Questions and answers

Radio Record, Volume V, Issue 3, 31 July 1931, Page 13

Questions and answers

W J.C. (Wellington): Hand capacity *is very bad on my set and specially on waves below 50 metres, in spite of the fact the plates on the tuning and reaction condensers are earthed. A.: Hand capacity is sometimes very difficult to eliminate. The usual prolure is to do what you have done, and if this fails fit a metal panel and earth. If that fails you will have to shield the set entirely and earth the shielding. However, we think the metal panel will overcome your difficulty, The use of a different valve and a different grid-leak sometimes effects an improvemen 2. The set fails to. oscillate on the broadcast band, although it is excelent on the short-wave. A.: You require more turns upon the tickler coil of your broadcast coils. 8. Would an accumulator be more economical than three dry cells? A.: It depends upon the valve you are using. If you are using one with a eonsumption of about .06 of an amp then it is just as economical to use the three dry cells, otherwise a small accumulator would be the better. You must not, however, everlook the fact that the cost of an accumulator would pay for many cells, Hes TOP (Auckland): I want to eonstruct the Differential One for short-wave work and do not want to go to the expense of buying a differential condenser, for I have .0001 and .0005 eondensers on hand. A.: You ean use the .0001 as a reaction condenser, but you cannot use the, ‘differential’ circuit-try the "Ranger. 2. On my one-valve set I have logged 14 Americans and five Japanese stations. If I add a further stage of audio will I get still more stations A.: Most decidedly, yes. You have done excellent work with the one-valve get,

3. My valve is 24 years old. Will l get better results if I install a new 201A% A.: Yes, but we would advise you to use a new 221 which is the modern equivalent of the 201A, B D. (Hawke’s Bay) : What is the cor- * rect type of 4-volt Mullard valves to be used in a four-valve B.D.-(a) with 2 dry B batteries of 90 volts; (b) with 135 vO. A.: PM8, r.f., PM4DX detector, PM3 first audio, PM4 second audio. These will be quite satisfactory for either 90 yolts or 185 volts B. 2. Are Mullard valves still imported into New Zealand? . A.: Yes, in very large numbers. information is incorrect as far as we now.

BRONNITUNE (Patetonga): What can I do to reduce static and loud erashes that oceur intermittently? I believe these crashes are from a nearby power plant. A.: Find out where the loud erashes are coming from. If you know of anyone in your neighbourhood who is using a power plant, your best plan would be to approach them and ask if they would incorporate filters, as these would undoubtedly overcome the noise. Filters were very fully discussed in the "Radio Record" about two years ago, and what was said then still holds good. It will be a case of using a choke condenser near the commutator of the power plant, which will certainly smooth out a great deal of the . noise and improve your reception. Failing this, shorten your aerial, but get it very high, pointing at right angles to the direction from which the noise is coming. A tone control will cut out a fair amount of the noise, or, rather, we should say, it will dim it. If your set is not fitted with a control, you could buy one for a reasonably small sum. This would eut out the higher notes and at the same time eliminate a great deal of the noise and give you much quieter reception. 2. Is is possible to have phones incorporated in this set? A.: Yes, as soon as we cam get the space we shall publish an article, on connecting phones to an a.c. set. Watch out for it, it will be along soon. 8. What is push-pull amplification? A.: Push-pull amplification implies the use of two power valves instead of one. The work that one valve would normally do has been divided between two valves, but the circuit is such that not only is the work split, but a greater amount of power can be fed to the speaker through the valves being connected in push-pull. The subject was very fully treated in the 1981 "Radio Guide." Ww J.M. (Auckland): I have a three- * valve N.Z.R. receiver, and am troubled by a continuous ‘low-pitched whistle. If I place my fingers across the secondary of the first transformer the whistle stops, and the tone and volume are improved. A.: Try reversing the primary. connections to this transformer. If this fails, connect across the secondary a one megohm grid-leak resistance, and we think you will then find your trouble will disappear. . What size coils, etc., are to be used for the accompanying crystal circuit deseribed im the "R.R."%

‘A.: That circuit is the "Rejecta Two," which was very fully described in the "RR." of June 19, 1931. A M.T. (N. Otago): Concerning the * matching of push-pull output valves to an output transformer are 2400 turns

correct for the primary of an output transformer if wound on a 1#in. core? A.: We are inclined to think 2400 turns rather too few. A better idea would have been to have wound on say twice the number and taken out several tappings so that you could more accurately match the impedance of the valves to the primary; iin. core is also a little large, probably beyond the optimum; 1 x 1 is usually considered satisfactory, and on this you will need at least 7000 turns,

Novick (Christchurch) : My set surges badly. Would "B" batteries be the cause? A.: In all probability yes, run-down "A’s" or a defective accumulator could also be responsible. A swinging aerial would also cause surging, but not to a very marked extent. You do not say how old your batteries are, but it would not be a bad plan to test them. A H. (North Canterbury): Is there @ * commercial shortwave adapter for an 8-valye a.e. receiver? A.: There are quite a number of them to be had nowadays, known ag super heterodyne converters. We think if you wrote to the Wellington agents for your set they would be to fix you up with the converter, We believe the prica is somewhere about £12 or £15, but we are not quite certain. . PpuRURI (Picton): Does my set Be up to 500 metres only? If so, there any way, without tampering with a factory-made set, that I can extend the: tuning range? : The only satisfactory way of Ox tending the tuning range is to add more. turns to the coil and this is not easy in a set such as yours. Hvidently you have just too few turns upon the coils as most sets range from 250 metres. to" 550, thus cutting out the empty space’ between about 80 on your dial and @ .~ AD (Te Awamutu): The shortwave | adapter in the 1 30 "Radio Guide" goes into oscillation satisfactorily on the to 90 m. coil, With either of the two smaller coils I cannot obtain oscillation on any part of the dial, I have increased detector voltage, but without result.

ORRESPONDENTS must attach "S ’ this coupon to all queries sent to the Technical Hditor (Box 1032, Wellington. Limit three questions, Dniews letter is accompanied by i/o ee. Name of set .....cccccccecscveccce Number of valves o.00000c2900000980 Name Address eeoc oso OODDO000DD OOOO COSO900 20000000000000099000089008 ©060000060500000000006800000080008080 COHOCOG OOH SSCHOGOSOBOOCOSOOBOODOSSOOOOSS Nom de plume eoeeesoceeoaoseoseseor To be kept in subsequent inquiries. Date cece Please Note:(1) Be specific and brief, tabu-_ lating, if possible. ; (2) Write legibly, and on one 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 specialisi’s fee. Oem een we neaas «*eoceen

A.: Use a 10 megs. grid lead and if this fails to produce oscillation increase the number of turns on the. reaction coils of the non-oscillating coils. 2. With the large coil in I can obtain oscillation over the whole dial, but at the’ lower end fringe howl develops om the oscillation point. Carriers of stations on this band are very bubbly and erratic and the stations difficult to hold. . A.: This seems to be due to a defective grid-leak, or a second rate r.f. radio frequency choke. If you experiment with different grid leaks and try altering the number of turns on the reaction coil, and so decrease the detector voltage, you will probably get some satisfaction. 8. Would it be satisfactory to connect phones direct to the speaker terminals? There is a filter condenser, in the circuit, but no choke,

A.: Yes, it will be quite safe to connect the phones where you indicate. ELECTRA (Auckland).-TI have built the "Sparrow Hawk One," as in the "Radio Guide," and it works very well, for sometimes KFI comes in faintly on the phones. I would like to use a midget condenser 50 mmfd. for tuning, and also one in place of the primary coil, Would an aerial condenser give as good results as a primary coil in the circuit? A.: It would probably give better results. Make a coil such as that deseribed for the "Kestrel Three" and try coupling it straight through without the primary. coil. 2. Would you please publish specifica tions for a 50 mmfds. tuning condenser and a .00015 differential reaction? A.: Yes, we will publish them next week,

(Note.-This correspondent adds that the Selectra crystal set in the "Guide" gives good results, It gets 1ZR, while 1YA is operating. Good. We like to hear of these successes. It cheers us up.-Tec. Hd.) G oO. (Auckland) : Would it be possible *to publish in the "Radio Record" a special page dealing, with crystal sets? this + logglity I diet 2YA, ise HC, 1YA, 2B and 1ZR on a erystal re there is a ‘erystal circuit which uses two crystals for getting double the strength. Could you publish it? .: You are very lucky indeed getting all those stations on a erystal set. There

is a full-wave crystal set, details of which were published in the 1929 "Guide." We reproduce the circuit here, but for full details you must look up the "Guide." Two coils are used, both being the same, and they are connected to two permanent crystals, which, however, must be accurately matched if good results are to’ be obtained... Practice has shown that although this set is a splendid one for experimental purposes one cannot. always guarantee success with: it. J. P.B.’ (Dunedin): I have acquiréd a s.g. valve and want to make up a five-valve set using s.g. rf. and s.g. detector and two audio. Can I get .a good circuit? A.: The- "Outspan Five" will be the set you are. requiring. It was described in the "R.R.’ of February 20. 2.I am now using a four-valve B.D. using A630,'in the first stage, A615, det. AG09, first ‘audio, and B605. power valve, but sometimes a erack of static makes the set break into oscillation. The station roars in and then fades back to its normal strength. What could I do to rectify this? A.: This appears to be originating ‘in the radio stage due to your using the A630 valve, which is not altogether | suitable for the B.D. unless specially matched. Such trouble is difficult to eliminage without reconstructing your r.f. coils. We suggest your using the "Outspan" circuit. 8. I am contemplating building the s.w. power pack'in the "Guide and Call Book." Is 36 gauge enamelled quite safe to draw 210 volts at 100 amps., and would the efficiency of ‘the transformer be affecied if the window were increased to 1/16" more than stated? ' A.: 36 enamelled, wire, would ‘be quite safe. The transformer May ‘warm ‘up a

little, but nothing to worry about, You could make the window 1/16" more. without having a detrimental effect upon the transformer, oe H. J.B. (Upper Hutt): I have made * three of the "Rejecta Two" crystal sets, and have been pleased with the results. The sets were constructed with various sizes of wiré, formers and -various settings for experimental purposes. One of the sets I found worked and is still working well with the first tap of the primary and tap of the secondary connected. On one I use a carborundum unit, which is decidedly superior to the caiwhisker which I use with the others. So far I have not been able to receive 2ZW with all my juggling with the coils, ete. In tg I know of no-one’ in this locality who as. . A.: We do not think that 2ZW is reeeived satisfactorily in. the Upper Hutt on a crystal. We have not: heard of it being done-certainly. not with a selective set such as the "Rejecta." It is quité understandable the set working well -with the top of the primary and the top of the secondary connected. You are then, to a large extent, offsetting the value‘of the primary coil.. Such a connection ‘destroys all selectivity. st T K.T. (Te Kuiti): I have a commer- * cial short-wave receiver which wil] oscillate satisfactorily on the 80 metres band, but not on the 40 metres band, unless I have 70 volts on the detector plate. There are 10 turns’ on the aerial coil Could I increase this number to cause the set to oscillate? mo, A.: It' is not wise to increase the num. ber of turns on the aerial coil, as. you will probably introduce dead spots. . Better nerease the number of turns on the regeneration coil, For the 40 metres band there should be at least three-quarters of the number of turns on the tickler as on the secondary. Sometimes an equal number are needed, though of course you can use finer wire for the tickler. Sometimes the use of a small value veriable |. condenser in the aerial will make the at oscillate. Say a,midget with a maximum eapacity of about .00005 mfds, O C.S. (Hamilton): I am using a three- * gtage hf. s.g. .a.c. receiver. Would you recommend plate detection, and if so, what value of grid bias would be necessary to use with the 227 detector valves? A.: Plate detection would, be satisfactory. The value of the resistance is 5000 ohms. It should be by-passed with a 5 mfd. condenser. ; 2. What are the specifications of a tone control to use in a set employing pushpull with 245 valves? : A.: Use a 1 meg. variable resistance in series with a .002 mfds. by-pass — condenser between the plate of the audio yalve preceding the push-pull stage and rth. (jpPsox (Te Kuiti): Are cireuits such as the one enclosed as selective as two stages of r.f.? In nearly every case, yes. 4 2. My "Sparrow Hawk’ one will bridg in 1YA, 2YA, 3YA, 2ZW, and 2B and 8L0O. Is this good? A.: Most decidedly, yes. 38. Why is it that-2ZW comes in with more volume than 2YA? . A.: This is rather difficult to explain, and must be due to some freak of your locality. 2YA is a much, more powerful station, and is generally received at greater strength than the weaker 2ZVW. B.D, (Blenheim)-How can I *. change the enclosed circuit from an untuned s.g. stage to a tuned s.g., stage? It employs a radio frequéncy choke between the aerial ‘and the earth. A.: In front of the r.f, choke employ a A { f transformer, such as the first transformer deseribed for the "Outspan Five," ‘and*ase } a condenser to match: the coil. It ‘will } then be all- right. Study the diagram/ published for the Outspan, and look, over the "Kestrel Three’ this week, and you might pick up something about tuned ri. , stages. .. aw 2. How many turns will I need on a

valve base coil for.the 80. metres band using a .00013 tuning condenser? A.: 154 turns of 26 d.s..c, wire, and for the secondary 14% turns of 36 d.s.c. wire for the tickler, 2. How many turns will I need for the broadeast coil? a A.: You will need two broadcast coils to cover the band efficiently. The first will require 80 turns on the secondary of 80-gauge enamelled: wire, and 30 turns of 36 dsc. for the tickler. The next eoil will require 148 turns of 30-gauge enamel] for the secondary, and 65 turns of 36 d.s.c. for the reaction. ILLIMETRE (Gore).-When will the a.c. yersion of the Outspan Five be published? . A.: We do not kiow. 2. What would be the most suitable valve in the last stage with two 224 rf. and two 227 det, and Ist audio in use? A.: One 245. If you want an a.e, circuit of the "Outspan Five" type, look up this year’s "Radio Guide" and you will \ find information that will interest you, the heading ‘of "The Radiogram ve, ADAPTOR. (Invercargill): I purpose ‘building the super het. ac. shortwave adapter described in the 19381 "Guide," but notice that certain of the values are not specified. Please furnish. A.: We reproduce the circuit with full details. This also answers the question of "Perplexed" (Putaruru). 2. Would this adapter give good speaker volume on shortwave when used with a midget set?-Yes, it would be quite satisfactory. CONTROL (Waikato): My set after going for about 10 minutes cuts off, hut when I switch it on again it goes, but erratically. The filaments are still alight when the set is off. On taking the set to an expert he could not locate the trouble. A.: It is rather strange an expert could not locate a trouble such as this. It may be anywhere, and itis difficult to tell you by correspondence where to look. Probably it is due to a loose connection or a puncturing condenser. It may even be in the speaker. One of these times when it is on take off your aerial lead and touch it to the post and note if the set recommences. The trouble may possibly be due to your aerial and earth shorting. Such a very simple. thing can very often cause a great deal of trouble. Hxamine your earth and make quite certain it is all right. When the set has stopped pull out the last valve and see if there is a plop in the speaker. If there is go backwards, noting which valve you can pull out without causing a plop. This indieates that the trouble is in the circuit of that valve, and it will probably be beyond your ability to repair, necessitating the aid of an expert. However, having located the trouble, get in touch with your expert, ¢ell him you have found out what is wrong with it, and ask him to put it right. HIRD Grid (Greymouth): In answer '..t0 my query regarding the purpose of the resistance in "A-" lead of the h.f. valve of the Radiogram Five, you state that its purpose was to provide bias for the H.F. valve. { fail to see this, because it would act as an extra resistance in series with the rheostat and just eut down the filament voltage, A.: By doing so it creates a difference {n potential between the filaments and the grid which is an earth potential. You

see there is a difference in potential on each side of the resistance; the side nearer the valve being at a higher potential than that near the earth. Therefore, the filament is at a higher potential than the grid, ‘which is what we want for bias. 2. You state the characteristics of the Ceco pentode given in the "Guide" are correct, but I fail to see how a valve with an impedance of 5000 and amplification factor of 575 ean possibly have a slope of 2000 mhos. A.: That is a nasty one all right, and it ig a case. of our middle stump being sent spinning down the field. Perhaps these characteristics will suit you better. The impedance of the Ceco- pentode is -800,000 ohms, 1. ite direct current were put across the heater of the cathode type a.c. valve, would everything, particularly the free bias, by means of a resistance between cathode and earth, be satisfactory? A.: Yes; but it would take a very heavy toll of your battery : 2. Does "double tuned i.f. stages" signify that both beat frequencies are tuned and amplified instead of one as used to be the case ?-Yes, 3. How can the load impedance be calculated in (a) resistance capacity coupled (b) direct coupled set? I am wondering if the optimum gain could be obtained from a valve with, say, a 50,000 ohms impedance by having a high plate resistance. and using a higher "B" to overcome the -‘inereased resistance. A.: The load impedance-in resistance capacity coupled sets is equal to the value of the resistance connected in the plate circuit. This should be — considerably higher than the impedance of the valve. In direct coupled stages the impedance is automatically matched, and you do not have to worry about it, you get your constants correct. As you suggest, it is better to use a high value grid leak with a plate resistance detector, and inerease your "B" in order to get the plate voltage at working value. A B.C. (Auckland).-Would it be pos- * gible to use a shortwave adapter in my all-electric receiver? Could it be plugged into the detector valve A.: Yes; both adapters and converters are used. Convertors are applied between

the aerial and the set and the adapter used in the detector socket, Both are obtainable commercially. Mipcetr (Lower Hutt): I have a midget five-valve s.g. set, and am employing an aerial 125ft. in length, 33ft. hi at one end, and 25ft. the other. ould raising the aerial by another 10 feet and shortening it by, say,, 40 feet. (a) reduce the rather broad tuning of 2YA, (b) reduce or increase the present dis-tance-getting capacity of the set? e A.: We are inclined to think it would reduce the broad tuning of 2YA, but it is possible it would decrease the distancegetting abilties of the set. The 10 feet increase in height is a splendid idea, but the shortening by about 40, while greatly improving the selectivty. would no doubt adversely influence the DX ability. On removing the earth altogether no difference is apparent either in local or distant reception. What is the explanation of this? A.: It is quite common with a.c. sets. You are getting your earth return through the electric mains. POWER (Auckland): I have an aerial 80ft. high, 90ft. long, including lead-in. Would a copper ball Gin. in diameter be any better? A.: Certainly not. There is very little that can equal a good aerial such as you are using. 2, Please supply information as to the kind and size of outside aerial that will bo suitable for short-wave work. A.: Yours is excellent. The only thing to watch in_a short-wave aerial is not to get it too long. How far away would a short-wave station have to be before it was audible on a small speaker worked from a twovalve short-wave set? A.: On a_ two-valve shori-wave set you will possibly get fair. speaker results from the N.Z. amateurs, 2 and 3ME. possibly PCJ, and America, but they will be rather weak, we are afraid. You ean rely on getting only the New Zealand hams on the speaker, and perhaps Saigon if it is coming in well.

WwW E.A. (Auckland): Is an aerial 50 * feet high and 50 feet long, double wire, spaced 6 to 8 feet, better than a single wire aerial of the same length and height? ; A.: No, There is very little to be gained by increasing your aerial more than 100 feet for a set of the type we note you are using. Such would possibly improve a erystal set, but it would have a detri-: mental effect’ on & multi-valve set. = 2. What is conductance and how. is it measured ? A.: Conductance is the ability of ¢ conductor to carry an electrie current. ig measured in mhos, and is the opposite E renigiance, Mho is the reciprocal of an Conductivity is equal to 1 divided by the numbers of ohms resistance. The word mho is formed by spelling the word ohm backwards. 8. Would a condenser of approximately 1 mfd.-inserted in the earth lead have.a effect upon static discharge caused by a leak in the tramways system? , A.: We are afraid not. 8.H. (Baleclutha): I have built the ¢ Ranger Two and met with grea} success, having logged all the main N.Z. stations, several Australians, and 2 Am¢ ricans and Japanese. I would like par ticulars of shortwave coils. A.: Adapting the set for shozteavs would entail more than making shortwave coils. You would have to use a series con; denser with the tuning condenser, . and, having done this,’ you. could then’ construet the regeneration coil described fot the Kestrel Three. You would couple th aerial to the grid of the shortwave cof through a neutralising condenser such shown, in the s.g. stage of the "Kestrel ree T P.G. (Waipukurau): I am avin ¢ difficulty in obtaining valves for my Cossor set. Could I use Mullard? so, are the following correct: PM? PM1A, 2? Aes Yes, they would be all right, bus we suggest you use PM2 instead of the PM252. t will give you greater lift. Don’t to ask for English valve bases when you get your valves, 22.0% IRELESS (Dunedin).-I have bil the "Ranger Two," but cannot it to oscillate on frequencies between #¥ a and 3YA? A.: Take off 4 or 5 turns froin’ ‘the aerial coil and you will find it will "ogelllate satisfactorily.