Questions and Answers

Radio Record, Volume II, Issue 39, 12 April 1929, Page 12

Questions and Answers

Ooil Specifications. RHfOstat, Westport, writes asking information on the following points: 1. Please advise specifications of coils for wavelengths 80 to 200 metres. In the set used spiderweb coils are employed, and I should like to wind up my coils to fit it. If these specifications are not available, would you please advise the different types of coils. A.: We regret that we do not have the specifications for spiderweb coils, but in any case it woulé be impossible for one set of coils te tune between 80 and 200. Solenoid coils would be better, and the following specifications should meet the correspondent’s case :-

The specifications for a _ ecoil to tune from 110 to 220 metres eould be arrived at by proportion and experiment by taking the above as a basis. Full details. appertaining to this have been worked out in our 1929-30 issue of the "Radio Listeners’ Guide," which will be published within the next three weeks. 2. Would a higher voltage than 224 "B" increase volume and distance? A.: If the set oscillates freely, there would be no advantage whatever. Concerning the Screen-Grid. pts (Napier) writes: "I wish to thank you for your answers to my questions published in this week’s

‘Radio Record’ re the advantages of condenser coupling for the screen-grid valve, and have read up your explanation, and feel satisfied I am making up a B D sereen-grid, and I would again ask you for an O.K. on the circuit of which I set down below. A.: The circuit is quite O.K., but in the "Radio Listeners’ Guide" we shall publish the last word in screen-grid circuits. The circuit to be described (erroneously called the Screen-grid Browning Drake, for where is Browning Drake when the slotted primary is removed? embodies the latest researches of the world’s technicians. 2. Would the radio frequency choke described in an earlier issue of the "Radio Record" be efficient enough for a screen-grid valve? The choke is 1000 turns divided into five slots of 200 turns each.-Yes, quite OK. 8. Which is the correct side of the filament of the detector to bring in the lead from the grid lead? A.: Always to A positive. 4. In the diagram on Page 30, volume 2, No. 36, the filament is shown connected to be plus 185 as well as to A positive. This I take is a draughtsman’s error. A.: Obviously yes. We regret the little inaccuracy, but take it that anyone desiring to experiment with the sereen-grid valve would immediately recognise, as did the correspondent, that the connection was erroneous.

5. Can you tell me on what date the Listeners’ Guide will be released? A.: Sorry, but at the present time we eannot. The technical staff is working on the publication as fast as possible, and there is every prospect of it being published within the next fortnight, as there are very heavy demands. Data for Moving Coil. I AM making up "Pentode’s"’ dynamic cone and am just about ready for winding. Could you advise me the number of turns on moving coil to suit UX112 in last stage. Would it be possible to rewind suitably a burn-out audio transformer for the output 25 to 1? I am making the gap 5-64.-J.B. (Christchurch). A.: See "Pentode’s" article on "Finer Details of Radio"; 112 would require-105 turns. Owing to the fineness of the transformer wire .and difficulty of gauging the exact quantity of wire it would be advisable to purchase one. An Appreciation. THANKS for tip in "Mail Bag" of March 15 re 10,000 ohm variable resister. I have sent away for resister, as I had no other means of reducing volume I had to cut down filament current. Needless to say I always read all technical news in "Record." I have only one problem at present and that is reduction of volume, and that should be solved when resister arrives. I am satisfied that if batteries are kept up to scratch most radio troubles are cured.

Away up here we can log in on five valves 1 9and 32YVA anv time. night or day,

except 83YA in summer daytime.-

H.

W.

Young

North Auckiand ) =

The Home-made Moving Coil. H F.B. (Auckland) has made the dyna- * mie cone speaker, and has been running it for the past week with excellent results, but has had to reduce the number of turns on the coil to 86 to get the best results from a 609-valve. Until the spool dried out it was found that the consumption was 3 amps., but it now requires 1.8 with 1-16 inch gap. We are asked to supply the following information :- 1. The number of turns required on the eoil with two 171s and the number of turns suitable for a B605. A.: See the special technical article by "Pentode" in another part of this issue. 2. The amplifier is now using OPC 3 and AF 5 C transformers. Is this O.K. with 171 valves?-Yes, quite O.K. The "AC" Dynamic Speaker, N D.B., of Pahiatua, writes suggesting * ‘that the current required for the speaker be drawn from the 230-volt mains. A rectifier would be required, and we have been asked to give information as to its assembly. A.: The construction. of a rectifier to supply current to both the amplifier and the speaker would be a job much too difficult for the amateur, for it would necessitate the handling of the voltages approximately 1000. An electrician could make a satisfactory job, and we could supply particulars to such a_ technician.

The "Rotorua" Portable. C F.M. (Auckland) asks the following * questions relative to the construction of the "Rotorua" portable, described by "Pentode" in our issue of December 14, 1928. 1. Is there any reason why the gridleak and condenser should not be screwed to the base board? A.: It is inadvisable to secure these to the base board, as there is a possibility of this being slightly green, causing a short circuit. This being the case, the higher frequencies or lower wave lengths could not be tuned in. 2. I have brought both ends of the wire out on the base of the choke by passing these down its side and cementing. them to the centre. Is this all right? -Yes. 3. I am making the box, 20in. x 6in. x;’ 14in., will this effect the number of turn on the loop and at what turn showld take the tapping off? A.: In the first instalment there was an error in the specifications for the loop antenna, the number of turns of which should be twelve. This was corrected in the second instalment. This being the case, there is no necessity to vary from twelve in the altered size, The tapping will be taken off at the fourth turn from the inside. 4, What ratio transformer should IL use if another make other than that specified were to be used? ' A.: There is no necessity to adhere to the recommendations, as far as makes given in the article, The ratio of the transformer should vary between 34 to 1 and 5 to 1. 5. It was not stated in the article how the speaker diaphram and unit were connected. A.: On purchasing the unit it will be quite obvious how the connections are to be made. Blue Print Wanted. CouLD you send me a blue print of a five-valve "Raleigh?" I have been unt

able to Obtain one here _

A.R.

A.

(Dunedin).

A.: In all probability you will have great difficulty in obtaining a blue print of this receiver, but try J. A. Smyth and Co., Wellington (who advertise in this issue), as they are the agents. Improving Tickler Reaction. W J.B. (Dunedin North), writes :-I *wish to improve my tickler reaction and, incidentally, tone. Shall I use a

Constructors of the dynamic cone speaker will find much useful information in a special article by "Pentode" on "Finer Points of Radio" on page 30.

J choke or a 25,000 ohm resistance in Gombination with 1 MED condensor between the tickler and the first audio transformer? . Loge A.: In our issue, volume 2, No. 30, this point was dealt with very fully, and the correspondent is referred back to that number, oo 5. Would I be right in using a PM8 for the first RF followed by a SG and detector A.: No. The correspondent has reversed the order. The screen-grid should come first, followed by the RE and then the detector. Cannot Increase Filament Voltage. C L.F. (Dannevirke), who sent us a few * points for enlightenment in Volume 2, No. 35, makes additional comments: "Z, asked what was the matter with my receiver that I could use only 4 volts on a/6-volt valve. I have turns off the tickler, but I still get continual oscillation, when I increase the voltage. Your reply was that the set was not neutralised, and it is only a 1-valve set. How do you neutralise such a set? The batteries are only four months old, and the valve is practically new. When the dial is turned from 10 to 0 a continual scream is heard. .: We regret that in our previous reply we missed your point regarding the neutralisation, but for this we cannot altogether take the blame, for no reference to the type of receiver used was made. A 1-valve set cannot be neutralised, this process being possible only when radio frequency stages are employed. We still think there are too many turns on the tickler, and the fact that the set oscillates violently on the lower wavelengths seems to strengthen this opinion. Reduce the number of turns still more. There is no indication how many volts are being used on the B battery. This ay be too high, as when a detector only is being used there is little advantage in exceeding 45 volts. If the correspondent has been using his batteries only four months, he would be advised to test them. The R.F.C. is preferable to the resistance. If the moving tickler alone is to be used and it is desired to incorporate the choke and by-pass condenser, make the connections as follow :- From the plate of the detector valve make a direct connection to the tickler. Connect the other end of this to the radio frequency choke and the free end_ of this to the audio transformer. From the tickler side of the choke take a lead to a one FD fixed condenser, the other end of which is connected to the earth or "A" negative. 2. I have not had much success with sereen-grid boosters. I have been using a RF choke of 900 turns in groups of 200 turns wound with 34 D.S.C. wire. Is this all right or would you suggest the names of some reliable chokes? "A.: This choke should be quite effichent, but at the present time there are wery many reliable makes of chokes on the market. The general concensus of opinion is that the English chokes, although slightly more expensive, are to be. vreferred to American for New Zealand conditions. 8. In your article on the application of the screen-grid in the issue volume 2, No. 36, it is stated that two RF chokes are employed, whereas your diagram shows one only. A.: By regarding the aerial coil as a choke and taking the aerial in at one end and a lead to earth at the other end, the difficulty of the non-agreement of text and drawing can be. overcome. 4, Does the shielding of the RF and the detector stages of: the receiver inerease distance-getting or not? +. A.: Shielding introduces certain losses, the chief aim being. to prevent interaction and distortion. As far as dis- ‘ tance is concerned there will be very little ‘difference. between the shielded and unshielded receiver, if anything, the shielded may be a little inferior to the unshielded, but as far as the selectivity, stability, and clarity are concerned, the shielded is far to be preferred.

Secondary ‘Ticklet Range turns. turns. (metres). 20 7 85 to 110 11 5 45 to 65 6 5 80 to 45 3 4 15 to 30