Questions and answers

Radio Record, Volume IV, Issue 1, 18 July 1930, Page 34

Questions and answers

"Wr." (Hokitika) asks what would . a De, the cost of running a 6-valve -all-electrie set. ‘ft "A You would ‘need to name the valves you are using, and state if you are using a dynamic cone speaker, and how: the field is energised. If you do this, we can give you some idea of the running cost. 6A -C.Y." (Auckland).-You sent a ""*. covering letter for your questions but! omitted the actual questions. Send us thesé, and we will endeavour to an swer them for you. ‘ "W M.M." (Glen Murray) has a 4- *’ valve set, but cannot get any short-wave stations. ‘The set is quite dead when the short-wave coils are plugged in. What is the matter? A.: Increase the detector voltage (probably marked HT1) to 45 or even 673 volts. The tuning is very fine, but needs some practice before stations can be tuned in. Removing the earth frequently makes the set more sensitive. A fairly short aerial is hetter than a long one. Wot. "

"H G’ (Hamner Springs) asks ‘us *when the D.O. eliminator is coming along. ; A.: We have been temporarily held up with it, but it should appear within the next month, Our difficulty in Wellington is that there is no D.O. current with which to experiment. "W H.H." (Barnselugh) asks us * where a switch which was deseribed in a previous issue might be obtained. A.: This was a news item concerning a piece of appartus which has just made its appearance in America. No doubt within a very sbort time it will be available in New Zealand. a B." (Longford) sends a report on * the frequency test, and asks if his apparatus, comprising a horn speaker and H.R. set, came up to scratch. — It reproduced well between 200 and 3000 _ cycles. A.: It appeared to be slightly lack.ing on the higher frequencies, with, of course, the usual horn speaker cut off of the lower. Other than this the per formance was normal,

"J H." (Sandringham) asks concern * ing the Daniells cells. 1. Would the charger supply 6 volts to the valve filaments if connected to the accumulator as shown in the diagram? A.: No, it would be quite safe. 2. Are the two extra cells in the charger to overcome all resistance — Yes. . 2. How many of these cells would be required to charge a 6-volt necumulator, and could they be left permanently connected to the accumulator? A.: Bight would be required. and they could-be permanently connected. 4, Would they keep the accumulator fully charged if .75 amps. were taken out per hour and the set used for three hours a day? A.: In all probability, yes. 5. I have constructed the eliminator for small sets, but it hums very badly. I have departed from the specifications in that I did not shellac the laminations as they were already insulated. I used 1700 turns on the transformer primary and reduced the other accordingly. A.: Probably the insulation is not sufficient. The laminations should "ave been shellaced as specified. It is ‘yjuite a simple task. You need only a jar of shellac and can put in the strips hy the dozen, and lift them out singly and leave them to dry. You should not have made the primary less than specified, for it would not have harmed the eliminator to have made a join. It seems that you will have to pull the transformer down and make it up again. "H. W.Y." (Herekino) asks advice on * certain points. 1. When I use "Round-the-World" Two as an adaptor plugged into the detector socket of a 4-valve Browning Drake I get nothing but a continuous whistle. A.: Reverse the connections to the primaries of one of the audio transformers. 2. I have built the 4-valve B.D. exactly as specifications, with the coils, aerial 74 turns tuned with a .0005 condenser, secondary 88 turns tuned with a .00025 condenser. There is a greut difference in the readings of these two dials. A.: The coils are not matched. Make them both ‘the same, preferably 74 turns tuned with a .0005 condenser. 8. Rotating the tickler makes no difference, A.: Try a .0005 fixed condenser across the primary of the first audio transformer. Try reversing the connections to the primary and to the tickler. 4, The neutralising condenser is full out. A.:)The capacity appears to be too big. Can you reduce it by removing fixed plates. | . 5. I cannot neutralise the set as de-

scribed because I cannot rece:yve signals with the R.I*. valve turned off. A.: Use the other system for regenerative receivers. That is, rotate the tickler till the set howls and then adyante the neutralising condenser to the howl. The method has been‘ fuliy described in several of our publications, including the last "Guide" and the "Record." " V.P." (Petone) has an A.C. set * which stops after from 14 to 2 hours work. The valves light, but chere is no signal from any stations. The maker’s agents have taken the set back and cannot find any trouble. A.: Unless your aerial-is defective we cannot suggest anything. Examine this equipment very carefully to see is not a possibility of the aerial shorting to the earth. We would advise you to have the representative of the firm come to your home and examine the conditions there. "NOVICE" (Nelson) wishes to construct the Daniells cell charger. and wants to know a’ few points: a 1. Is the battery charger left cdnnected to the set all the time, even when the set is in use?-yYes. 2. How are the connecting wires joined to the copper and zinc rods? Is it best to solder them direct or by brass terminals to the copper and the zinc? Ihave plenty of brass terminals from dry "A" batteries. Could I use these or the strips of zine off the old dry "A" batteries to which the terminals are already attached?

Av: Any of these methods would do quite well, It is merely a matter to spit your own convenience, My set is a 4-valve regenerative, ahd the valves 4-volt ones. What capacity accumulator should I buy and about what price should it be? A.: If you are using valyes which are very light on "A" current the system will quité possibly succeed, if you use a 20 amp-hour accumulator. This would cost probably about £2. 4, If I cannot get the Lelanche pots what can I buy in their place? A,: There are no substitutes for them. They are unglazed pottery jars about the gize of cocoa tins. 5, As this system is so effective, cheap and reliable, why has it not always/been recommended to country people who cannot get accumulators charged? A.: It has its limitations. It has not a very great capacity, but is admirably suited as a trickle charger for fairly small sets. It is very messy, and many people will not take the trouble to bestow the constant attention that these cells require. There is a fair upkeep with them, because the zincs may be eaten away fairly rapidly, though the system would be cheaper than dry "A" batteries. Particularly if the cells are neglected does trouble arise, "RB." (Auckland) is using a 3- * valve screen-grid set, and wants particulars of valve-base coils to cover the 20/30 metre band. A,: Please state the value of condenser used in this kit set. "VM. P.A." (New Plymouth) desires to * change 4-volt valves to 6-volt, "and asks for the changes in the components of the set, A,: There will be no need for any changes. Just substitute the valves and the battery. 2. What valves do you recommend? A.: Radio, 221A or 201A type; detector, 221, 200A, or 2014.3 first audio, 221 or 201A; second audio, 112A or intermediate power valves. Hox more information on this point, see the 1930 "Radio Listeners’ Guide," Section 6, and page 101.

B.S." (Whangarei) has a stan- * dard H.R.4, and asks if he can use parts of this to build a screen grid set. He wants to use the wire from the coil. A.: This will be quite in order, The gauge usually used in these being No. 22 3.W.g. 2. Can I use the transformers and the two .00085 condensers where .00025 are specified? A.: The 00085 can be used quite well, but to reduce their capacity to .00025 use a ,00875 condenser (or near equivalent) in series, The exact yalue ean be built up by separate condensers paralleled. These should give quite good results. J.8." (Wellington) asks if a .0005 * and .00015 condenser may be used in Round-the-World Two, A.;: .0005 is too large-but .00015 may be used for tuning. It will slighily alter the tuning on the wavebands. It would not be necessary to alter the coils, if anything they must be made slightly larger, You may try to use the .0005 for reaction by using in series with it a .001 condenser. "R. M.D." (Auckland) has a factorybuilt A.C. receiver which hums ‘padly. He wants to know how he can stop this. A.: We do not advise amateurs to interfere with factory-built receivers.

There is always a danger of shock, and furthermore, the regulations do not provide for inexperienced amateurs interfering with receivers. Consult your dealer, "G E.W." (North Auckland) has a * six-valve factory-made set using batteries, and asks: 1. Can I connect a pair of ’phones in place of the loudspeaker when signals are weak? A.: No. Besides damaging the phones you would hear a disproportionately great amount of background noise due to the action of the set,

2. The set is placed against an outside wall, and earthed immediately outside. A veranda has now been erected over the wall, and the ground is get ting dry, Will it be all right to dtive another pipe down outside the veranda in a wet plaice. A.; ¥es, Where possible, long leads are to be avoided, but in this case 3% would be better to have one, "NEw CHUM" (Tauranga) is. doubtful about the advantages of low loss coils. he asks: 1. Will low loss coils improve either _tone, volume or selectivity? A.: They will very slightly. It is now considered that they are not worth the trouble they take to cone struct. 2. Are they worth while in a district noted for its poor daylight recep~ tion?-No. 8. In the 2-R.F. B.D, should the coils be yertical? A,: Placed vertically they take less room than when horizontal. and they do not come so near to the shield as they would in any other position. 4, In the 1980 "Guide" "Valve Equi yalents" AC3 and 1021, Mullard are given as equivalents of 226 and 227, but they are missed from the other list. Why?

A.: They aré the exact equivalents of 226 and 227. There was no point in doubly reproducing the characteristies. oer an

5. Why are AC. ‘eacelvers in the 1980 "Guide" and All-Hlectric confined to one stage of R.F.? A.: Because more than one stage ‘ as A

makes construction very difficult, and are-generally speaking not worth while for the average amateur. 6. Do-the grid leads of both R.F. valves have to be wrapped in tinfoil when separate shields are provided for each valve? A..:It may not be necessary, though this is usually a preventative of hum. % In. some factory-built sets why are the R.F. and A.F. stages intermixed? A.: Manufacturers have reasons: of their. own for doing these things. Undoubtedly it is the best way for the particular circuit they are using. -"TNCUBU" (Waitaha) has a battery set which requires a 4volt "A" supply. ' He wishes to know if he ean use a 6-volt battery broken down through resistance. ‘ A.: Yes, there are two ways of doing this. You can either take a lead from one of the terminals provided and one from the cross bar, farthest from this tap, or you can use a series resistance. To calculate the value, find the total "A" consumption of the set in amperes by referring to the valve characteristics, and divide it into two (2). : 2. A friend hasan H.R. set on which the first condenser tunes very broadly. A.: This is a characteristic of this particular’ circuit; and it does not usually indicate a fault. There may, however, be some stray capacity, due to the wiring. "W S." (New Plymouth) has built *»-4-valve screen-grid short-wave receiver, but cannot get the set to oscillate. A.: Increase the detector voltage and take off the earth. 2.. There seems to bea short cireuit. Continuity tests are all right, plate voltages seem O.K., but when one is plugging into the phones there is a chance of a shock. A.: This is quite in order, and quite normal. 3, Ifa voltmeter has one tip on the. B+- side of the phone jack and the other on the panel, there is a reading of 120 volts. What does this indicate? A.:A perfectly normal state. Your A-+- is connected to the shield, and as far as resistance is concerned the "A" pattery may be neglected. A-, which for our purposes can be regarded as A4-, is connected to B-, so in effect you are connecting the voltmeter across the "B" supply. 4, Am T likely to burn out the valves with the set in this condition?--No. 5. Is it possible that B- is getting to earth through the "A" battery? A.: As we have explained, yes. "POWER-VALVE" (Palmerston North) wants to add a powervalve to a 4-valve set. He is using Philips valves, but is thinking of using an Osram in the last stage. A.: The two power vaives suitable for your set are Philips B509 and Osram P610. *D M.B." (Stratford) has seen in an * Pnglish journal that a 4-volt "A" battery .can be charged from a car 12-volt battery by: changing over at half charge so that the drain from the battery is equalised. He asks :- | 1. Is this method likely to injure either the car or wireless battery? ; A.: You would have to be very care- ; ful in the charging. Keep an hydro- : meter handy and test the car battery to gee that it is not run down too muca,

2. Is the enclosed wiring correct ?- Yes. 8. Would the change-over at halfeharge equalise the current dra from the car battery ?-Yes. 4, Does the car dynamo have to running before the "A" battery. will charge? ' Av:.No, providing the car battery -is fully charged, it will do the job. "K 0.P." (Carterton) has . experi- * enced a puzzling phenomenon. ‘Attached to his set is a battery eliminator and a charger. The cases of these are earthed, and there is no eonnection between B- and the case. . Reception has fallen off un'il now he finds that on removing the earth from’ the set the signals are again built up to: their former strength and tone. He asks why. A.: On. the face, it is a difficuit and puzzling phenomenon.. We can only offer a theory. The correspondent’ has said that A- is not earthed, and we presume that in normal cases there is an earth return in the aerial coil. This provides a return for high frequency current but none for the low, for which the A battery must be earthed. The audio frequency is probably earthed through the secondary windings and back into the primary of the eliminator. Furthermore, high-frequency current can get away through.the condenser resulting from the proximity of the earthed casing.’ There is, with the earth removed, a state of unbalance, there being no direct return for audio current. On removing the earth, the balance is somewhat restored. A- or A+, B- and C+, must always be earthed directly, and apart from open or short circuits within the set, the 5 will then work better with an ea Frequently, sets using eliminators' or A.C. sets do not require an earth. Then, of course, there is the simple explanation, that your earth is in some way connected with the aerial. "GIMPLICITY" (Wanganui) wants certain questions cleared up with regard to a 2-RF B.D. i. Is the variable condenser in the aerial designed to dispense with the tickler? — A.: No, it is merely to sharpen tun- . ing. It is not altogether essential. 2. Can I gang the other condensers? A.: Yes, you can gang as many as other than the reaction condenser. Generally speaking, the first radio should be kept on a separate dial, second radio and detector on another, though the three may be used on the one dial if a balancing condens~ er is connected across the so condenser.