Questions and answers

Radio Record, Volume IV, Issue 5, 15 August 1930, Page 25

Questions and answers

I B. (Christchurch) asks where he might obtain information concerning the Hammarlund-Roberts screengrid receiver. ae y John’s, Lid., Auckland. A630, ‘A609, and two B605’s suitable for the above receiver ? "A.: A680 would not be ‘much use for a screen-grid receiver. A609 can be used in almost any position except as a power valve, with one of the 605’s in the last stage. T McR. (Wanganui) has an 8-valve electric set which hums when the power is turned on. How could he reduce A.: You would ‘need to consult the agent who sold you the set as hum reduction is not a simple matter for an ~ amateur. > G O.C. (Christchurch) asks us for some : advice regarding commercial re-ceivers-he cannot decide which of three he would like. ‘A.: We are afraid that if you have heard the three we eannot decide for you. They are differently priced, and as far as we know are graded to the price. We tested No. 3, and the report appeared in the "Radio Record" some e back. CONSTANT (Wellington) has constructed the "Combination" receiver described in the "Radio Recc 1," and now wishes to add an extra valve, SO as to have the following combination :- Crystal receiver, crystal and one-valve amplifier, valve and an amplifier. ow®

A.: Add a stage as was shown in a diagram last but you will have to use a special jack before this stage to operate as a crystal and one-valve amplifier. Such a jack can be easily obtained by: asking for one to plug in to a first stage audio. GPARKS (Dunedin) asks one where might I obtain constructional details of a set suitable for receiving Australian and New Zealand stations, such set to use batteries only. .: The four-valve Browning Drake described in the 1929 "Radio Listeners’ ane" would’ suit your purpose admira by Is a kit set available for such. A:: Not to our knowledge, though the parts are readily available. HORT-WAVDE (Gisborne) asks ‘the. following questions concerning a single valve receiver described in a recent publication. 1. Can this set be used as a short-wave receiver ? A.: Yes, if you use special coils. .We would advise you, however, to use. the ‘Round the World" circuit described in

the "Radio Record" this year, but leaying out the second valve that is in .place of the first audio transformer; use your headphones. You would find it better to use a .00015 condenser, and use the numbers in last week’s "Radio Record." You would then not have a separate primary, but would come on into the secondary as shown in the accompanying diagram. Other than this there is little difference in the sets. 2. Will .003 condenser do instead of a 00085 and a 00025? A.: Yes, but you will need to alter the number of turns on the coils. We advise you to construct at least the detector stages of "Round the World Two." W G.S. (Napier) asks if he might at- * tach a short-wave adaptor to his ac. set to save expense of a separate receiver. A.: Yes; you cvuld quite easily | make an adaptor such as was described. the

1930 "Guide," but if you: prefer not to du your own construction you could either have one made or could purchase a commercial ‘one. EN. (Waimate) -Send along a’ dia- ° gram of two positions for aerial. Tn one he can get a 100ft., but would need to give the lead-in a twist. On the other way the aerial would be only 45ft, in . length, and the aerial would come in . straight. Which would be the better? | A.: Providing you can get a reasonable light to No. 1 it would be the better theoretically, although in practice there | would be little perceptible difference in the signal strength. ANGUS (Wellington) has . recently — wound broadcast eoils for his shortwave set. He igs having trouble with the reaction; everything works quite well for a part of the range, but on the lower frequencies the set will not oscillate, having more turns induces violent oscillation on the higher frequencies, A.: The trouble lies with the small valus of condenser used to contro] regeneration. Now that you have broadeast eoils you will need a bigger value condenser, say, a 00025 MARCONI (Blenheim) wishes to know afew points about the "Round the World Two." 1. When I first hooked up this set I put 224 volts on the plate of the detector. It would not oscillate; then I tried 45 volts and it went all right. Why . does it not oscillate on 224. volts? A.: It depends on the valve; not every one will oscillate on 22% volts. 2. I intend to shift the tuning condenser back from the panel on account of hand capacity. May the extension shaft be of bakelite,’ ebonite or other non-contacting material, or would metal do? A. By using metal you would be defeating your own ends. Bakelite or some non-conducting material would be necessary. 3. My set has the fringe howl badly. How ean I stop it? That question was fairly comprehensively treated in an article in last week’s issue. 4. How do I eonnect' a choke output to my set? Would the secondary of an audio’ transformer do for a choke? A. A diagram shows the connections, providing you are using only two valves the secondary of a transforrher would probably be all right, in any case it is worth trying. Is the plate-voltage control method of as efficient as condenser control? A.: Yes, but you must be careful to

select the resistance. The following are suitable: Centralab, Frost or Hlectrad, probably 50,000 ohms would do. ISTENER-IN (Alfredton) wants us to draw a comparison hetween a 6-volt A accumulator and a Columbia "Hotshot" battery. .

A.: The Columbia is merely four of he standard number 6 cells wired ready ‘or. use, 2. How would they do to run a sixvalve set? : They would be inadequate. 3. ‘How do you compare this battery with a 45-volt B battery? A.: They are different in size and purpose, though the 6-volt battery consists of four cells wired in series and:a 45-volt B battery comprises 30 much _ smaller cells wired in series. . W.Y." (Herekino) asks the follow. * ing relative to short wave :- 1. Concerning the four-valve screengride received in 1930 Guide, the theoretical diagram shows the grid leak lead to A positive and the under baseboard wiring to A negative. A.: It rests with your valve which one you use; ; try the positive. 2. Wiring planned.shows the R.F. coil to earth, the under baseboard shows filament and plate terminals to earth. A.: Filament negative is earth.

3. A whistle is heard which increases in intensity with the adjustment of the rheostat, A.: Do you mean that the set oscillates and continues to do only more violently as the rheostat is advanced? If so, you will have to reduce the number of turns on the tiekler or the detector voltage. If the whistling is independent of oscillation reverse the connections to: the primary of the first audio transformer. If this dots not stop it try earthing the cores and the frames of the transformers. . ; 4, I can get no on the 19-28 metre coil, . A.: Does the set on this band? [f not, inerease the detector voltage and then the number of turns on the tickler. 5..I am taking the plate lead through the slot at the top of the can, and ‘by another route to the grid of the next valve, Is this O.K.?-Yes, 6. Reaction works smoothly on the pS-88 metre coil with one type of valve, ut the. others will not work. Why? A.: These may need a higher voltage on the plate, and they may be older than the one that works. 7. Is the plate wiring system of "Round the World Two" correct ?-Yes. 8. I applied a voltmetré between the plate terminal of the valve and the B,and there was no reading, although the set was, working. A.: There must be a reading if you ate making the contacts properly and

your meter is quite O.K., otherwise the primary of your transformer is burnt out. 9. What is the gauge of the enclosed wire? \n A.: 40 S.W.G: * 10. Would a .5 mfd. condenser be O.K. across the reaction resistance?-Yes, 11. An anode resistance I had would not allow any current to pass unless it was full ins Was it defective? A.: Apparently so, though the resistance might be too high for the use to which you are putting it. B.O. (Nelson) has two condensers, * one with 7 fixed plates and 6 moving, and .the other has 10 fixed and 11 moving. What are the capacities? p jak Probably .0002 or .00025 and 2. A, .00025 condenser broke down in my set and I cannot get another one. ill a .0003 do?-yYes, " F." (Auckland) wishes to know *the number of turns for broadcast eoil to be wound on a 3in. former and tuned with a .0002 or .00015 condenser. A.: That was given in the details of coils for the "Round the World Three" when it was published some time ago. WwW H.P. (Wanganui) submits a sketch ° and asks the number of turns for the coils but he has failed to state the value of the condenser. If he will do this we might be able to help him.

2. I get good speaker reception from 2BL, 2YA, and 2FC, but cannot find either of the Wanganui stations. A.:-It appears that your condenser coil is not covering the band. Hither might be too small. Please restate your probless, noting where the three stations are logged. . 3. What are the voltages for each valve? : A.: I£ you have 180 you can put these on the last valve, but you must use 12 volts bias. XPHRIMENTHR (Christchurch) asks for the number of turns for primary and secondary of screen grid circuit sent in. A.: The aerial coil is about 20 turns tapped at the centre, and-L. 1 you can get from the "Radio Listeners’ Guide"; it is the same as the ordinary Browning Drake. The primary of L.2 might give you some trouble, but try 20 turns to start with. The secondary you can also get from the "Guide," and the tickler will be the same as for an ordinary valve. 2. Is this suitable for the Loftin White aroplifier? A.: Yes, providing you get the coils matched and properly shielded. 3. Will a transformer be necessary between the tuner and the amplifier. A.: A resistance unit coupling would do better. ; 4, Will a better tuner be described in the Record?" A.: We hope to describe a tuner for the Loftin ‘White’ shortly.

5. Are these coils obtainable in New Zealand? A.: If you submitted the circuit to Johns, Ltd., Auckland, they would possibly be able. to make them up for you. R H.R. (Wanganui) is building a four- ¢ valve set, and asks if two .0005 condensers are suitable.-yYes, 2. What voltages are best applied to the following :-- R.F. equals 674. Detector equals 45. First audio equals 90. Second audio equals as much as you can give "it. 8. Are the following valves suitable: HL 610, L 610 (either of these for the R.) A 615, B 605, and a super power valve A.: Hither H.L. ov L. 610 would do for the R.F., but you could not use B. 605 as first audio, you need use a general purpose valve such as L. 610. Which ratio do you recommend for the transformers ?-3% to one. 5. Is an amperite necessary with 6volt valves and 6-volt battery?-No. FEF H.Q. (Milton) wishes to convert a e 2 amp. charger into an A eliminator. A.: The method was described in "All About the All Hlectric," the choke for this has been described in the 19380 "Guide." C W,. (New Plymouth) submits a plan e for converting his charger into an A. eliminator. A.: The choke must come before the condenser, and the lead from the plate of the valve is A-, 2. Would the plan on page 59 of the 1930 "Guide" do with two separate filament windings? , A.: There is no plan on page 59. We cannot quite see your point. The sketch submitted would all right, providing the polarity as mentioned in this answer is observed. 8. There are two spools on the charger ; the bottom one goes to the mains and

the top three wires go to the valve as shown. A.: The charger is designed with primary and secondary on either leg, and the" three. wires Agel two filament and t¥ie secondary. -he plan on page 59 of the 1929 "Guide" is right. To make it an eliminator you merely shunt a condenser between positive and negative and put a choke in the positive lead between the condenser and the transformer. A G.C. (Te Aroha) built an "A" bat- * tery charger the resistance lamp of which burnt out; it was replaced and now the replace has burnt out. A.: You have too much voltage on the secondary. Take a tapping from fewer turns. You mayj have to undo your charger to get at the winding. A V.N. (Grey County) asks what would © be the charging rate of nine 10volt blocks of B. cells connected in parallel. The charging rate of the whole} 90volt block is 4 an amp. . A.: By connecting them in the capacity is increased and as such the charging rate can be increased. 2. My generator charges at about 5 amps. hat resistance could I put in to lower the. amperage without deereasing the voltage? A.: You cannot decrease amperage without decreasing voltage, but providing your voltage will remain constant there will be ne need for a resistance. To be safe, usé 2 3-ohm. rheo which would pass 5 amps. Probably only an ohm. would be needed. ot 8 D C.D. (Auckland) has’not had success ¢ with the Loftin White. The quality is poor though everything tests O.K. The plate current is 42 mills. A.: Constructors have been singularly fortunate with the L.W., though there is one source of possible trouble, and it seems that you have lighted upon it. Unless the grid-leak supplying current to the coupling between the 224 and the 245 is of the correct valve, the amountwof bias on the 245 is wrong, the plate current of that valve high, and the tone, to use your expression, rotten. Though many of these leaks are marked at 500,000 ohms, it is to be regretted that many do not come anywhére near this value. Try another leak of the same value, or, if a megger is available, have it tested. No doubt this will solve the trouble. 2. It appears that the tapped choke output is the better. Where can I put same? ‘The choke replaces the primary of the o.p. transformer and the condenser is placed in series with a lead from the plate end of the choke to the speaker. The other side of the latter is brought to earth or any other suitable return. Can you advise a transformer to match the Output of the N. & K. inductor dynamic speaker? I understand it is between the red and the green spot of the Farrand, = = . A.: Your safest plan is to obtain one of the tapped secondary o.p. transformers, which are now quite. common, and match the impedance from this. WwW A.G. (Taranaki): See the rly to ° ARB. (Clevedon). EF H. (Auckland) is using a Tungar * charger for a 6-volt battery; he gets a shock in standing on a concrete OOF. A.: Providing your charger is working all right we would not go as far as to say there was anything wrong. If it ig not quite right we should imagine there to be a breakdown in the insulation between primary and _ secondary. Wither this or the. secondary is not at earth potential. ONSTANT Reader (Wellington) wishes to add another valve. to_his combination receiver, but wishes to use only one valve -when he is using the erystal. The other is for use with the valve detecting only. A.: A sketch of the connections for another valve was published last week. To use the erystal and the one valve it

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would be necessar to use a jack before the extra valve. This jack is specially designed so that’ when the plug is out the secong stage is connected in the circuit, and@#hen it is in the second audio valve is eif-off. Examination of the jack will show how the connections are made. 2. I get a shrill whistle hen the set is operating on loud signals. A.: This sounds like a defective gridleak or speaker. A R.B. (Clevedon) asks if a tungar A * and B charger can be used to run a set. of 4 valves. 1. Could A and B currents be drawn simultaneously? A.: You would have to go to a considerable amount of trouble, though you could use the transformers in the charger to make your eliminator; you would need separate rectifiers. wei What values chokes and condensers I need? : These were given in the 1929 and 136" "Guides. " 3. Would the necessary apparatus be expensive? : It would cost you several pounds. 4, Would I need an electrician to assemble the apparatus? : So the regulations state.