Questions and answers

Radio Record, Volume IV, Issue 34, 6 March 1931, Page 13

Questions and answers

~ ny A.: You could use four 221A or 201A O I. (Christchurch-: What are the * capacity of the following midget condensers;.one with seven plates and the other with eleven? A.: Probably ..000025 and .000035. ECTRODE (Invercargill): I have a B.D. set which has lately developed rough tone. I have tested valves, speaker, batteries, etc., but the set -con- . tinues to behave the same. I am using two PM4’s in the audio stages, with 45 bg on the first audio and 44 volts grid jias, A.: You say the roughness has developed only lately. This is hard to understand because from the combination of valves you have it should have -been rough always. You should not use a power valve such as PM4 in the first audio, Use PM38 or PM4DX. ‘The bias of 45 volts should be only 14, but-why put the full 90 volts on the first audio biassing it with three volts. This would be ample for the first audio stage. Where was your coupon? "2. (Turua): I have a five valve. American receiyer, and would like to know the correst valve combination. ,Valves. The former are the better, but ‘they are rather difficult to obtain, As for the power valve you can use.one of the 171A or B605 type. ARSITEH (Te Aroha): I have constructed a powerpack with a core of 1.5/8 square. I designed it for 250 volts, rectified and smoothed, for it has 2200 secondary turn centre tapped. My voltage seems rather high. A.: Yes, we think it would be about 2. Is the low number of primary turns (720) accountable for the hum? A.: No, you are using the correct number of turns for-the sized core. 8. When testing the transformer without load, hum was inaudible until the right ear was placed within a ‘few inches, but when placed in a metal ease

with the rectifier and smoothing choke the hum can be heard all over a large room, A.: It seems as though there is a loose lamnation or some loose metal, about the transformer. How does it go when on load? ‘It should not be operated in anything but a low condition. 4, Will you be bringing out an allwave electric set in the "Guide"? A.: There is a shortwave all electric set, but specifications will be given for broadeast coils. 5. When will the "Guide" be out? A.: The middle of March. 6. Could the L.W. Three be converted to an all-wave set? A.: As far as we know, no. It is not suitable for high frequency ‘work. 7 What adjustments do you _ think necessary to change my powerpack over to the L.W.? A.: It would be suitable if used with half-wave rectification. ; C H.W. (Invercargill): When will ¢ you publish details of the Differential Four? A.: It will be a few weeks before we ean find space for it. Our programme, as far as we can see for the present, is: Next week, the conclusion of the "Pentode" article; the week following, the "Kestrel Three"; then some more crystal circuits, with probably the powerpack for the Home and Country: Portable or the Differential Four the following week. 2. Do you think it worth while scrapping the B.D. to build the Differential Four? — A.: It is very questionable. The Differential Four would, of course, give you all-wave reception and would not need neutralising on account of the sereen gid. Furthermore, it employs a smoother control of reaction and. has the advantage of the lift given by the g.g. valve. . 3. What method of coupling will be employed between detector and r.f.? A.: In all probability air-core transformer. 4. Will the set radiate?-No. ECORDITE (New Plymouth): Can you tell me how to neutralise the "King" neutrodyne model 25? A.: In the centre of the chassis are two cartridge-like condensers. On these is a seal which you must break and then adjust the handles until the seit is neu tralised. Actually, to carry out the neutralisation process, youu must tune to a strong station and then shut off ihe element current to the r.f. valve nearest the detector. Move the neutralising condenser nearest this until the signals are weakest, then re-light the filament and dim the filament of the first r.f. valve, repeating the process with the neutralisation condenser nearest it. NOTH.-Would correspondents, when referring to previous correspondence, kindly re-state their problems. In this case we are not quite sure if the question asked was the method of neutralisation. If there are any others, would the correspondent please communicate with us again? Ww,pa: (Inglewood). --My dynamic speaker has recently develo; a serious rattle. oat

A.,: It appears that the centring device has become limp or out of alignment, and the voice coil is touching the permanent magnets. You probably will not be abie to rectify the trouble yourself. The diagram shows some of the points in which trouble may occur. 2. My aerial is 7O0ft. high and 150ft, long. Would I get better results by shortening it? A.: In all probability,. yes. It is too long. ‘Try the effect of a .0001 fixed condenser in series with it. 8. My set is an all-wave four-valver, using ©6083 as a power valve. Could I obtain a better combination? A.: B405 would give you a greater lift and handle all the volume you fequire.

MESTIFIED (N.P.): Is it possible that an all-wave set may work weil on broadeast and very poorly on short wave? A.: Yes, quite possible. . An all-wave set is either designed for maximum efliciency on either short or long wave and rarely gives maximum efficiency in both directions. | 2. Do you think locality would have this effect? A.: Locality has a big influence in reception of both short and long waves, but whether this is your trouble is another question. 83. Can pou tell me if my make of receiver is suitable in New Zealand, and do you think the Differential Four would be better? A.: From what we have heard your set should give quite good results, but we are inclined to favour the Differential our. ° (CAMERA (Hastings) ; I took my commercial receiver apart and constructed the Night Hawk Two. Is it necessary to have my license changed to that name? A.: Not until you renew your license, 2. There is a very pronounced buzz in the phones. A.: We suspect your transformer, for though it is brand new it is of an unusual make. 3. The transformer has the following markings: P).P2. secondary, and Pl. primary. A.? t does not give us a great deal of indication. The secondary terminals are G and GB, while the primary are P and B+, but which is which we do not know. Connect them up and reverse them if resulta are not quite as they should be.

D.B. (Tolaga Bay): Are the seconNe daries of the rf. . transformers wound with 26 or 24 wire? A.: Either, but 26 is the more usual. 2. The sketch of the regeneration coil places some terminals on top,’ the article on the bottom. Which is correct? A.: It hag been shown on the top in the sketch merely for clearness. We would have had a fine old jumble had they .all been put at the bottom: and half the bottom broken away to show the primary coil. You could quite easily leave them on top and carry the leads down to go underneath the set. 8. Can 82 gauge wire be used for both primaries and tickler?-Yes, 4, What diameter and height are the coil shields? hi &: 4tin. diameter and 4h to Sin. 1¢ 5. suggest building my set mounting the valves horizontally and using in-ter-stage screens. Will it be necessary to shield the coils? A.: No, unless oscillation trouble is encountered. Is the L.W. still king of amplifiers and how is it that it is not more used on commercial sets? A.: It is still the best amplifier. Tt is not used on commercial sets because of patent rights and secondly because it is fairly solid on the output valve. This valve is called upon to operate, for some seconds without grid bias. . I have two 222 valves and am doubtful about their efficiency, What is the usual test? A.: Connect them up with grid bias, and see if the plate current is as specified by the manufacturers. The application and shorting-out of the grid-bias battery would cause a swing over severai

ORRESPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Box 1032, Wellington). uestions arriving without it are likely to go asiray or be delayed. Name of set £002900000008000800008 Number of Valed ..ccocecce0ee0see Name 000000000009000800608O000008 Address 2@0060080000000000 o 020000000000800000008% ® ‘e 200000006 000000029 0808 88S008080999999 Nom de plume SOAOGHOCOSSOGSRSeRe To be kept in subsequent inquiries. Date " ecccccccccvcccccccceioses 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 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 maiter supplied, which may be quite inadequate, Intricate and involved specifications csgnot be supplied without a specialist's fee, _.

milliamps. The exact number can only be obtained from the valve curves, but they, are not easily obtained for your type of valve, Connect the screen voltfgets shown on the valve maker’s specifications. ; "+8. What would be the total plate conpuinptio#:of the Tongariro Portable using two volt. valves? Would the efficiency and plate consumption be increased greatly by using four or six volt yalves? ~ > "AY: Neither efficiency nor plate consumption would be greatly inereased by vusinghigher voltage valves. . The total plate consumption is about 10 m/as., but you, could check this. up for yourself quite’ easily. . ‘9, Which superhet. short-wave convert"er is -the: best to: construct, your. own, "Radio News,’ July, or Q.S.T., July? As: We do not know anything about those described in the overseas magazine, but their circuits are, of .course, quite sound and there is no doubt: that they will work well, We have tried the "Radio Record’ one and there are ‘no complaints to make, so we presume you would be safer to follow our direcions. ‘A. J.J. (Ongarue): When I was buy * ing the components for my set I wan told that amperites were not necesry. A.: If the voltage of your "A" suppl; is equal to that of the valve you are using, ah. amperite. is not necessary; otherwise it is. Sometimes it is used to provide bias for a s.g. valve. " 2. On some evenings there is a gurgle

in the speaker like a man with a bad | cold. Can this be put right? ‘ A.: It is difficult to explain why ‘il should be present some nights and absent others. We suggest that ;cu test your battery, your valves, or it may possibly be due to a defective grid-leak. 8. I eonnected two coils »f my crystal set in parallel. One had 40 turns and the other 80. This quite doubled the volume. A.: It is not that you have come across a new circuit, but your tirst coil did not haye the right number of turns, and the addition of the other one brought the total inductance to whet it should be, or thereabouts. You would get the same result if you removed ihe 40-turn coil and varied the. tapping to the erysta; until you got good results. ; Could I turn my all-wave set into a screen-grid one or would it be better to rebuild it? A.: It would be better to rebuild it along the lines suggested in last week’s ‘Radio Record," or to the Ceéesign of the "Kestrel Three’ which will appear shortly. P T.W. (Dunedin): I have a 0 to 100 * milliameter and wish to make a valve tester. A.: This was fully described in the "Radio Record" of February 1, 1929. We reproduced a sketch of the hook-up to

the milliameter showing how this is aecomplished. The second sketch shows a suitable connection when not using the set supply. Test the valye by switching in and. out the grid-bias and check up the difference by the valve maker’s curve, 2. Would a six-volt accumlator be of any use?-A. : In what leads, should I place the ? A.: As shown in ‘the second sketch. 0-100 is a little too big, however, for this test, and you cannot get a true indication of a valve’s with so small a-scale. ~The valve ig tested ‘y duplicating the conditions set out in the makers’ list’ or in tablés and comparing the milliamperage with the anode current shown for the particular voltages tsed. T. E.P. (Lower Hutt): Where can I obtain particulars for valve base coils for my 'Cossor’ Melody Maker? A.: You can make up the ‘coils described for the "Differential One," but omit the. primary coil and wind the see-

ondary and tickier upon'an English base, watching that. you bring the wires out to. the’ correct pins. You will have. to cheek up on your circuit for this. 2. The aerial has 50° turns and the secondary 69, A.: In the case of the screen-grid valve you will need a primary and sec ‘ondary as deseribed in the "Radio Record" out tickler. Use a .0001 mfd. condenser in series with the tuning condenser to reduce the capacity to a suitable value. WENTY-FIVE" (Wellington) : Does a wave-trap decrease signal strength? A.: Not if it is well designed. = An up-to-date one appears in "Radio Guide" to be published shortly. 2. What valve should I use for my fivevalye neutrodyne? A.: Four 201A type or 22’s and a 605 type power valve. 8. How long should the earth wire be for efficiency? A.: It should be as short as possible and in any case not longer than 6ft. WIRELESS (Dunedin).-Is the onevalve set in the "Guide" superioy to the differential one? No; the differential one is the best we have yet brought out. 2. Is it possible to make it into a twoyalver? . A.: Yes, by using a transformer and another . valve, 3. When I have the tuning condenser full out and the reaction condenser in, the set howls. Why?-A.: It is oscillaing. 4. Would a fixed condenser across the phones make any difference to reception ? Yes. If the condenser is large enough, it will cut off some of the higher frequency ani give a lower, weaker tone, 5. Is it possible to incorporate a filter condenser in the set? A.: Is is not worth while, when running a two-valve set. GNEI (DDO (Whangarei).-Where can 1 procure the necessary transformer’ for a Bremmer Telly set? : You could use good 33 to 1 transformers, as you will probably have considerable difficulty in obtaining the ones you mention. You would need to use good transformers with an AIFS in the la t stage. You are, however, taking a big risk in using three transformers, as it is very difficult to get them working correctly. Are you quite certain your "transformers are not impedence-capacity units? 2. Is it possible to design a stage of rf. to give a constant gain over 200 io 500- metreband ? A.: A relatively constant gain‘has been achieved by some of the better commercial receivers, though it is almost impossible for a home constructor to do this. 3. Which gives a better coupling over the whole musical scale-a choke condenser output filter, or an output trans: former of a 1-1 ratio? A.:' The -transfarmer_ is general considered to be the best. but it should be matched to the speaker in use and not chosen haphazsrdly.. Took out for our article on matching in the 1931 "Gnide." [PROVEMENTS (Pahiatua): Is it worth while to convert my factorybuilt set into the Outspan Five? A.: The Outspan Five would give a far greater pickup than your own set. You would probably have to purchase a large number of extra parts. Take stock of your own set before you pull it to pieces, and then you can decide if it ig worth while making the alteration, 2. What is the method of neutralisation employed im the accompanying circuit? Has. it any advantages? A.: It is not a standard measure, and we have not encountered it’ before, . 3. What is the capacity of a variable condenser with 21 plates? ' A.: If the plates are: large it ‘is " 0005, if they are smaller, it isa : Probably it will be a .0005.

4, Could I employ differential | action on the set ag it is? : A.: In all probability, yes. Wind the’ 25 turns on a 24in, former, using: 50-gauge d.s.c. wire, and slip it’ inside the 3in. former of your detector. coil so that the turns are under those at the grid end. . Then make the: connestions ; shown in our dagrams,

A Sunple Vawe lester. Valve Tester-2, ° 5. What value rheostat must I use to control the volume by dimming the filaments of the first two r.f. valves, which are 221's?-30 ohms. 6. Is there a better method of controlling volume? A.: For your set, no, 7. What is the output of B605 87 watts. & What is a suitable screen grid valve for use with the Outspan Five? rs A.: There are several which will do quite well. We noticed that those firms\ who advertised with the article quoted several valves which are quite to be recommended. A B. (Petone): Can IT use .0005 vari*able condensers for the Night Hawk? A.: Yes. Wind about 10 turns less on’ the broadeast secondary coil, _ five turns less on the primary, and leave the tickler the same. The short-wave’ coils need not be altered. OLLITEH (Pio Pio): I intend to.build a foundation set, but I do not wish to spend too much at-the present time. Av: Try the: Differential Two or Three laying ‘it out as described by "Cathode" in the recent series so that a fourth stage

Yr ¥ , / 4 @an be added later on. The three valves i would probably give the main Australian stations on the speaker. L D. (Te Awamutu): My set howled * loudly when tuned to .a station, and when the volume was tuned, passed beyond a certain point. : __ 4.: This is a case for a serviceman. Jt is probably a defective bias resistance in one of the radio stages. This might be a soft detector valve or a defective acteen grid valve. These things can be ‘determined only by testing. 2. When the pickup is attached it will ne bring out the high notes of the reeord, A.: This is due to the design of your set, and it can be overcome only by a special transformer used with the pickup. 8. How could I improve the selectivity of my set? A.: Use a wave trap as is described in e R.R. this week. URIOUS (Timaru): Is a driving rod of a cone speaker made of brass or steel ?--Usually brass. 2. How many-turns for a 20 Henry ehoke? A.: On a fin. square core, 10,000 turns, a gap of .08in., and 36 gauge Wire. : 3. Why must I have a laminated core? A.: Undesirable magnetic effects are set up when the core is solid. These are reduced to a minimum by the use of a laminated core. 4, My aluminium panel is corrodedwhat can it be coated with to prevent this? A.: Clean it with Brasso. 5. What gauge wire is used in loudspeaker coils?-40. VALVES (Whangarei): What valves would give the best results in an American five-valve neutrodyne? A.: Four 221’s or, failing these, 201A’s, and a medium power valve. G T, (Ngaio): Would a 66 R. unit or ® an inductor dynamic speaker be the better with the L.W.?--The latter. 2. Which is, in your opinion, the better of the two inductor dynamic speakers I mention? A.: We do not know anything about them, having tested only the Farrand. 8. Is the Loftin Three selective?-No. 4, Can a 30 Henry choke be used for Se. output instead of a 1-1 transformer? -Yes. ~ 5. I have some 36 wire. What would be the resistance of a choke? A.: If you are using a 1-1 core with ‘about 6000 turns the resistance will be approximately 350 ohms. . Can -R8 in the tuner be entirely éliminated if B plus is taken from the eentre tap of the 245? A.: As far.as the voltage is concerned, yes, though it may be necessary . tc incorporate a high frequency choke in this lead to prevent radio frequency cur-~ \went getting-into the power valve. Pk UP {Mata Mata): Would cop- ‘& per shield cans be satisfactory for the coils of the Outspan?-Yes. 2. If 32° gauge wire is used for the primary, how many turns will be requir-ed?-The same. yt, 3. Ig there any advantage in using a larger differential condenser? A.: A .0002 or .00025 are the best sizes for the Outspan. 4, What ratio output transformer should be used with a magnetic speaker? A.: Usually a 1-1, though we give this subject full consideration in the 1931 "Guide." 5. Were Lotus Vernier dials used on the original?-wNo. 6. Is the following valve combination satisfactory: Two S610, two L610, and two M256 in push-pull?-yYes. E H. (Karori): Can the specifications ©. for the 250 valve power pack described in the "Radio Record" some time ago Pg adapted for all-wave xectifica-

A.: Yes, the 280 may be used by connetting the high voltage secondary one end tg each plate and the centre tap of the filament winding (5. volts) as high tension positive, c 2. Can I use the 7.5 volt. winding by taking off suitable tappings? ~ A.: Yes, the best plan is to use one centre tap for both and make the limits of the 5 and 7.5 windings on either side of this. This would not take away the efficiency of the vack. 3. What initial voltage would be necesgary to obtain 250 volts rectified ?-About 4, Is a former 14 by. 14 equal to a former 1 3-8 square?-Yes. 5. How many turns would be necessary on this former to give 2.5 and 7.5 yolts?-12, 24 and 36 respectively.

"MLECTRODE" (Ponsonby): Can I work a screen-grid valve with power detection straight into a push-pull stage? What is the value of the resonating condenser? ’ ’ Av: About .25. The system should work quite satisfactorily. ay . Can I use a centre tap speaker direct into the output? A.: It is not wise, especially with the big drain of 60 mills, FIAROLD (Dunedin): (1) Could you give full details ag regards the Pierce earthing system? , A.: See elsewhere in this issue. 2. The shortest and most direct earth is the best. Is this correct?Yes. 8, Is the size of earth immaterial? Will 7/18 insulated do? A.: The bigger the actual contacting surface with the earth the better; 7/18 will do splendidly. 4, At what height must an aerial cross an iron roof so as not to be weakened by it? A.: It hag little effect above 20 feet. 5. At a- distance of about 14 miles from 4YA, Dunedin, should a long aerial (say 180 feet) bring 4YA in the background of all stations reveived? A.: No, with a selective set it should interfere only over twenty degrees, and be heard weakly over thirty or perhaps orty. 6. Will a microphonic valve always give trouble, no matter what stage it is used in A.: Rarely. It must be a bad valve to do this, 7 OUNTERPOISH ‘(Christchurch).-I cannot place my hand on the "R.R." relating to the counterpoise earth. Would you repeat the information? A.: A counterpoise .is: really a second aerial erected just far enough from the ground to be clear of all objects that are likely to encounter it. It must be carefully insulated as-the aerial itself, ‘and must be with the ground

terminal, the norma] ground being disconnected. Ren (Cambridge).-I can tune Weljington at two places on my dial. wy! ? this second place is what is known as a harmonie-it is a wavelength half as long again as the original one, and is sent out by every transmitter. It is heard only from powerful transmitters, such as 2YA or one nearby.