Questions and answers

Radio Record, Volume IV, Issue 12, 3 October 1930, Page 24

Questions and answers

Kao (Invercargill) : Can a blasting loudspeaker be remedied? A.: The blasting may be due to several factors-the speaker may be totall inadequate to handle the volume you expect it to; there may not be a power valve in the last stage, and the bias may be unsuitable. You may be expecting the set to carry too much; and some of the batteries may be flat, particularly watch the "CO" battery. 2. I am troubled with a hissing noise. Is it the fault of the aerial or the valves? Remove the aerial and see if it persists. It may be dte to electrical interference, or a defective component. It is probably not due to valves being unmatched. The set may not be neutralised. 8. Would an eliminator be of any use? A.: An eliminator would give you a tonstant supply of "B" current without the worry of batteries. You would prole.

bably be able to get much higher voltage than would be practicable with batteries. [D®TECTOR (Invercargill): Can an ordinary crystal detector be, used in the place of the semi-permanent detector shown in the Radcord crystal set in the 1930 "Guide"? A.: Yes, but you would have to alter the lay-out slightly. JC ILOCYCLE ‘ (Bluff): Can you tell me how to add a stage of screen grid r.f. to a four-valve B.D.? -: We have forwarded you your sketch with.the screed grid valve shown. It: will be-necessary to shield each stage of r.f., and the detector or the coils and the screen grid valve in cocoa tins, . Have you a cirenit of a screen grid booster?

A.: We do not advise the use of a s.g. booster. It was a good expedient before the use of the s.g. yalve was properly understood. It,is now better by tar to incorporate this valve in the receiver itself. It may be used in either the first or the second stage and should for preference be transformer coupled with about 25 turns on the primary, when a 2tin. former is used. ; 38. How many turns would a 409 require on a 23in. former? , A.: About 12. 4, When I, put more than 16 or 17 volts on the detector the second. last stage motor boats. A.: Reverse the connections to the primary of the first audio transformer and if this is unsuccessful try the second audio. It may be that you are drawing too much from your eliminator. 5. For local reception I usually use PM4.in the first audio. A.: This is quite wrong. A general purpose valve will be quite sufficient. ""NBIRTY-THREE" (Timaru): When I connect a pick-up to my set a whistle is heard in the speaker and the music becomes distorted as though the speaker cannot carry the volume. A.: Perhaps you had the volume control turned on too full, If this is not the case see the dealer who sold you the set and ask him if you have connected the pick-up correctly as you do not give us sufficient details. Can I use a short-wave adaptor on my set, and where could I obtain one? A.: If you do not care to make your own, conimunicate with Mack’s Radio, Wellington, who are specialists in shortwave sets. 8. Which is the better pick-up of the two I mention? A.: Sorry, but we cannot discriminate between two commercially-made products. "GAINING EXPERIENCE" (Queenstown): If I remove some turns from my coil would the lower limit of the tuning range be lowered? A.: Yes, but you would clip off some of the higher waves. Your best plan would be to get specifications of coils for the wave band you wish to cover. If you let us know the capacity of the condensers and the size of formers we would design them for you, but try your dealer first. 2. I have two plug-in coils; which one should I reduce? : A.: You must take an even or pro- portionate number of turns off each secondary oil. 8. When I connect the pick-up across G. and IF. of the detector valve my tuning was altered about 10 deg. A.: This was due to the extra capacity the pick-up introduced into the grid circuit. Providing you (disconnect this apparatus when it is not in use your tuning range should not be affected. "T ONGWAVE" (Auckland): What 4 voltage should I use for a two-valve set to get really good results? A.: As high as possible, but if you are using batteries 185 is a practicable value, 2. Can I obtain a copy of the "R.R." containing a description of "R the W Two" ?-yYes, from our office. 3. If I construct a R. the W. two, how

far should I be able to get on the loudspeaker? A.: Not yery far, as the set is primarily designed for phone work. Do not:be disappointed if you do not get any shortwave stations on the speaker -gatisfactorily. 4. What valves would be needed A.: The two you have should be satisfactory if they are of the 201A type. Pickup (Hastbourne).-I am building an amplifier with two stages of pushpull, What is the nearest ratio of Ferranti output transformers to supply a unit with a resistance of 1300 ohms? t A.: Use an output transformer with 4 tapped secondary, and see which one suits your reeciver best. ° 2. If I use two of these speaker units would the ratio need altering? A.: Yes, and herein lies the value of the tap transformer, 8. I intend using a trickle charger to supply the filament current. Would you give the specifications of a suitable choke? A.: You will find a fairly full description of a suitable choke on page 65 of this year’s "Guide." If you have not a copy and cannot borrow one, write us again and we will reprint the article in the "Record," as quite a number hayg’ asked for it, . 4, What is the approximate capacity per sq. ft. of surface of first-class aluminium, and also the best solution for a condenser? A.: Your question about the capacity of aluminium is most indefinite. A short ~

time ago we published an article about electrolytic eondensers. You should find all the information you require in that. Regarding the making of the condensers, we regret that. this is not suitable for afnateur construction, and we have been asked. by the authorities to draw attention to the dangerous nature of such ex-~ periments. ‘ . N H. (Te Awamutu).-What book * would be suitable to assist me in the construction of a three-valve set? Av: The "R.R." of February 28, and March 7, "Round the World Two" and "Three," two. very successful sets, are described. If you do not require the shortWaves, you could make only the broadcast section. . POWER-VALVE (Palmerston North): There are three rheostats on my home-made set, but. the one controlling the detector is the only one of any use to control the volume. How can I make the other two. work? . A.: You could use a 30 ohms. rheostat on your radio valve, and none on your audio, or a better way would be to control volume by means of a 500,000 ohms. resistance’ between: the aerial and the earth. The most. successful system of yolume control is by increasing the bias on the radio valve. This is the method incorporated in most of the new American a.c. sets. W Q. (Whangarei): My electric set ¢ distorts badly. Should I change the valves? A.: No; consult the. dealer who sold you the set. , S N.K. (Miramar): I cannot under- ® stand the references to time express-. ed like this-400 to 800 G.M.T. How does this correspond with New Zealand? A.:> For scientific purposes the clock is regarded as being graduated from 0 to 4 hours, commencing at midnight. Four figures are used to express hours and minutes, e.g., 4 a.m, should be wriften 0400, ereas .9.45 p.m. should be 1745. The G.M.T. refers to standard time at Greenwich, near London, and is a basis for calculating time throughout the world. Every country is so many hours ahead or behind this-time is based on . geographical position. Our DX __ clock would simplify the reduction of times to New . Zealand. standard. The numbers you quote, 800, 1400, and 1800 G.M.T. (allowing one hour summer-time in England), are 8.80 p.m., 2.30 a.m., and 6.30 a.m., New Zealand time. UBRISTS (Napier): What is the ~ value of a condenser with 21 plates? A.: This is an unusual size, and is probably either .00035 or .0005. If the plates are large it is the latter. 2. How many plates should be removed to alter the value to .0002, and would double spacing be an advantage?

A.: Bleven plates are required for a 2 condenser, and double spacing would certainly be an advantage, .but you would need less plates. The easier plan would be as you suggest to use a series condenser of .0045 or :003, according to the capacity of your condenser. . . 3. What is the diameter of a test-tube? -About . fin. , pAk4MA _(Palmerston North): Can. ' an.a.e. short-wave adaptor be used with an a.c.?-Yes. . Lot M BH. (Nelson): Will you explain how * to make a saturated solution of bluestone for the Daniell’s cell charger? A.: Take about half a pint of water,

preferably warm, and add the bluestone crystals, stirring all the time. Leave them to soak, and when they have disappeared add more until you reach the point where additional crystals will not dissolve. You will require about a pound of bluestone for the six jars. 2. Will six cells be enough to keep my battery charged?-If it is a four-voit one, yes. 8. My set works best with the detector rheostat turned a quarter on. Why?: A.: At this point the valve is heated to its optimum temperature and further increase does not improve matters. AvAX (Auckland): I enclose a circuit. Is it suitable for New Zealand conditions?

A.: /It should be’ quite an‘ interesting circuit to try out, though it will not have the amplification that a B.D. type of set would have with a stage of screengrid r.f. transformer. coupled. 2. Are the. coil windings . correct?es: 8. Could L2 be wound on L8? If so, how many. turns?-yYes, 20. 4, Are L2.and L3 wound in the same direction ?-Yes. x ‘5, Is my valve combination correct ?- es. G. G.L. (Palmerston North) : I enclose a circuit which I have pieced together. Would it be satisfactory? A.: When one sees your set .there is a strong temptation to quote Grey"Let not ete. _ You would be very lucky if you managed to get this’ set working. ‘Two stages of screen grid

should be ample for your requirements, for you must realise that when a certain point is reached the amplification of noise is increased to such an extent: that the gain one way is offset by the other. 2. Is the power supply adequate and are the coils satisfactory? » A.: Yes, these seem to be quite in order, but we advise you to use smaller formers, say 2ip , 8. Is the volume control satisfactorya 200,000 ohms resistance across. the first transformer? A.: It would’ be better to incorporate some form of y.c., which prevents overloading the detector. Adjustable bias on the s.g. valves would be better. ‘ 4, Is the coupling between the stages correct ?7-Yes. NOTH.-Your results. would be better if you used anode bend-‘detection. ' QPTIMUM (Wellington) : I cannot get more than the local station; and then only very poorly. I am using a screen-grid four-valve set. When the s.g. valve is moved from the socket there is no difference. A.: Bring in the aerial to the grid of the detector valve, and see how reception compares with the full set. No doubt there is something wrong with the screen grid stage. Do you have the connections made correctly? The plate in your valve is at the top. _ Check’ over the wiring once more and look for dry joints. If everything is in order have the s.g. valve tested. The list of valves you append are not entirely satisfactory. ou have not stated the order in which > —

they are placed. If one is to take it that if they. are in the order nominated in your letter you will no doubt be encountering. trouble on this score. B205 is a last stage valve, A209 first avidio, and ,.if anything, detector, though A215 .be. a far better, valve. for this socket. than 225. 2: What stations should I be able to receive on this set A.: This on your locality, but you should be able to hear Australian and Japanese stations quite easily. ° ATOM (Wanganui).: I am a little disappointed with my set, for when i attempt to increase the volume there is a terrific noise. A hospital with an X-ray. plant is not more than about a mile away, and freezing works are also

é : handy. What steps can I take to reduce the noise? I cannot get the Australian stations in the daytime, nor the Ameriean stations. ‘A.: You appear to be in a very bad area and the noises are due probably to electrical interference over which you have no control. Take off the aerial from your set and then turn it up to its maximum point. If you get any noise then your set is probably defective and you should call in the agent who sold it to you. If it is outside interference you might approach the hospital and ask them to take steps to prevent the noise. It can be reduced by a suitable filter. LAMBDA (Wellington): How is a midget condenser placed in parallel with the tuning condensers? A.: You connect the fixed plates of the midget to the fixed plates of the tuning eondenser of your set, and the moving of

each to one another. A midget condenser will contain 7 to 9 plates. 2, When the detector is just about to oscillate there is a tingling sound when -_ table on which the set rests is touched, . . A.: Use a non-microphonie valve socket, or, preferably, see the expert of the firm who handles your sets, H™ (Blenheim): I am troubled with noise which I think I picked up from high-tension lines which run about feet from our aerial. A pole nearby sends out sparks on a wet d*y. The power board engineer said we ould always get noise if we are near high-ten-sion wires. A.: The sparking should be prevented, as it is a very lively source of interference. It can quite easily be overcome, and if you have not mentioned this to the power board engineer do so immedi-

ately. A certain amount of noise will always arise from high-tension lines. Af ©. (Dunedin): I have an old model * battery set and wish to know if the -uumerated combination of valves is satisfactory. A.: It is decidedly unsatisfactory, providiny they are placed as you indicate. B605, first radio, and A609 first audio, should be interchanged, but make quite sure B605 is not now in the power position. You should use A609 not A409 for a detector. BG605 with 100 volts should have a bias of 9 volts at least. GTINGY (Petone) : I wish to get better results from my set. Would a change to other makes of valves improve matters? A.: No, if you are using the same make of valves as your set you are using the best possible. If you think your results are unsatisfactory, call in one of the experts of the firm who run these sets. 2. Does the DX Club imply the use of short-wave receivers?-No; ordinary broadcast. O K. (Napier) : How can. one obtain a "e license for operating a B class station ,and what is the cost? _A.: It is necessary to pass an examination set by the P.M.G.’s Department; for information write District Radio Inspector, Wellington. : 3. Can a semi-power valve such as DEP 410 be made to oscillate?--Yes, 8. Can an.a.e, directly-heated valve be used in a transmitter of, say, 24 watts? ---No. 4. How many volts would be needed on the plates of the oscillators in a 2}watt transmitter?-Approximately 250 volts 20 m. amps. \icroamMp (Gisborne) : How can two rectifying valves be connected to the a.c. mains without a transformer? A.: To a this would be contrary to the regulations, A transformer with a separate primary must interpose between the mains and any radio apparatus. 2. What is the inductance of the following filter? A.: The sum of the separate inductances. Do you know that as it is at present it contravenes the regulations? 8. Would two chokes be better as a filter if used in parallel and how would this affect the inductance? A.: The total of inductances in parallel is equal to the sum of the reciprocals of the separate inductances and in series to the sum of inductances. Note: We advise you to construct a "RB" supply on orthodox lines, as details of such a system are always readily available. F, (Gisborne): Can a screen-grid * valve be added to the R. the W. Two? A.: Yes, see the description in the 1930 a ruide" on the screen-grid short-wave set. 2. The set goes in and out of oscillation with a plop. A.: A short time back we had an article dealing with troubles on short-wave such as this. ‘Try varying the "B" voltage, the capacity of the grid condenser, and even change your detector valve, preferably to one of the special detectors. E J.H. (Napier): Would I get better ¢ results with my three-valve kit set by adding another stage or by using sixvolt valves? A.: Six-volt valves would give you slightly stronger signals, but adding another stage would be better. An additional stage has been described in the "Radio Record" recently. B S.D. (Wanganui): Will the voltage ¢ divider in "Megohm’s" 250 power pack have to be changed if a Raytheon valve and the 250-volt tap is used? A.: Leave out the extra resistance of 12,000. 2. Would ‘a Raytheon B.H. supply enough ‘current for two 245’s in push-pull?-Yes,

3. What will be the resistance to break down the current for (a) one 227 in the amplifier and two 227’s as radio and detector? A,: For the audio. valve 4500 ons and for the other valves 2000 ohms. . The two. resistances are connected in series, the 4500 ohms first. ‘. 4, Will the bias potentiometers have to be altered? A.: For bias for 245 about 1000 ohms variable would be required. The two 400-ohm potentiometers could be wired in series, one made variable and used for the two 245’s. 5. Would any pant of the pack need altering? A.: Only the chokes, which should have a smaller gap. 6. With the original pack was it possible to get 130 volts grid bias-by using 250 valves?-Yes. 7. Where will I have to tap the secondary to get 250 volts rectified and smoothed? ": A.; At the 1800th turn. a, 8. How many\turns and what gauge wire will I need to supply 2.5 volts for a torch bulb pilot light? A.: Sixteen turns of 30 d.c.c. 9. How many pieces of each size stalloy are required for the transformer? A.: This depends upon the thickness of the stalloy. If you ask the dealer for stalloy of the two sizes, each enough for a pile 24 inches high, you will get the right amount, 10. I wish to use the metal rectifier to supply speaker current. Can I have a winding on this power transformer to supply the eight or nine volts necessary? f so, what size and covered. wire? A.: You will require 55 turns of 26 gauge d.¢.c. wire. 11. Will the gauge of primary wire need to be changed for this?-No. 12. Where should this winding be?-On the outside. 18. Would I get better results from the speaker if I used a separate trans-former?-Not necessarily. vy G.N. (Lower Hutt) asks certain ques- * tions regarding an A and B trickle charger described in the "Radio Record" some considerable time ago. A.: We regret that we cannot now advise constructors to make this piece of apparatus, as we have received advice from the electrical supply authorities that apparatus of this nature presents a certain danger and should not be made. Your best plan would be to make a charger such as described in this year’s "Radio Guide." ; QOEAPUA (Gore-Chatterton): When I attempted to advance the volume control, especially if I am operating on the lower wavelength, the set sets up a vigorous howl which stops if I hold my hand over the second radio valve. A.: Your set is not neutralised. If you ean locate the neutralising condenser adjust the one controlling the second radio valve slightly until this trouble disappears. W.-©. (Timaru) : Please cup War * ticulars for building a transormer for stepping current down from 230 volts a.c. to 180 volts, to light thirty-three sixvolt lamps in series. A.: You have not. stated the amount of current to be taken by each lamp. The transformer would have a core 1} x 1 with 1070 turns on the primary and 1025 on the secondary. The gauge of

wire would be that necessary to carry the current of one lamp as they are to be used in series. Full details on transformer construction can be found in the 1929 "Guide." ' 2, Can a 280 volt a.e. dynamic speaker be used with a battery set using an eliminator? . A,: If you mean one that uses a rectifier, yes. It is necessary only to connect the a.c. mains to a suitable lighting point. "J. J. (Dunedin): Can a wave trap cut out 4YA and bring in 2YA when the two are separated by one point on the dial? A.: It is very doubtful, especially as you are right in Dunedin, ‘put you could try the wave trap in the 1930 "Guide."