creases or decreases the bias oscillation is caused to stop and start. By placing a coil in the eireuit and disconnecting: or insulating the grid pin or contact; loéal’ broadcast can be heard, Try it, {O0K-vP (Matamata).-lIf I increase the number of turns on the primary for the "Outspan," what should I expect as regards amplification and selectivity ? A.: Generally the effect of increasing the number of primary turns is to increase sensitivity and decrease selectivity. ° There is a point, however, at which further turns on thé primary only cause the set to become unstable, 2. What value of resistances and condenser should: be used in an r.c.c. outfit using s.g. detector? A.: The exact value will have to be found by experiment, but the following will give you a basis. Use a 100,000 ohms resistance in the plate circuit, a .5 coupling cox condenser and a .5 megohm grid resistan 8. H 610, followed by S 610, and hooked up to L 610, make a satisfactory set ?-Yes, (Note--I find the "Outspan" very sensitive, and quite selective. I have had several Americans at speaker strength.)