Questions and Answers

Radio Record, Volume III, Issue 14, 18 October 1929, Page 30

Questions and Answers

In Brief. W. J.P. (Hokitika) --You can obtain a suitable unit from Rodger Importing Company, Christchurch. I, BILTON, Lowburn Ferry, Otago, * would like to hear from a listener who has assembled a 4-valve A.C. Brown-jng-Drake in regard to volume, range, etc. Speaker Terminals, AIKAWA" (Picton) asks the rea- ‘ gon of three terminals on a Baby Grand speaker. He states that he heard an explanation from the Gisborne station one evening, but the critical part was spoiled by static. : A.:-The three terminals consist of a common and an alternate for the positive, which connects with either of two inmpedances so that the speaker impedance may be as nearly matched to that of the last valve as possible. hy does a Ford generator run hot after charging for one or two hours? A.: This is quite usual in this type of generator. On some occasions they get so hot that it is impossible to bear’ the hand on- the body. Obtaining an Earth. ISHING to obtain the best earth pos- ' sible "Harth" asks if the following is quite in order:-I intend running a wire under the aerial, buried about a foot and connected to four Plates, each about six feet under the surface. Is there any need to have the lead wire of each plate to the main earth exactly the same ength and spaced evenly? A.: This will make a good earth. It is, of course, working on the large condenser plan, and the plates may be regarded as eing one over the other. In this matter

optimum capacity of aerial and earth system may be obtained. There is no need to have the spacing‘and the wires exactly the same. ‘The correspondent states that he will have to deviate slightly from his original plan, and take some of the earths to an angle. This will not appreciably affect the working of the set. ‘+ * Coil Design. "W. H." (Otago) finds that he cannot _’** tune the whole of the broadcast band with his .00015. condenser, so asks the specification of a suitable coil. He states that 4QG is the lowest frequency station he can gét. He just misses 2YA. A.: With a small condenser it is impossible to cover the whole broadcast band with one coil. Condensers of this type have the advantage of being able to separate the stations better an the larger type, but it is impossible to construct one coil that, in conjunction with this condenser, will cover the broadcast band. Another will have to be designed. and although the correspondent has not stated the diameter most suitable, we offer the-following. as.a suggestion :-Diameter 2hin.. size of wire 24 gauge DSC, number of turns 100. The lower, fre- quencies may now be covered satisfactorily. Circuit Required. (ouLD you oblige me with the circuit for an Airzone 8 coil tuner? asks "A Radio Enthusiast." A.: It is not clear which you mean, one for'a DC or an AC,set. As soon as possible we shall publish a circuit of a three-coil tuner for an AC set. If you require DC, please write again. ANOTHER correspondent, "G.M." (Christchurch) asks for such a cir-

cuit as the above to work from AO, This correspondent should see the answer to the above, Pentode’s Push-Pull Amplifier. INTEND to construct the push-pull amplifier described by ‘"Pentode," writes E.H.B. (Inglewood) bat am not clear on two or three points. In my set the A+- and B- are connected together, while on another terminal B- ground ana C+. In the push-pull amplifier A-, and OC--. are in the same terminal. What shall I do? A.: Take your A-+ off the B- terminal, and put it on one by itself. Con-

nect A- to B-, C+ and ground. If this is not convenient, disregard the Bto the amplifier. Another Push-Pull Problem. HAYIN G read your article on push-pull, I would like to incorporate it in my set, which uses 201A and a 603 as a power-valve. Can I use another 603 in push-pull? A.: Yes, C603 is the equivalent of 171A, and is quite a good power-valve, especially when used in push-pull. Transformer Problem. L.C. (North Otago) wishes to *construct an amplifier using Ferranti Transformers, but he is at a loss to know the designations of the transformer: to use. A.: Input AF38C, output, if an ordinary speaker OP8C (1 to 1). If for a dynamic cone speaker, which is not provided with an output transformer and yet requires one, an OP4C with a 25 to 1 ratio. If the correspondent wishes at some future date to use a dynamic speaker he could use OP4C and disregard the secondary terminals connecting the plate terminals of

the primary directly to the speaker. ‘This is a method of output filter arrangement now becoming very popular. It is technically known as choke output. ; 2..Can you supply the data a structing an REF choke? va .: As several correspondents. have asked for these details, "Cathode" will devote an article to this subject. See notice published on this page. Circuit: for a Charger. (oULD you supply a circuit for a Tun- , ani bulb? asks "F.G.V." (Masteron A.: Sorry, Mr. "F.G.V.," but this is a trade circuit, and we are unable to publish any details; in fact, you are not allowed to make it up, A and B battery chargers can be obtained for a ridiculously low sum, and it would not be worth while making one. Connecting a Microphone. Hew ean a microphone be connected to an amplifier, asks "S.B." (Taranaki). ; ; A,: There are two types. of phones, having two and three connec tions. ‘The cheaper carbon microphone has. two terminals, one is connected directly to the primary of a suitable transformer, and the other to a small battery to the transformer. Such a transformer has a step-up ratio of 1 to 100 or 1 to 200. Their manufacture would be very laborious, but they could be obtained from certain dealers. Try Hartle and Gray, Auckland. 2. What is the amperage of a 6-volt dynamic speaker? What trickle charger would be hest to use for it? A.: About half an amp., so that an ordinary 3-amp. trickle charger will do the job. If there is any hum, shunt it with an electrolytic condenser, or one with ‘a very high capacity. | 8. What B battery do they work best with? ° A.: As high as possible. Very satisfactory results can be obtained from 135 volts upwards, but for power amplification 250 at least should be used with the UX245 type of valve. 4. Which will give the more volume, C603 or BG605? ; A.:- The latter, but: it will not & to handle the same undistorted out}*it as C608.. Unless used in push-pull, 605 is not suitable for a power amplifier. Connecting Audio Transformers. wrt are the correct connections to . the audio transformers of a neutrodyne set?-T.H. (Houghton, Bay). A.: The terminals’ marked "G" on each transformer are connected directly with the grid of the following valve. GB is connected ‘with a grid bias negative of suitable value, or, if this is not provided, to F-. Of the primaries P is connected to the plate of the preceding value. and B is connected with the power supply in the case of both transformers. _ In the first case, it should be about 406° volts, in the second, about 90. ' . I have found that amperites are uxnecessary with Philips valves. A.: Yes, . naturally. Amperites are merely resistances, designed to eut down the. voltage from a- 6-volt battery to 5 volts for various American valves. Where the. voltage of the value is 6, and is used with. a 6-volt battery, a resistance is obviously unnecessary. A simi-

Questions and Answers UR Question and Answers Depart-’ ment 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 Editor, "Radio Record," Box 1082, Wellington. 2. Be brief, do not make apologies for writing, and where possible, set out the questions in tabular form and _ state name. 8. 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 wil] be used in answering. ~

@:.: state of ‘affairs exists with the 4-volt ‘battery used on a 4-volt valve, ‘8. Why do some sets require much less grid bias than others? A.: This depends on the last valve, which does not always adhere to the maker’s curves, "Cannot get Distance. Wwax ean we not get past Australia? asks "O.1." (Motueka). We have a five-valye Neutrodyne with batteries al- * most new. One and a half volts are applied to the valves. The earth and the aerial are both good. : A.: The valves in ‘your Neutrodyne should be supplied with 5 volts on’ the filament, but we think you must have made some error, for you certainly could not get Australia with 14 volts on the filament. Getting past Australia is not particularly easy, especially with a Neutrodyneé of only five valves. ‘There is no method of getting Japan and ica "other than good aerials and earths, fully charged batteries, and good eomponents. If your set has been in use for some considerable time, you should have the valves tested. Origin of Morse. T is the origin of the Morse received sometimes. with. station 2FC,’ and at times on 220 and 550 metres, A.: This is a question of no mean ' dimensions. Morse may originate from a hundred and one difference sources, and unless you or a friend can identify the source we cannot help you. Matching the 245. HOw many turns should I put on my moving coil loudspeaker to match the impedance of the new 245 power valve? . A.: The impedance of this valve is 2000 ohms., so that. 70 turns will have to be put on. . , 2. Will thé output transformer carry the current? A.: Yes, OP4C will carry the current from this valve. It is doubtful if cheap transformers would. The writer has been using the Pilot transformers for some considerable time with a pair of these valves. There has been no tendency. to choke, and no semblance of a burn-out. Short-wave Adapter. We have a screen grid 7-valve all-elec-tric set, writes T.N, (Nelson). Is it possible to use’ the screen grid_short- . Wave adapter described if the ‘‘Listeners’ Guide"? : A.: Yes, quite. It will required, how- . ever, the use of batteries. All electric short-wave adapters are possible, and when we get the opportunity wé shall deseribe one in the "Radio Record." An adapter used with an electric set connects just as with a DC set. ‘ , 2. Would it be better to use the dynamic speaker supplied with the set, or earphones? A.: For use with the adapter alone use ’ the phones, but when the amplifier is attrehed it will, be necessary to use the spemker. If telephones are ysed after ‘an amplifier noise is amplified to a very great extent. This spoils the reproduction, and makes listening unpleasant. Coils for the All-Electric. WwH: eannot low lost coils be used for the AO Browning-Drake? asks "New Chum" (Tauranga). . A.: We remarked in the article that eoils that were previously used in a ‘Browning-Drake were quite suitable. This includes space wound coils. These are more efficient, but with the efficiency of modern valves and transformers, one ean afford to lose a little efficiency on the coils. For this reason, they are made as simply as possible. 2. I read in the "Record" ‘that the B.-D. was notoriously unselective. Is this a printer’s error? A.: No. The Browning-Drake as described is certainly a little unselective, but thie lack of selectivity will not wotry those people who are away from a powerful station. Selectivity is gained by a Joose coupling in the aerial coil, and

more than one stage of radio: frequency. 'Fhis type of Browning-Drake has neither of these attributes, though the introduction of a fixed condenser in the aefial lead will quite frequently sharpen _selectiyity, and with certain coils increase the volume. . R.F. Booster. "QCREEN GRID" (Ngaio) complains that he cannot prevent feedback when using a §.G.R.F. booster. This is unshielded, ,and situated some’ 10 inches from the set. A.: Shield grid boosters are not always satisfactory. The. trouble occasioned by feedback is very gréat. The corregspondent should have some shielding arranging it so that the plate wire from the. top of the valve is entirely shielded from the grid. The lead should be encased in lead, and this grounded. The best arrangement is the valve built in the set, and shielded from the coil as described. xy