Questions and Answers

Radio Record, Volume III, Issue 12, 4 October 1929, Page 26

Questions and Answers

In Brief, Ss A.M. (Hamilton). Your case is one ~* of unusual dimensions and within the next week or so we shal! give it close attention. Amplifier. a i, (Christchurch) asks the probable * cost of the A.C. amplifier described in our special issue. A: The cost could be computed by ‘reference to uny of the advertisements. It would be somewhere about £12 if the constructor made his own transformer, 2. Would this arrangement be satisfactory with Pentode’s moving coil speaker, which I have made and which is giving excellent results? | A: Perfect results should acerue and if the amplifier is carefuily made there should be practically no bum. A.C. versus D.C. 1s there any difference between an A.C. and a D.C, set? asks "Argument" (Napier) as far as tone, volume, and sensitivity is concerned. A. The characteristics of modern A.C, valves are equal if not better than many of the D.C. types, and thus the all-elec-tric receiver is equal, if not superior, to the battery set. Valve for valve efficiency can now. be obtained. The extra valve in the A.C. set is employed only because there is more power available. The superiority of a seven-valve all-clec-tric over a six-valve D.O. ean be exemplified by reference to our D.X. columns. Reception of American and Japanese stations with the all-electric Neutrodynes are now quite common occurrence, where-

as in the days of six-valve Neutrodynes they were less common. 2. Does the variation in "A" current cause any notable results? A.: With the indirectly heated cathode type of valve, where the internal construction is such that small fluctuations in temperature have little effect, the result of temperature variation is almost negligible; Flowever, in very many sets automatic line voltage control is provided as a safeguard. Valves for a Small Set. I HAVE a thre¢e-valve set, but find that although the volume is still good, tone has fallen off considerably, writes "E.W.W." (Ellerslie). The valves are twelve months old. JI have been using special valves for each position. . Is this satisfactory? , A.: You haye been using a good combination. We suggest that the grid bias battery has not received the attention it might have, You appear to have strengthened your "B" batteries and neglected the "O,’’ with the result that tone is impoverished, An Aerial Problem. [* I raise my aerial another 10 to 20ft. at the high end, would I be able to get further afield? asks ‘Listener’ (Southland). A.: It is not the height at the higher end that counts; it is that at the lower. If the aerial were raised at this end, an improvement should be noticed. Valve Combination for a Reflex. C4 IT improve my valve combination in a Roberts’ Reflex, as "D.H.R." (Christchurch).

A.: An improvement should be apparent if PM4 in the first audio stage is replaced by PM3, and the detector C509 replaced by A415 or PM4D. This valve ag detector is unsatisfactory from the point of view of consumption of "B" battery. The transformers become saturated when a power, or semi-power valve. is used in anything but the last stage. Aerial Covering. " R.P." (Christchurch) states that he * has a double aerial, one side of each being separate lengths, one covered and the other insulated. Should they both be bare?

A.: The question is a little vague. The meaning of double aerial being ambiguous. As for the covering, there will be very little difference whatever the combination. Coil Design. HAT are the number of turns, spacing, and gauge of wire required on a 2-inch former to between 15 and 100 metres?-Mack (Johnsonville). A,: Four coils are necessary, built up of 18 gauge wire, double spaced, with tuning condenser .0001.

Hixperiment with the number of tickler turns may be necessary, as the yalues are approximate only . 2. Will a Philips Trickle charger charge Mefohms "B" batteries eight in a row? A.: Philips have several trickle chargers, and it is necessary to know which one is intended. To charge eight in a row the output voltage would have to be sixteen volts. The best plan is to series paraliel them, arranging them so that the voltage is equal to that delivered by the charger, but the amperoge that of the numbers of rows in parallel plus amperage of an individual cell.

8. Who are the New Zealand agents for the Ward and Goldstone Alterno Rectifier?. . . A.* Extensive inquiries to locate the agents. | A Improving the Browning Drake. if HAVE a 4-valve Browning Drake, writes "Brownie" (Stratford), and I would like to know which of the following will give me the best résults. The addition of a screen grid as H.F., parallel feed, adding another stage of R.F., or another stage of audio. A.: The result of any one of these additions will rest largely with the ability of the constructor, but adding another stage of triode R.F’. should be the most effective. Both radio stages should be parallel fed. ‘Two audio valves could quite well be used, but no more than this if transformer coupled, unless, of course, push-pull is used in the last stage. 2. Would a short-wave adaptor work satisfactorily from this set? A.: Yes, providing the adaptor itself is satisfactory. . Why can I not get 2YB, which is about 25 miles distant, when I Pan get 4QG, TZL, ete.? / A.: It appears that the inductance of your tuning coil is too high-take off a few turns of the secondary. Note: All queries should be addressed to the Technical Bditor. Strengthening the Browning-Drake. WISH to strengthen my BrowningDrake, says "Screen Grid" (Ngaio). Which is the best way to do so-using the screen grid bdoster or building the five-valve Browning-Drake? . A.: Building the five-valve BrowningDrake, but see the reply to the above correspondent, 2. Gould I use my present .0005 condensers for tuning and use 3in. coils?Yes. 3. Is it necessary to use a short aerial if the screen-grid booster is used?

Questions and Answers QuUR Question and Answers Department are always prepared to give consideration to the problems of our readers. This service is gratuitous, but a few courtesies are asked. 1, All technical correspondence should be addressed Technical Bditor, "Radio Record," Box 1032, Wellington. 2. Be brief, do not make apologies for writing, and where possible, set out the questions in tabular form and _ state name. 3. When diagrams to be returned or a query. of a nature that is not suitable for discussion in the columns is sent, include a stamped and addressed envelope. . 4. In general we cannot undertake to discriminate between competitive trade apparatus. 5. If you prefer to use a nom-de-plume keep the same one if writing again. This helps in the entering of the queries for future reference. All letters must be signed with full address. Initials will be used in answering.

‘\ : It is not necessary, but it improves phe selectivity and cuts down the noise evel. te: The 2-R.F. Browning-Drake ocecasions some people a little annoyance as far as neutralising is concerned, but with the addition of parallel-feed and by-pass condensers between ."B plus" R.F. and earth no difficulty should be experienced. The writer has heard several of these sets that have been carefully built. They have been neutralised and give very fine results as far as sensitivity and tone are concerned, Questionable Batteries. HAVE a two-valve all-wave regenera- — tive set, writes "OC. McK." (Dun- | edin). My "A" battery is new, but a 45 volt "B" battery has had ten months’ use. Reception has weakened considerably of..Jate. A: place the 45 volt block and add anothém, io make 90 volts. Examine the — grid condenser and grid leak. If handy to a dealer, have the valves tested. Carefully examine the set for poor contacts between the valves and the sockets for corroded joints or terminals. Inspect the tuning-condenser for a short-circuit. Tetrode Amplification. WISHING to help a beginner with his } first set, I would like you to answer the following questions concerning the Tetrode Crystal Set and Amplifier de--scribed in your issue of August 9.-"Kau-spanka" (Hastings). 1. Would this set give good speaker reception at Hastings of the New Zealand stations?

A.: This is asking rather much of a one-valye set, It give good reception from Wellington and "phone reception of the other stations. 2, Would a screen grid give better results and could this be on 224 volts?-Decidedly not, 8. Would the amplifier for a crystal set from the "Listeners’ Guide" give as good results? A.: The two circuits are identical with the exception of regeneration added to the more recent set, which is the better. 4, Would a galena crystal. be more sensitive to louder reception? A.: Being more sensitive will be slightly louder, but would be very annoying when tuning. 5. There is no earth shown in the diagram, Is a connection to "A-" as advised in the "Record" the only one required ? A.: It has been shown in the lay-out diagram, but unfortunately omitted in the theoretical diagram. Terminal No. 5 should be grounded. 6. Is a tetrode suitable for an R.I. amplifier ?-‘"‘Yes."’ Adding a Stage of A.C. Radio. ¢ A. C.9" (Palmerston North) who owns a three-valve A.C. set, wishes to add another stage as a He does not wish to alter his set. A.: Accompanying is a diagram of such a booster, using the 227 type of valve.

[ts construction should not present any difficulties. Transformer Problem. " D.P." (Southland) asks concerning * power transformers: 1.. Would a rectifier of an "A" battery eliminator rectify high voltage at approximately 400 volts, passing 100 mil-liams.-No. 2. Could a battery charger transformer be reversed to make a _ step-up transformer for transmitting purposes, providing a bank of resistance lamps were placed in series parallel to break down the voltage of the lines to that of the normal transformer secondary? A.: A bright idea certainly, but unfortunately not a very happy one as far as the lighting bill is concerned. By this method, you are running to waste about five times as much electricity as you use. Construct a properly designed step-up transformer as per the "Listeners’ Guide." 8. Which is the best way to rectify the current? A.: For very high voltage two 281’s, or their equivalents. 4. What is the drop occasioned in yoltage by the rectification. A.: With a valve as rectifier, it is very small. Take the difference between plate rating and output rating.

Metres. Secondary. ‘Tickler. 100 to 65 28 10 65 ‘to 45 17 8 380 to 45 9 6 15 to 30 . 44 D