Questions and Answers

Radio Record, Volume III, Issue 37, 28 March 1930, Page 28

Questions and Answers

Nettralisation Difficulty. I HAVE a five-valve Browning-Drake with impeda formers in the amplifier. Frequently reproduction becomes fuzzy and the set goes into oscillation. A. temporary cure can be effeéted by flicking the tickler coil half a turn or intertupting the "B" supply, states "Stator" (Christchurch). .: The set is not neutralised, and back coupling is taking place between the radio frequency coils. Hxaminé the neutralisation system for defects, and try reversing the connections to the primary or to the neutralisation condenser. If possible, try substituting this for another one. Two-Valve Set. qT the Guide there is a circuit for a erysta] set with valve amplifier employing the double grid valve, states "Beginner" (Napier). 1. Can a valve be substituted for the erystal? A.: Make up "Round-the-World" Two using the broadeast coils and employing the double grid valve in the last stage. 2. What type of speaker does the set work best with? Would it work a "Baby Grand" or a home-made exponential speaker? A.: Neither of these are suitable for @ set so small, especially as it employs only 224 volts. se a very light cone. According to our tests the small ‘""M.P.A." should be suitable. . Logging the Americans. wr: eannot I receive the American stations on my eight-valve A.O. set? asks "S.H.B." (Auckland). I have a good aerial and a good ground.’ A.: Most of the American stations are received by sets using regeneration, and it is possible for the operators to hetrodyne the station. This gives them some indication where the station is. In your case the only way is to dial very carefully and listen attentively for any suggestion of a station. Watch the "D.X." columns to ascertain which stations are coming in well, and at what time. Logging Cards. (ouLp you supply me with a card for getting stations? asks "J.ET." {North Canterbury).

A.: You require a call-book in which all particulars about the stations received in New Zealand are given. At present there is not a complete list, but in May we propose publishing a very full list which should help greatly. Short-wave on Broadcast. AST week I received a_ short-wave amateur well down on my tuning dials. Why? asks "Radio Fiend" (Hltham). _A.: Unless the amateur was transmitting on the broadcast band, it is very difficult to say. We suggest, however, that the high frequeticies were striking some nearby object and being re-radiated at the natural frequency of that object. You would then pick them up on your band. 2. My dials do not appear to match on the higher frequencies, but come into resonance at about 2BL’s frequency. Why so A.: Have you read the article on aerials in last week’s "Radio Record?" It is quite likely that the capacity of your aerial system is adding to that of your aerial circuit, and putting an unequal amount of capacity into that circuit. consequently the readings do not agree. When, however, the capacity in the aerial circuit is increased by turning in of the condenser, that in the aerial circuit becomes more or less negligible and the readings agree. However, it is quite likely that either the condensers or the coils are not matched. As you suggest, taking a turn off the tickler end of the secondary might improve matters. 8. When the aerial tuning is below 30 degrees, a continual howl is noticed. Should this be? A.: No, the set is not neutralised. Try adjusting the neutralising condenser and see if this howl can be cut out; if not, reverse the connections to the primary. It may even be necessary to readjust the number of turns on this coil. "Round-the-World" Two. HAVB constructed "Round-the-World" Two, and cannot get results, states "Carborundum" (Wellington). 1, I am using an aerial 150 feet long, and cannot get thé set to oscillate in tuning on shortwave.

A.: Your aerial is far too long. Reduce it to 50 feet. The very long aerial is introducing damping and preventing the set from oscillating. 2. Should the radio frequency choke be home or factory made. I have one made from 1000 turns on celluloid. A.: Your choke should be 100 to 150 turns on celluloid for shortwave. You are using a broadcast choke. 8. 2YA comes in all over the dials with the broadcast coils, and also with the shortwave coils. A.: This is the result of an aerial that is too long. In any case, you will not be able to éliminate Wellington on the broadcast band without a wave-trap. It is asking a little too much. However, it should not worry. you on the short-waves. A Noisy Set. I HAV® a seven-valve A.C. all-electric set, and for séveral months it has given good results, but I have had to change the detector and now the set does not work properly. It is very noisy past a certain point. A.: We regret that it is impossible to give you much help. Amateur servicing of factory-made sets is not the task for an amateur. We advise you to contact a reliable service man.. Had you given us your address we could have recommend-. ed one in your district. What More? I GET splendid results from 1YA and 2YA on "Pentode’s" . érystal and valve, states "C.M.," Auckland. Reception from 2B, Sydney, is rather distorted, and there is nothing from 2FO. ‘How can I improve my reception? A.: You are really very lucky to get Australia on a crystal set. Possibly your crystal is not a good one. Wiring "Round-the-World" Three. "FR.D." (Greymouth) asks regarding "Round-the-World" Three. 1. What gauge wire should be used in wiring Round-the-World Three? .: It is immaterial providing it is not less than 20 gauge. Use glazite or similarly covered wire, to make the neatest and most efficient job. 2. Is the amperite not necessary in the extra stage? A.: Not if six-volt valves are used; with a six-volt accumulator or with a four-volt valvés with four-volt .aceumulator. If, however, you are using five volt valves, put a six (or lower) rheostat in the circuit, and keep it well advanced. Condensers for Round-the-World Three. ould I use .0005 variable condenser for "Round-the-World" Three, as I intend to use broadcast coils only, asks "R.B.R." (Wanganui). A.: Yes, you must use one coil only and that of 60 turns of 20 D.C.C. wire on a 38-inch former. 2. Could I use a twin-wire aerial to feed two sets providing each wire was separated from the other? A.: Yes, provided the wires were about 6ft. apart. No damage would he done if they were closer, but you would not get as good results as you should. Transformer Trouble. I HAVE a regenerative set using three general purpose valves and a power |

yalve in the last stage. I am continue ally burning out the second stage audio transformer, states "G.R.W." (Rodiey). A.: In all probability, biasing the Airst audio valve would cure. your trouble. This valve needs 44 volts when 90 volts are used on the plate. : Could I replace last stage with two stages of resistance capacity coupling? A.: Yes, but if you are using only dry batteries you would be raising both "A" and "B" current consumption to no purpose. The tone might be improved slightly, the volume would be increased a little, and the c»st would be greater. You would need the materials indicated in a sketch in last week’s issue, omitting, of course, the push-pull stage. Various Points, I AM using a Ratheon as a rectifier, and wish to know if the voltage in it: drops as the drain upon it increases, "A.M.P." (North Otago). . A.: We have not made these measurements ourselves, and cannot obtain any authentic figures. Hxperience has shown, however, that for drains up to 100 milliamps the voltage is fairly constant. 2: What A.C. voltage must I arrange to keep the rectified output at 300 volts? A.: About 350 colts, 8: In the amplifier I intend to use Hnglish valves, and have decided on Marconi P625 for push-pull. Is this obtainable in New Zealand? A.: Yes, we believe so. Try Amalgamated Wireless, Wellington. If not, its exact equivalent Osram P6253 is readily obtainable from most dealers. 4, In selecting the first audio valve IT believe that I shall have some trouble in

etting a valve that cannot be overloaded . by the grid swing from the local station, 2YA. _Furthermoré, I am expecting to have difficulty in arranging the amplifier so that the two final valves cannot be overloaded. \.: We would advise you to use the ordinary 227 type of valve, either singly or in push-pull for. the first stage, followed by the two P625’s, Any usual ¢ombination of this nature can be overloaded by the local station. When it is considered that a signal as great as half a volt can be obtained from the aerial and magnified through the detector, the first audio valve, the transformers, and the last stage, it can be seen it would require more than two 225’s to accommodate the swing. You will find the combination provided by MH4 and two 625’s Ample for your requirements. " 8B I propose taking bias. from the lower end of the potential divider, and presume .this will be satisfactory. A.: The best méthod is to keep the grid at negative potential and the cathode in one. case, and the filament in the other at a higher potential. . 6. Is enamelled resistance wire able in New Zealand? . A.: We cannot say. Try Johns, ited, Auckland. Pentode Difficulties. yp order to get greater volume, I pur- = chased a B448, and on placing it in its socket, found that an, unsuppressible howl resulted. How can I prevent this? asks "O.B." (Nelson). : A.: Wxamine the batteries, fot in otder to run a pentode batteries must be kept up to the mark. If these are all right, put a grid leak of 4 megohm in series with the plate lead from the detector to the following: transformer. Round the World Two. I SHOULD esteem advice on the following points relative to the construction of "Round the World Two," asks C.J.C. (Frankton). 1: I have PM 5, PM 6 and 608 on hand; can I use any of these and be assured of good results? A.: Yes, use PM5 in the first- socket and PM6 in the last.

2: What is the voltage for the Bdetector and amplifier? A.: re about 30 volts on the détector and 90 on the amplifier. You will probably have to vary the detector value. 8: I note that Amperites are of different values for different valves. How can I determine the value of the Amperite to be used in this case? A.: Amperites are to be used only when using a five or 5.5 valve on a six-volt battery. In your case one is not necessary; you are using six-volt valves. They are resistances, valued so that when used with certain valves they will pass the eurrent required by it and at the same time reducé the voltage accordingly. Short-Wave Difficulties. wat is the use of the midget condenser in series with the aerial o Round-the-World Two, writes "H.W.Y." (Herekino). A.: As this set does not employ a primary ¢oil the selectivity is not sharply defined. The midget condenser is a means of obtaining selectivity. 2, Using .0001 variable condenser with a .00025 reaction, should the readings on a booster employing the same value tuning condenser and the main set agrée.

A.: They should more or less. The fact that they don’t merely indicates that the coils are not matched. Providing you can cover the wave-bands there is nothing to worry about. 8. Can you explain why coils made of 18 gauge tinned copper wire will not oseillate while the Round-the-World Two coils A.: In all probability the tickler coil is connected round the wrong way. Reverse the connections to it and then try. 4, I have to take the wire from the variable condenser to the coil closé to a copper shield. Is this lessening the efficiency of the set.-No. oe 5. Would the addition of a 14 grid bias improve the set? A.: Ié put on the last valve, yes. It will improve the tone and keep down the battery consumption. Battery Trouble. wat is the matter with my set when the valves light up, but there is no sound for a few seconds, and then the polices are muffled, states "K.H.H," (Picton). A.: Many things may be wrong, but in all probability the B batteries are low. If you have had them for more than six months, you may be fairly certain that this is the cause of your trouble. Other causes may be valve or valves may need renewing, for although they light up, this is no indication that they are working properly. There may be something wrong with the speaker, or there may be a defective component in the sét. Probably, however, your "B" batteries need replacing. BWxamine them carefully to see if they are sweating, for diréctly this commences the life of the battery is at an end. Tone Hard and Forced. J HAVE installed a pentode, but the tone is hard and forced, and when I reduce volume by turning down the rheostat it becomes worse.-Mons" (Palme*ston South). Av: Try a .01 to .006 condenser across the speaker, aud if this has little effect,

a 4 megohm grid leak between "G" of the last transformer and "G" of the valve socket, 2: In a reply concerning a kit set, you told me not to interfere with my transformers, but a local firm are changing the transformers in the same type of set which they are putting out. Could théy change mine? A.: Yes. When these jobs are in the hands of expert technicians they can be done satisfactorily, but we do not advise the amateur who knows little about radio to attempt them. That would probably have a great effect on your trou-ble-too much harsh volume. We think your best: plan would be to get in contact with dealers and ask them to put your set right rather than attempting to do so yourself, 8: What do you mean by load on an eliminator? A.: The amount of current taken from the ,eliminator by the valves. : I cannot get my battery to show full charge. I blamed the hydrometer, but it is O.K. The cells appear a little sulphated. A.: This is the cause of your trouble. A sulphated battery will never show a full charge. Wither take your battery to a service mar, or carry out the instructions laid down in the, 1929 edition of the Radio Listeners’ Guide. 5: Gan I use the parts of my set for a Browning Drake? A.: Yes; except the sereen-grid valve. Radio-Gramophone Questions. HAYE constructed the amplifier deseribed in "All About the All-Hlec-trie," and it is working a dynamic spéeaker excellently, writes "J. McM." (Arapuni). I now propose to construct a radio gramophone incorporating the above, and want advice on the following quesions :-~ 1. What type of tuner would you suggest that I build to meet the following requirements :-Single dial-selectivity, A.C., sereen-grid, good toné, but distance not essential. * A.: Our experience regarding screengrid has been that it is not generally suitable for amateur construction, and for this reason we have left it severely alone. Other than this, the A.O. Brown-ing-Drake tuner should méet your requirements, This will be, we hope, redescribed in a modified and improved form in the 1930 Radio Listeners’ Guide. Another guitable circuit would be twe

stages of screen-grid without reaction but employing a small condénser to tune the aerial coil. A three-gauged condenser balanced on the first coil is necessary. . 2. Do you consider that the 227 in the first audio stage can handle a sufficiently large grid swing to meet the full load réquirements fér the following 245 valves in push-pull? A.: A 227 is capable of overloading two 250 valves, so that you will have no: difficulty when followed by two. 245’s. However, if you are in doubt, use two 227’s in push-pull, 8. In order to obtain the necessary voltage for the screening grid, of an 8.G. valve, I have tried connecting the 500,000 ohm poténtiometer across the power pack, -and am taking a lead from the slider to the screening grid. Although I can adjust the plate current, the valve refuses to operate. A.: The value of the potentiometer is not suitablé. In screen grid valves the screening grid consumes a small amount of cutrent and needs a resistance that will pass 1 or 2 milliamps. as a potential divider; 500,000 ohm is far too much You must work out the value by Ohm’s law. Divide the voltage direct from the rectifier by 2 and multiply the result by Interference, "G J.W." (Greymouth) asks if there is °* no way to prevent intetferérice from a D.C. light genetator. A.: It can be treated very successfully by the methods described in our issue of September 13. We wish to thank you for the details of the short-wave tfeceiver sent in, Power Leakages. I AM using a seven-valve set, and I am almost drowned out every night by power leakages, although a man ttvo doors away escapes. What do you suggest doing?-"Dynamie" (Lower Hutt). A.: We can do little to help you. You must communicate with the secretary of the Hutt Power Board and give him as many particulars as you can. Look over your aérjal system very carefully and be quite sure thére is not a loose connection therein. 2, What would be the approximate cost of an all-electric short-wave adaptor? A.: If you had to provide only the valve, condenser, éte., without power equipment, it could be had for about