Questions and Answers

Radio Record, Volume IV, Issue 35, 13 March 1931, Page 13

Questions and Answers

NO8THENDER (Dunedin): What voltage should I wind on a tramsformer to supply two 240 volts from the last: valve, allowing for drop in choke in the rectifying valve, and in the biasigg resistance If you wind your transformer for 800 volts, you will have ample for P625. It requives 24 volts bias. at 250 volts plate, and will require a 1000 ohms biasing resistance. This evidently is the valve you require for the last stage. . 2. Does automatic grid bias result in drop in plate voltage? A.: Yes. A drop in plate voltage equal to the amount of bias supplied results when automatic bias is provided for. You must always add the amount of bias to the plate voltage. 8. Can you give a method of positive ly biasing an a.c, valve? A.: There is no need to do so, A.C, valves are constructed for a negative bias, even when acting as detector. Should you require a positive bias, the easiest manner to obtain one is with a battery inserted in the grid return of the valve. ‘The positive will be toward the grid, while the negative goes to the yrathode and earth.

CE (Kakahi): Is the number of pipes used in the Pierce earth system of any importance? A.: Pierce, the originator of the earth system says thait his reception improve as he added more pipes until he got to about a dozen, when more were not worth adding, Measurements, however, show that unless the pipes are separated by six feet‘or more, there is nothing to be gained by putiting down large numbers. 2. Does the bottom pipe of the radiator have to be blocked to hold the water? A.: It should not be watertight, but allow the water to leak away slowly into the ground, 8. What length should the pipe be?Six feet.

GOREEN GRID (Trentham): Can I use a pair of phones in parallel with the speaker?-Yes. 2. Would it be possible to reduce. the a.e, hum in the phones? A.: Not without greatly, interfering, with your set. 3. The set works just as well without an earth. Would. I get better results if I improved my earth? A.: Probably not, many a.c. sets work better without an earth than with one.

B.D. (Mt. Hden): I could get only a .0002 differential condenser for the Night Hawk. What effect would this have upon the set? A.: It would be an improvement. The .0002 is more serviceable than the smaller capacity condensers. MA* (Auckland): I can get hardly any strength on by dynamic speaker, though I get quite good volume on the horn. Would a power valve improve things? . . A.: If you have connected up the field supply and are quite certain that this is getting to the speaker itself, i.e., that the cords are not damaged in any way, you should have your speaker tested. The horn speaker would be more sensitive, but not greatly so, A power valve would not strengthen yourself. You can get a much better tone through its use, but strength would if anything be weakened. om (Timaru) : What is the resultant capacity when a .0001 and a .00035 condenser are connected in series?.00008 approximately. 2. How is a fixed condenser placed in parallel with a moving condenser? A.: By connecting the one terminal of the fixed condenser to the moving plates and the other to the fixed plates. 2. What do you consider to be the better type of reaction control for shortwave, (a) differential, (b) resistance? : There is very little to choose between them if the resistance is of good quality. . . 8. Would a .00025 or .00015 condenser be suitable for the differential short-wave adapter for tuning? A.: The coils are designed for a .0001, or really for a .00008 moving condenser, though .00015 could .be used without alteration to the coils. J A.B. (Wellington): I enclose a dia- _* gram of a three-coil tuner which I wish to make into an all-wave receiver. Could you supply a diagram? You cannot make a short-wave set from a broadcast three-coil tuner. Better scrap the set, saving what parts you can and make up the Differential One-valve et. SYD. (Otahuhu) : I am unable to obtain five-prong bases for the differential Series. A.: You ean use four-prong bases it you use the circuit that is given in last -week’s "Radio Record," and wind your eoils as shown in the accompanying dia: gram, OWL (Wellington): Can phones be connected to my all-electric set? A.: Yes, but you would need an electrician to do the job.

DANIELL (Nelson): Can I add a further stage of screen grid to my three-valve screen grid job without altering the present wiring? A.: It is not an easy job to add a further stage to a commercial receiver. No doubt it could be done if you were very familiar with the circuit. 2. What amount of current would the Daniell cell charger deliver? A.: About half an amp. 8, What amount of current does my set use?- A. + amp. 4, Can I better my valve combination? -No. X 4 (Wanganui): Can I use 34 . gauge enamelled wire instead of 36 or 38, and will the number of turns remain constant if I construct a celluloid choke? A.: You may use 34 gauge and the same number of turns, but your choke wil be much bulkier than if you use H. ©. (Dannevirke): Could the Differ- * tial Four be better than my commercial kit set? A.: We are inclined to think it would. 2. Would the Outspan Five be better than the Dif. Four? A.: The Outspan Five is the culmination of the series of articles:of the Diff. series. It is better than any of the smaller ones. ,8. Could I use any parts of my set for a Diff. Four? A.: Yes, quite a number of them. 4, What ratios are the transformers in my set? A.: As far as we know, 34 to 1. E.S. (Hastings): I am interested * in shortwave work, but am quite an amateur. Could you suggest a book on shortwave receiver construction? f you are interested in shortwave you should get in touch with the Sec. of the N.Z. Shortwave Club. Suitable books on the subject are the "Radio Guide," and for those a little more advanced, Handy’s Handbook.. WO-VALVDER (Christchurch): Is the enclosed circuit the combination of the detector circuit of the B.D. and the amplifier in, the Crystal Valve and Amplifier, satisfactory? Yes, we have indicated the position of an essential condenser and one or two values. 2. What would be the value of the biasing resistance for the 171A’s?-2000 ohms. 3. Would a 112A valve in the last stage give more volume? .: Yes, it might even be better for you _to use a high gain power valve such at B605. This would require a biasing resistance of 2250 ohms. ADIO (Mosgiel) : Has automatic volume control any decided advantage? _ A.: It is a handy refinement, but that ig all. . . : ue Can anything easily go wrong with it? / A,: We are not in a position to say, as we have had no experience of this new feature. 8. I am told tone control is not satisfactory. Is this true? A.: Tone control is only a method of introducing a certain amount of distor1

ion, but you need not use it unless you ike. A set although provided with a tone ontrol, can be equally as good for real ‘eproduction as one without it. Note--We cannot answer your last juestion as we have had no experience of he receiver in question. CO G. (South Dunedin): I imtend to * build the Outspan Five. Would the tickler ‘coil be satisfactory if placed nside the secondary at the opposite end o the primary? A.: Yes, if about 60 turns are used. 2. Is my valve combination suitable? A.: Yes, but bias the second last valve with 44 instead of 9. " 8. I am using .0005 tuning condensers. Will 60 turns be correct for the secondary, 80 for the primaries, and 25 for the aerial coils? ; A.: Approximately, yes. The primaries may. possibly be reduced to 25, but try 20 to start with, and if the set is difficult to control reduce to 25. Would a separate rheostat for the detector valve be an improvement? A.: It would not be worth its inclusion in the circuit. . ; 5. How is the third aerial terminal connected? A.: It is connected through a .0001 fixed condenser to the plate of the sg. valve. 6, I have two af. four transformers and a 5-1 of another make. How should I use them? A.: Use the two A.F.4’s. 7, Which is correct for the differential condenser, a .002 or a .0002? A.: The .002 was a misprint; it should have read .0002. . 8. I have a 10-1 dial and two ordinary dials-which would be better for the detector stage?-The 10-1.

ORRESPONDENTS must attach this eoupon to all queries sent to the Technical Hditor (Box 1032, Wellington). Questions arriving without it are likely to go astray or be delayed. Name Of set ....ccccccecececcccers Number of Vales .ccceccevccsecces Name eee veverecversescseeeseeeeee Address aeccccucccccesacsensvosers fa POCO Oe HOHE EOSE SHOES SOOOEEHEDSEESE®S Nom de plume acesesessseenccses® To be kept in subsequent inquiries. Date ac aacccsccvecsesce evceree Please Note:(1) Be specific and brief, tabulating, if possible. , (2) Write legibly, and’on one side of the paper. (8) We do not design circuits. but accept suggestions for feature articles, Solving trouble, as different fron advice, is difficult by correspondence: and while letters are given every consideration, answers are nol 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.

9. Would a 38-ply panel be satisfactory? : A.: Yes, if coated with Sapolin or some insulating paint of that description, 10. Would two shielding boxes as used for the old B.D, Five be satisfactory? . A,: Quite, only they are a little more cumbersome, ‘ ; "11. I have an rf. choke. of 1000 turns wound in slots. Is this satisfactory ?- Yes, ; J H, (Auckland)-Would a low. loss *"* oil be sufficiently efficient for the wave-trap described by "A.H.H.?’-Yes, 4. Would the efficiency of either the si, or the wave-trap be impaired if. the l, "er were constructed in a cabinet? ".; Yes: the wave-trap should if possible be shielded and away from the set. 3. What is the capacity of the varin: ‘a condenser? ' Ay: .00035, but see our amended article last .week. 4. Would you tell me the. method of determining the capacity of variable con"déeasers? * : Av: The capacity in mfds. is equal to 0885.x N x divided by 1,000,009. Where S is the area of a moving vane in sq. centim. D is the air gap and N the number of vanes. — . 5. Would d.s.c. wire do as well or better than d.d.c.? A.: ‘The silk covered would be the better though, there is very little difference, > .

6. What is the best earth? A.: We cannot do better than refer you to a yery complete article which was published in the "R.R." on June 6, 1930. GNORANT (Johnsonville): I can get numerous stations on the higher wavelengths, but below everything seems distorted. I am using 5 201A valves. A.: It seems that your set oscillates on the lower frequencies owing to there being no adequate. means of controlling feed-back. Your set is evidently an old one, and should be taken to a reliable radiotrician for overhaul. Some type of power yalve should be used in the last stage, and because of this the connecting link. between the bias of the last two valves should be broken. If your diagram is-correct then your © battery has been reversed. ©- should go to the grid return, that is. the g-b. terminal of your transformer, A different amount of.

bias should be"placed on each valve, the er one having much more than the : 2. Is it possible to alter the set to include a s.g. yalve, and would this give better results? A.: It is possible urgess your set is awkwardly laid out. It would not be a very simple matter, but it could be done by anyone who knew something about the job, and it would improve your long(listance reception. 8. When I rotate the two ganged condensers I get a loud crackling noise in the loudspeaker. A.: This is due, probably, to the mov-. ing and fixed plates touching one another. See that there is no dust between the vanes, and examine them very closely, at the same time moving the condenser. 4, Would_a power valve improve quality from 2YA? A.: It would very much improve the tone, but as we have pointed out before, would need to be biased correctly. Is there any difference in the construction of the following fixed condensers: By-pass, grid, blocking, and between the following variable condensers: une, reaction, straight line frequency? A.: Very little. It is really only a difference.in their use. The term "by-pass" is used generally with reference to condensers that are between a high tension supply and earth to conduct any stray high frequency current that may be where .it should not. Grid condensers are of small yalue, and are used in the grid circuit of the detector valve. in conjunction with a grid leak to cause the valve to rectify. Whereas the capacity of a by-pass condenser ranges from .25 upwards, the grid condenser ranges from .0001 to .0008. A grid condenser can be used in other parts of the set, such as across the transformer primaries. A blocking condenser might be anything, because any condenser blocks 1.f. current. When the term "blocking" is applied it generally means that the condenser is placed in a circuit so that it will prevent. the passage of low tension current, i.e, the current used for ordinary power: werk, having a low frequency, if a.c. On.the other hand, hf, current has a frequency of many thousands and is rarely measurable in volts. As for the variable condensers, a tuning conderser is any condenser that is connected across the tuning coil to make it resonate at any particular frequency. In other words, it is the condenser which,

when used with a coil, enables a station to be tuned in. A straight line frequency condenser is a. type of tuning condenser so named because of the shape of its plates. A reaction condenser is, in construction, the same as a tuning condenser. It is used to control the amount of h.f, energy fed back into the detector coil, this process making the set more sensitive. There are a few special reaction condensers that are not used for tunins. Outstanding among these is the differential condenser. A FR. (Auckland).-I am using’ com- ° mercially made shortwave. coils, and reaction is controlled by .0005 differential condenser. Could I replace this by, 2 differential without alteration to the coils? A.: Jn all probability, no. The tickler would probably be too. small. 2, I wish to add another stage of amplification to my two yalyes, Would it -be better with a s.g. as in the Diff, Three or a stage of: audio? ‘A.: It would be better to use the s.2. hf. stage. . 3. I am using a 7 meg. leak. Woutd a 10 meg. be better?’ A.: 10 megs. is the better for shortwave work,

USSIE = (Wellington).-Interference from power leakages is completely ruining by reception. -I am told a counterpoise improves matters. (Could you possibly deseribe one. . counterpoise is a second aerial slung very carefully underneath the proper aerial. It is about 10 feet from the ground and every care must be taken to insulate it from anything grounded. By a lead-in it is brought to the set and attached to the ground binding post. The ordinary ground must be disconnected. E J. (Coleridge) : Is it possible to pur- . chase "A," "B," and "C" eliminat or A.: We do not know of any particular "A," "B" and "C" eliminators, though the Atlas people turn out a "B" and "CO" eliminator and a trickle charger combined. This is arranged so — that when the eliminator is switched off the charger is switched on. In _ reality, of ,course, this is an "A," "B" and "c" eliminator, If you can obtain a "B" and "CO" eliminator you can always haye an "A" eliminator made up, or can purchase one. You must use a voltmete: and keep a fairly constant check to see that it is not. delivering a voltage that is too high, otherwise your valve filaments will suffer. OHM (Oamaru): Is the enclosed circuit suitable for shortwave? A.: Yes, but substitute the Cl and C5 with a single small value condenser such as .0O01. 2. What Osram valves do you suggest for the circuit? A.: H410 and L410.

3. What is the difference between a differential condenser and an. ordinary one? . _A.: A differential condenser has two sets of fixed plates and one set of moying plates. An ordinary condenser has one. set of fixed and one of moving. INETY-ONE M. (Otorohanga): I have been troubled with heavy interference which entirely blots out reception. A neighbour living about a mile away suffers equally badly. A high tension transformer runs right up to the house about a chain distant from the aerial. I have complained to our local power board. but they cannot offer any solution, Could you or any of your readers assist in. identifying the inter ference? A.: We suspect the transformer. If you have complained to the power board and do not get satisfaction, write direct to the District Radio Inspector and restate your complaint. Perhaps other listeners in that district seeing this note may care to write us on the subject. FRESH (St. Heliers): Could you advise me where I could get a book dealing with the subject of radio right

from the beginning? I want to know’ something about the "How and Why’? A.: Probably the best book in your ‘ease is ‘Wireless, the Modern Magic Carpet," sold by the Te Aro Book Depot, Wellington. The "Radio Guide," published: by ourselves, contains a big section devoted to the elements of. radio, but is not, of course, as full as the ‘Magie Carpet." : H J.P. (Blenheim): Will the values * of the fixed condensers, audio choke and variable: resistances in the "Guide" be satisfactory for my yalves? -Yes, quite. .2. What is your opinion of the s.g. as a detector? A.: It is quite a good valve: for the purpose, especially for shortwave reception. MEG. (Mangapuroto) -My Daniells cell charger will not keep my six-. valye set supplied with current. If sets, the aceunmulator run down. The cotgh, sumption of the set is approximately . amp and a half, A.:. This is far too much for the Daniells cell charger. It is only a 4 amp charger, It is not intended for a heavy eurrent such as your accumulator will be called upon to supply. We advise you to keep it on your accumulator, though, now you have it in operation, asit will no doubt charge it slightly, and pay for itself. You, of course, will have to take it along to the battery charging house regularly. It appears that you have: made it up quite correctly. If you want to use lower consumption valves you should try the new 221’s where you have 201A’s, and a B605 where you have the 171. You would find, then, that the charger will keep your accumulator charged for a very much longer’ time. Only the other day an accumulator was shown us which had been kept charged by Janiells cells for three years, and theze was not a trace of sulphation upon the plates. The set, however, was drawing only 4 amp. F t ats E J.G. (Marton).-when I first bought ° my set a buzzing noise could be heard in the speaker when the grid leak was removed. Now it does not make any difference, A.: You have omitted to tell us the essential fact. Is your reception any weaker now than when the buzzing occurred, and if not, the phenomenon need not worry you, as it is due probably to characteristics of the detector valve. E M.F. (Invercargill).-I have a seven- * valve a.e, receiver, and I wish to build an a.c, shortwave adapter. Could I use one such as shown in the enclosed circuit? A.: Not without introducing a great deal of hum. A.C. shortwave adapters have not proved themselves satisfactory. We have a good one in last year’s "Guide," . but it used a small "BY" battery. The trouble generally is hum, as it is almbe. impossible to keep this out of reception unless special precatitions ate», aken, . :

t GANIC (Timaru) -Can I use power } y grid detection with PM4DX valves? nae 2. Would there be any gain in amplification if I-altered the connections of my audio transformers to the shunt feed ‘audio, as shown. . . A.: You may gain-in though you will lose in volume. If you are to e1aploy it in the last stage it would be: advisable for you to apply grid bias by breaking the return between plate terminal of the transformer and earth, and inserting therein. a grid bias battery with .the positive terminal toward the earth. The value of this battery will depend upon the bias to be applied to the valve, ; Hee (Greymouth) : Can I use .00035 condensers for the Outspan Five? Or could I. reduce their capacities by removing plates? yx, A.: You could use .0005 condensers for Ap the Outspan quite well by winding about ‘ten turns less on the secondary coils. The primaries will have to be reduced by approximately two turns, but the tickler should remain the same. 2. Could you forward a rough -sketch of the eoils somewhat similar to the sketch of the regeneraformer? A.: The coils were really so simple that we did not bother with a‘sketch. If ‘you send a stamped and addressed envelope we will send you the necessary diagrams, 3. Does the diameter of the coil former affect the number of turns? .A.: Yes. If the former is larger than that specified there are fewer turns, and if smaller: there are more. Could I procure a blue print for the Outspan Five? A.: No, a blue print has not: béen is+ sued. If you have any difficulties we will try to clear them up in Q. and A.,, i necessary devoting a special article "to their elucidation. Would any other cor- \ respondents who have experienced any % difficulty with the Outspan Five let us know as soon as possible, for this is .really a splendid sét and very easy to ‘make, The fact that our diagrams were very badly treated in the printing process has caused a certain amount of confusion among readers, If there is the demand we shall supplement the previous article with a redescription of certain ‘points that’ may not be clear, and suitable drawings. G L. (Auckland): Which is the better, ‘* a single or double-wire aerial? A.: A double wire, providing the wires are spaced at a greater distance than six feet, and that the total length of the wires added together do not exceed 100 feet. 2. Which is the better, enamelled or bare aerial wire? A.: Hnamelled is the better because bare wire corrodes,

, & Which is better to have in series the aerial, a midget variable or ‘® fixed condenser? A.: A midget variable, because it is easier than to alter the capacity of the aerial to suit the particular frequency being received. . 4, For efficiency is it better to vary the grid bias to suit different valves? A.: Most decidedly, yes. Some valves take more bias than others. 5. Which is the best way to vary the grid bias voltage? . A.: By shifting the wander plug to . the most suitable tap. Another method is to.connect a potentiometer of 2000 ohms acros the bias battery and vary the centre tap to give the best results. This method, although it permits of finer adjustment dof grid bias, places a certain bleed upon the battery. It is, however, } very small. 6. Are 4-volt valves as efficient as 6volters? A.: For all practical purposes, yes. 7. What make of transformers do you advise and what ratio? A.: It pays always to use high qualrah transformers haying a ratio of 34 or 2

BE J.L. (Ohura): I want a long and Aue shortwave receiver..Do you know of one? It must be battery operated. A,: The Philips is:the only one we know of, although. you could have one of the R.R. receivers made:up by any experienced radio dealer. A.T, (Invercargill) : My commercial * -gsuper heterodyne receiver produces a ploppy sound inthe speaker ° when the announcer pronounces a word with an accent on the "t" or "th." What is the reason and cure for this? A.: The reason is due probably to oscillation caused by the high frequency sounds of the consonants you mention. The cure can be brought about only by the agents for the receiver. RA DX (Waikato): Which is. the best method of coupling valves-by transformer, as in the Outspan Five, or through a condenser as in September 5 cireuit? A.: The Outspan Five is the better method of coupling s.g., valves. It is conducive to greater selectivity, though sometimes the pickup is not quite as great. 2. For a primary I use 30 turns of 80 d.s.c. on a 2in, former..Is this enough to get maximum results? A.: It sounds about right. 8. Would it be worth while changing the detector coil to the regeneraformer as in the Outspan Five?-Yes. 4, Would the regeneraformer make the set more selective? A.: Reaction always tends to make a set slightly more selective than a set not using reaction. 5. Does painting a- coil with shellac lower its efficiency? A.: Shellac will dissolve the enamel of covering of wire, but for a space wound cotton or silk-covered wire, shellacing does not have a detrimental effect.’ GTHP DOWN (Taumarunui): Could you supply data for shortwave coils mounted on valve sockets? A.: A complete table was given in the R.R. of January 30, 1981. Furthermore, details were given in connection with the Differential One, and these could apply to a .0001 condenser. %. Can you give me information on constructing step-up, step-down transformers to work on 110 volts, 50/60 cycles a.¢.? A.: This information was given in the 1980 Guide, and a table will appear in ‘the 1981 Guide.