QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 18, 13 November 1931, Page 13

QUESTIONS AND ANSWERS

FRANK

KEE

The Technical Editor will, through these columns, be pleased to ‘help readers experiencing trouble with their sets, Queries are limited to three-for more than this .a shilling fee is charged, and a similar fee is payable for queries answered by post, Supplying layouts. circuits and solutions of intricate theoretical problems is beyond the scope of this service. A coupon must accompany all requests for information. Nonappearance of the coupon in any issue cannot be regarded as a reason for its not being used. Address all queries, The Technical Editor, Box 1032, Wellington.

ft ‘ JON (Wanganui): Would the "Home ;.and Country Portable" give good loudspeaker strength on 1YA if used within five or ten miles?-Yes, 2, Could I use a spare transformer as a choke? If so, what would be the inductance? A.: You could, although it would not make an altogether satisfactory one. We eannot find out the inductance. 3, What combination of Philips fouryolt valves should be used? A.: A415 detector, A409 first and second audio. "TONE (Hawke’s Bay): On the lid of my five-valye battery set is written "Use only 3-ampere yalves." I am now using low filament consumption yalyes in all stages, bnt the yolume control does not appear to work. Otherwise the set is better than what it was previously, A.: You can'safely use the valves now in the set, but the volume control, which is a resistance in the filament circuit, will not be of a sufficiently ‘high value to control the voltage when the smaller current taken to the-improved valves is to be passed, If you take out the resistance and replace it with a 30 ohms one you will find it will work again satisfactorily. . 2. I am using 12 yolts grid bias with 108 H.T. for a BG605 valve, Is this too much?-No. 8. Must I put in a special g,b, tap when using the 221 in the v.f. or first audio stage? .: No, Sometimes a slight saving ean be effected by biasing valves other than the last, there is rarely any improvement in either distance or tone. This does not apply to.a.ec. sets. I S.G. (Christchurch) : We do not know *anything about your particular receiver, but we presume it would be as sensitive as others in its class. Ags to whether it will receive European stations,

however, there is some doubt, as the reception of these stations is due mainly to the locality, P*10800 (Hindon): What would be the range of an ayerage short-wave two-valve set? A.: You could get most of the stations that are worth while hearing on phones,

including G5SW, RV15, Rome, and the Americans, Would a "Night Hawk Two" receiver be the best circuit for a two-valve set for short-wave? A.: The "Night Hawk Two" is a splendid s.w. set, If you want it for these bands only, use .0001 tuning condenser instead of the .00085 specified and omit the .001 condenser in series, 3. What would be the best valves to use in such a circuit? 7 A.: The new 230 tyne would be excelent, L2CtRO (Masterton): Your adapter should work satisfactorily from your broadcast set. 2. In making the electric "Radiogram Five" the "Guide" does not specify what transformers are necessary; would a 3-1 more or less old-fashioned transformer do in the first stage with a 50 for the last?

A.: Yes, we think your transformer would be quite satisfactory. If the tone is poor substitute the old-type for a more modern transformer, For more information concerning the super-het. a.c. receiver, see section devoted to it in "Questions and Answers in Radio," a booklet just published by the "Radio Record." H C.O, (Cambridge) has made a © Majister short-wave unit from the October, 1929, "Radio News," and he would like to get into communication with any other constructor who hag built the same or a similar set. ‘ L M. (P.N.)-How many turns, and * what size of wire are required to bring in 175 kc. with @ .00035 condenser ? A.: 220 Turns of 30 dse. wire on a 3in. former would be quite satisfactory. Ai W. (Dunedin) -We are redesigning ® the oscillator stage of the super het. shortwave receiver, and it should be ready in a few weeks. However, the "Cathode super" will be ready, we think, before EB A.B. (Hastings).-A d,c. lamp is one * that uses direct current. These are the same as the ordinary lamps used in the a.c, socket, If you have fused your lights look over the connections very carefully, because if the lamp ig in serieg you cannot do any harm. If, however, you have put the lamp in parallel, you will cause trouble, Are you quite certain you are clear on the meaning of "in series’? One side of the mains is connected to one side of the lamp and to nowhere else. The other side of the lamp goes to your helix and back to the main again, If you make this connection your light cannot blow out, but should you connect the side of the lamp which is eonnected divectly with the mains with any part of the apparatus other than with the lamp socket itself, you will have trouble. By the way, do not forget that in experimenting directly with the mains you are contravening the regulations. FE P. (Christechurch)--The Browning- ° Drake, Five was fully described in the 1929 "Radio Guide." PARTON (Ohuka)-My set has . stopped, although the batteries are in good order. All the valves light. A.: There are many things that could be wrong with your set, and we advise you to consult a chart such as that published in "Questions and Answers in Radio," a booklet which is available this

week. Have you tried another speakex, for yours, being an old one, may possibly be the cause of the trouble? A brokendown condenser would also bring about the symptoms which you describe, also one defective valve, for, although a valve lights, it may still be at fault. It would not be a bad plan to have them tested, It may be that a filament resistance has burned out, or rather developed a high resistance connection, in which case the valves would light dimly, as you say. However, if you do not know anything about a radio set it would not be a wise plan to attempt to find the trouble, You may incur more serious trouble in the process. ANC. (Musselburgh).-For each coil you will requine from 24in. to dim of 2in. former and 14in, of 1din. former, but the regenaformer will require 34in. The coils for the sets are exactly the same. Your yalves are quite satisfac tory, and the bias resistance can be found

Information Coupon © crs be used with all requests for information.) Name of set (teres enosroeeeaearons Number of valves eecessepeeceeenefe Name "TC ePC OOS OHEEROOC SOE SOO SOLS Address *COOPCPEDeEL OOOO EBOBOROODS seem eee eae re cserseeospegoeosorseynee SFP HOH He we SHSESESESEOHISSEOHOHOSZO ESE Nom de plume ..ecesccccccenvees: To be kept in subsequent inquiries Date swomecavessces eoocveenves Please Note:(1) Be specific ont briet, tabuting, if possi (2) legibly. and on one side of the paper. (3) We do not design circuits, but aceept suggestions for feature articles,

by looking at the table in the back of the "Guide." G G.S. (Hawera): The "Guide" is not * incorrect; you are confusing two different principles, that is all. It would be beyond the scope of "Q. and A." to explain the position more fully, However, in "Questions and Answers in Radio," you will find a good deal about that subject. H G.. (Hamilton): I have an S8-valve * super. het. commercial set, and the earth wire of this is connected to a galvanised pipe by a 15 feet wire. I understand that 9 feet should be the maximum, Is this correct? A.: The rule is that the shorter the eonnection to earth the better the earth. However, with these modern sets a little difference such as you describe would make really very little difference in performance. 2. Could you recommend a publication for amateur purposes, dealing with radio set, with explanations from A to Z in understandable language? A.: The first section of "Radio Questions and Answers" should be exactly

what you want. It describes the functions of every part of a radio set, without technical terms unless they are first fully explained. [ENAzZ (Te Kuiti): Since fitting a pick-up to my electric set I cannot get the stations I could before. A.: We wish you had given us the circuit you use in connecting up the pickup. for, although we told you how to do it, yet there are many ways in which it could have been done and we could have shown you perhaps another circuit to try. We wish that correspondents in referring to previous questions in the columns would either state the case fully or eut out the original query and pin it to the letter. It would save us a great amount of trouble. 2. Your idea for transposing the chassis, phonograph and speaker to a separate eabinet will be quite in order, It is advisable to leave the back of the cabinet open. BLUEY (Napier): We do not know where you could obtain a diagram of the circuit. It is probably an ordinary °

neutrodyne type of circuit with resist-ance-coupled amplification in the last three'stages. If that is the case the first three valves are correct, the next quite wrong-it should be R.O.A. 221 or its equivalent; the next would be better, as the and 221 could go in the socket which A615 previously occupied, and 171 is correct in the last stage. You can tell the detector by giving it’ a smart tap, in which case a fairly loud ring will come through the speaker. ‘The other valves will ring, but not to the same exten 2. The set has no earth terminal, ‘""B-" goes to "A-+."? Where can I put the earth wire? A.: To the "B-’-"A--" connection.

WAVELENGTH (P.N.): I have constructed a one-valve set and cannot get it to oscillate, although I have tried taking off a few turns from the tickler. A.: Instead of taking turns off you should put them on to get a set to oscillate. Reverse the connections to the tickler, that is, interchange the wires that go to the plate of the valve and the fixed plate of the condenser. Reduce the number of turns on the aerial coil to, say. 18, that is. if interfering with the tickler has no effect, and try a different gridleak, say 3 or 4 megohms in value. Make quite certain that the grid return, that is, the wire that comes from the movingplate sof both condensers, connects with " fe, ,

pick-up (Gore): Your aerial and earth system is not satisfactory. It would be better to place a mast near the house and use an ordinary "TI" aerial without the fancv business of running a loop from the distant end through the sround to the set. VALE: While we cannot say definitely whether your set would give you loudspeaker strength on the New Zealand and Australian stations, yet we have every reason to believe that it would. We made one up, and were able to'bring in those stations without any difficulty. As to whether or not it will be light upon batteries will depend entirely upon the tyne of valves you use. If you use the 221 type or: the 230 type in all stages except the last, where you use a medium power valve (not a big. one, which will use a lot of current), your batteries’ will last quite well. It is certainly a troubleproof set. Furthermore, you can add a stage of s.g. to it and make it very much more powerful:than it is at present. Details of this were published in the "Radio Record" a short time back. Mc. (Port Ahuriri): We can design a circuit to fit in with your components. If you could let us have the circuit of the adapter so much the better. You would, of course, have to send the fee for a reply by past.

BALDY. (Ohakune Junction) : Is the electric Radiogram in the 1931 "Guide"? as good as those published in the vEadlo Record"? : Yes. the Radiogram is an excellent set ‘embodying many new features, It would be as well to use a 245 in the last stage if you can get the voltage to operate it NOVICE (Christchurch) : When the 44 volts "©" battery is connected the filaments of the valves light even although the filament switch is turned off: A.: There is undoubtedly a wrong connection with your "C" battery. From what we can make out-by comparing your sketch with the circuit, the connection between C- 44 and the by-pass conden--ser, which apparently is connected to the neutralising condenser, is quite wrong. The leads from the C-s should go direct to the F- terminals of each of the two transformers. They should not go to any other point in the set except in the ease .of the second transformer, across which is a condenser. One side of the condenser is connected with F- of the transformer, You appear. too, to be shorting out the first "C" battery. We would not advise you to use your set without the "CO". If you cannot rectify the

trouble yourself, get someone who can,., It should be done. PFREQUENCY COP (Auckland): You have not told us the size of the former. We do not happen to have specifications of all commercially-made apparatus at our finger-tips. 2. I have a pair of phones that are extremely sensitive when tested with the electricity of the body, but when they are connected to my four-valve set I can hardly get any volume through them. A.: How do you test them with the electricity of your body?-The best method of testing a pair of phones for sensitivity is to take a smail battery, say 14 volts, and connect the terminals across this. If you get a loud healthy click the phones are quite all right. If. the click is weak, then there is something wrong with them, as the valves you are using in your set are quite satisfactory. J

R F.C, (Huntly): How many turns * must I wind on a short-wave r.f. choke on a glass former 11/16in. diameter ?-About 150. 2. How many turns on the same former for an average broadcast receiver i. choke?-About 1500. . f 8. The gauge of your wire is 42 enamel. ANxXt0Us (Khandallah) : 2YA spreads itself between 60 and 100 on the dial of my new super-heterodyne set. I have tried both "Knife-edge Rejecter" and "Dual Wavetrap" without success. A.: In the first place we think there is something wrong with your set. super-het. of the type you are using should not do this. 2YA should disappear cleanly. As a matter of fact we have tried many modern sets in your neighbourhood, and even when using a very large aerial we can eliminate 2YA in favour of 4YA. A wavetrap should effect an improvement, and we suggest your taking your wavetrap along to a Wellington dealer, say Fear and Co., and asking them to try them out. If ther. aré satisfactory, have another try to just them. But your ser should not need a wavetrap. FCONOMIC THREE (Gisborne): Is there any way of tejuvenating a PM.14?-No. PD Menosis (Taihape): How long will my standard "B" batteries last with a four-valve set using 201A valves? A.: If the set is operated three to four hours a day, your batteries will last approximately 6 months. 8. Concerning the three-coi] tuner mentioned in the "R.R.," Sept. 25, page 16, how is this drawn? A.: The same as is shown in the lower right hand sketch in the diagram, only

he conventional variable condenser symaoe is shown between the aerial and the 8. How do you detect the detector? A.: Usually if you tap the detector you can get a ring in the speaker. Other valves in the set will also give a ring, but the detector is the most pronounced. Furthermore in the lead to the grid of the detector valve is a grid leak and condenser, It is usually preceded by a coil and followed by a transformer, S;: SUPER (Taranaki): The valve * of the resistance is really not important. 100,000 ohms will be quite sufficient, although 500,000 ohms would do equally well. 500,000 ohms would make a good volume control used in the manner you suggest. It is really preferable to vary the voltage on both the s.g. valves, in order to control the volume, GID (Whangarei).-Yes, our specifications were slightly confused ; 2100 was the maximum, that is, 1050 on either side of the rectifier. This will allow a slightly bigger margin than would the 850 specified. As, however, if you use half-wave rectifications you will need only 1050 turns, which will leave a big margin. TALLOY (Thames).-Using a stalloy core, 3 x 8, what number of turns per volt must I use on the transformer to be connected with the 230-volt 50-cycle mains? A.: You will require one turn per volt. The transformer will be 95 per cent. efficient. 2. Where could I get an uninsulated transformer steel? A.: Try the National Electric Engineering Company, Wellington. 3. The shell and core pattern transformers are usually considered to be _the most efficient.

TEP-UP (Greymouth).-Your questions are beyond-the scope of Q. and ELOX (Auckland).-Please send your cirevit and we shall amend it as you desire. M (Miramar).-AIl you can do is to * keep experimenting with different numbers of turns. We tried out a very large number, and those we published give the best results. If this is not the case with you, you can only do as we did, try various adjustments. . G.T, (Wellington).-Does the mut- * ual conductance of the s.g. and pentode valves vary as the impedance? A.: The impedance has a bearing on the mutual conductance, but it is wrong to say that the mutual conductance varies as the impedance, m.c., can be measured in mhos. by dividing the amplification factor by the impedance and multiplying by a million, It can be seen from this that m.e. will be constant for a given amplification factor and impedance only. Anything that tends to diminish the impedance or raise the amplification factor will better the m.c. Conversely, anything that increases the impedance or diminishes the amplification factor will have the opposite effect. 2. Does the mu refer to the plate current only or to the sum of the extra grid and plate currents? A.: Indirectly to both, in that the extra grid influences the impedance which in turn influences the mu. The actual calculations are made from the plate current only. Mu, of course, denotes amplification factor, and is derived from the curve of the valve by comparing changing current brought about by change in anode voltage, divided by change in grid_ voltage responsible for the same variation in the plate current. . 8. If reducing the ht. on the extra increases the impedance, and therefor nn

the amplification factor, why not use no voltage on the grid? _A.: If you increase the impedance of a ven valve, you will diminish the amplieation factor. A valve has a high amplification factor, not because it has a high impedance, but because of the arrangement of the elements within the valve. Consequent upon this is the high impedance, B B. (OC. Otago).-Using a 2-amp tun*gar bulb, what is the correct voltage to tap off the transformer to charge a 6-volt accumulator with 2 amps? A.: The drop through the bulb passing 2 amps. is about 10 volts. Consequently you will require 16 volts, A better plan would be to make provision for 20 volts, and me a resistance to regulate the charging rate. 2. What voltage would be required to charge (a) a 50-volt accumulator, (b) 100-yolt accumulator at an eight of an amp? .: The drop through the valve would be about, 15 volts, consequently the secondary must be capable of delivering 65. (b) The drop will be from 20 to 25 volts, hence you will require 120-125 volts, from the secondary. The hook-up is the same in all three cases, and the rate of charging for the "B" accumulator will be about 00 mamps. oy ©. (Dunedin): Yes, to your first two * questions, and no to the third. ({REEN GRASS (Pelorous’ Sounds): Would three "B" batteries give me better reception than two?-We think so. CARAVAN (Wellington): Our Super Six is somewhat similar to the circuit which you are using, which, by the way, contravenes the P. and T. Department regulations. As we shall be publishing full details of this set in the near future, we advise you to use your parts efor it.

W A.L. (Rangiahua): What can I do * to stop the noise from a lighting plant coming through my set? A.: You can overcome the difficulty by usitig chokes and by-pass condensers in the manner described in the "R.R." for August 14. OwL (Greymouth): Volume from the "Kestrel Three" is very weak. Tuning is very sharp and the set oscillates when the differential condenser is earthA.: There is a fault in the set somewhere. Make the following tests: See that the moving vanes of all three condensers are actually connected with earth, that the fixed vanes cannot touch the moving or earth, that a condenser is not being shorted out through its proximity to shielding. Bring the aerial in to the fixed plates’of the detector condenser and see if there is any improvement in either volume or operation. If there is, then there is definitely a fault in the r.f. stages. Have you remembered that the cap of the s.g. valve connects with the plate? Are you getting screen voltage to the xf. valve? 2. Is it in order for a spark to occur when I connect "B-’? It occurs only once after an interval. A.: This indicates that the condensers are charging and it is quite in order. PDeEXW: Is it necessary to earth the dead end of a Beverage aerial to a resistance, and why? A.: We said so, and gave the reason in the article on the Beverage aerial published in the "R.R." on October 3, 1930. 2. Is fencing wire as good as the standard copper aerial wire? A.: For the Beverage aerial, yes. RIKI (Blenheim): Your present aerjal is the better. ’