Questions and Answers

Radio Record, Volume II, Issue 42, 3 May 1929, Page 12

Questions and Answers

IN BRIEF. L J,. (Wellington): Your coil would ¢ tune between 12 and 17 metres. Two turns of 18 spaced 3-8 wound upon @ valye base would tune down to 10 me: tres. An Output Choke. 'A JJ. (Auckland) writes:-1. Does *. a choke coil and condenser filter arrangement connected between set and speaker (in order to eliminate D.C. from speaker windings) from the H.T. B battery through short-cireuiting same? A.: Not if. properly connected. . 2, Could you give an approximate idea how long two medium-sized 45y. batteries (90v. total) should last on a standard 5valve neutrodyne set, 5 201A valves {no power valve), set being used on an average of 80 hours per week? A.: That is not easy to say, so much depending on factors difficult to caleulate, probably three months. Try a power walve for quality (171A). 3. A.: When all batteries are connected to set aud the B minus lead is disconnected and connected again, I have several times noticed a very tiny sparkling, although this does not happen every time, but usually does after battery is left connected for a time. The set is completely "dead" when I have noticed this, the speaker not being connected and filaments of valves not lighted. Is this in order? The same sparkling takes place when set is rooting signals if any lead is disconnected, but of course that is usual. A.: The high-tension wires may be short-circuiting, in which. case the battery would run down rapidly. If possible ‘connect a_milliamet:r in the negative "B" lead. If it registers there is current flowing. A bypass condenser may have broken down. If this is not the ease the spark could be caused by condensers changing up. Additions to Crystal and Valve. N’ the "erystal’ and valve with 8 valve performance" would it be possible to use a valve, in place of the crystal, as the detector, also could a radio frequency stage be added to this circuit. If so would you please let me have a diagram of this circuit. A friend of mine using the circuit given by you has logged the main Australian stations on the speaker, and also JOAK, Japan.-H.R.F. (Blenheim). A.: If the erystal were replaced by a valve and another valve added the set would be converted into a three-valve rebeeen gnepeeeeeemesneenseee — _

ive of the usual type. It would Se be the "crystal and valve." If there is a demand for a three-valve set "Pentode" would describe one. Try the booster contributed by a correspondent. The Beverage Aerial. " C.F." (Gisborne) requires full .de- ¢ tails of the Beverage aerial. These have been given in the Listeners’ Guide, but we shall repeat them in brief. About 600 yards. of uninsulated wire, either copper or steel, are required. Hrect this in the usual method, high enough from the ground to be just clear of stock Run’ towards the stations to be receiyed; that is, if- the Australians are required in a north-westerly direction, while ’ if for the Americans.in a north-easterly direction.. Keep well insulated from trees. Connect the distant end, to earth through a potentiometer or other resistance of the order of 400 ohms. The lead-in is brought in in the usual manner, but it is preferable to incorporate a small condenser in series. . The same correspondent asks for details of the "Loveless" aerial, but we cannot get data on this type, and-would appreciate any remarks eoncerning it that any correspondents might have to offer. It was described in "Radio News" of January. ’ "F wish to establish ‘Radio Research Receiving Station’ for short waves. Can I do this without special license?" A: See P. and T, regulations, published in last year’s "Radio. Listeners’ Guide." Under these, the correspondent may experiment as far as receiving goes, so long as he does not energise his antenna and cause disturbance? He is advised to refer to them before commencing. Extracts will be reprinted in the new edition of the "Guide."

ithe "Combination" Receiver. I HAVE constructed the "Combination Set" recently described in the "Radio Record," and have obtained excellent results. When using the crystal and amplifier, 2YA comes in with. great yolume. while clearness and tone are all that could be desired.. When Wellington is

off the air the Australian stations can be picked up with ease, writes "Bug’’, (Wel- ‘ lington).He asks the following questions: When using PM8 as detector, what is the correct plate voltage? A.: Plate voltaga should not exceed that which will cause the valve just to oscillate. Is it possible to make the set into a two-valye receiver, and still retain the erystal and amplifying qualities? A.: Yes, see the article in the latter part of this issue. What is the foreign station below 4QG, Brisbane? A.: Probably JOGK on 280 metres. In conclusion, I wish to express my appreciation of the R.B.C.. and wish you continyed success. Matching Impedances. | . F.B." (Avondale), who has made * Pentode’s Dynamic Cone Speaker, states that it is giving. him fair volume and exceedingly clear reception. He is intending to use it with a push-pull transformer, and asks which would be the better, two 603’s or two 605’s. A.: The 603’s would be the better. I take it from the "Finer Details of Radio," that the impedance. of the moying coil for 603’s in push-pull wonld be 3300, necessitating 1650 turns on the field coil and 65 on the moving coil. Is this correct? A.: The former, No.--the latter, Yes. There is no need to vary the number of turns on the field coil, for when 6 volts are continuously used, 1000. turns will be necessary irrespective of the impedance of the valve. Different From Specifications. "G I." _(Hastings), writes: I intend * building the "Record" short-wave set, and find that the components I bought when in town.last are not right. How many plates shall I take off a .0003 m.f.d. and .00015 m.f.d. to make them correct eapacities. of. .00025 and .0001.

A.: See tables in the "Beginner Corner." Would a 8 megohm leak and .00025 condenser do for detection purposes ?- es, Would 28-gauge D.C.C. or 26 enamelled be the best for the choke?-28 D.C.C, Would 24 D.C.C. do for the ticklers? A.: Yes, but if 28 were used for the choke it could quite well,- and to advantage, be used for the tickler. Low Charging Rate. " D.P." (Dunedin) constructed a * trickle charger, and worked it from a transformer delivering 22 volts. It would pass only about .33 amperes, so he purchased a dry electrolytic rectifier, and found this, although rated at 2 to 2.5 amperes, would pass no more than 4. He asks now if he could use a lamp resistance. instead of a transformer to break down the voltage. A.: The difficulty is that the transformer is not delivering sufficiently high voltage, and should be stepped up much higher. To deliver, say, 60 volts, the lamp resistance would not be suitable. Request for Circuit. A NEW subscriber, "H.A.B." (Helensville) writes: Could you: tell me if you have at any time printed an amplifying circuit, employing either one, or preferably two, valves? A.: "Megohm" described: a two-valve amplifier in our issue Vol. 1, No. 28. It has been slightly altered, and appears with full details in the 1929-80 "Radio Listeners’ Guide." So also appears an excellent one-valve amplifier. "Round the World Two." "G. T.K."' (Wellington) writes: I am ¢ constructing the "Round the

World Two," which was ‘described by "Pentode". in the "Record" dated 4u/+/2, Couid you answer the following quesiions tor me? :- (1) 1 have a variable grid leak 4-10 mcgo., could L nse that instead of loose ones ?-Yes. (4) Could a rheostat be used in place ot the amperite on the audio valve?es. . (3) Could the R.F. choke. be wound with gauge 28 D.C.C., and’ how, many turns extra-would be needed. A.: 28 gauge could be used winding about 120 turns, but there would be the . difficulty of getting them on the testtube. (4) Would it improve reception if a .0UL m.f.d. condenser was placed across the phone jack ?--Not perceptibly. Receiving Lower Wavelengths. "(\ROSLIEY" (Nelson) writes come plaining that he is unable tu log stations on wavelengths of approximate, ly 200 metres and lower, while he is yet ° able.to get Japanese on the higher lengths. He states that his variable con-' densers have very few plates, about 7, though there is nothing to indicate their value, A.: The difficulty, no doubt, lies in the fact that the condensers are unable to tune in the whole broadcast band, and consequently cover only the upper wayelengths. Keplacing these by ..0005, or even .UUU85 mid. condensers, would probably enable the-lower wavelengths to be tuned in. Identifying Stations. . BB." (Otorvhanga): has asked us to * identity two stations on 31 and 28 of his dial respectively. He says: "We heard very faint music, singing, and clapping from the station ot' 31, while trom the station on 28 we heard the announcer suying thut we. had been listening to St. Louis orchestra, and that they were how closing down and wished all good morning without giving code‘call. This was at about 7.30 p.m." A.: Krom a list of stations and dial readings given, we take it that the. stutions are on approximately 270 and -278 metres, In all probability the stations are ZEY Sydney (on 81),,and KMOX, St. Louis, Montana, America (on 28). The fact that he signed off by saying "Good morning" would seem to strengthen the probability that he was Amer-

ican. Making Dry Batteries. " B." (Wadestown) has been very * successful with articles described: in the "Radio Record." He now wants full descriptions so that he may make his own dry batteries, or, rather, fill the cases of his old batteries. A.: It-would not be worth while. There would be difficulty in making the paste to the correct. consistency, and. it is’ very doubtful if the zines would stand a second filling. It is usually the zincs that give out: and becotne .corroded; in fact, it would be very. difficult to remoye.the old paste from the containers without ruining them. Mounting a Speaker. " A N.W." (Glenmore) has purchased * a magnetic cone unit similar in shape to the dynamic cone and asks :- (1) What should he the size of the baffle-30 x 20 x 1 or #}? (2) Should the opening be cut to the exact size of the cone or to the size of ring: of composition or felt which is glued to the cone? A.: To allow of movement of the cone, cut the opening to the greater circle, that is, to the rim of felt, leaving the cone free to move with the rim. ° (3) Is there any particular way of. connecting polarity of these speakers? A.: Unless they: are-specifically marked, no. (4) Is it advisable. to use an output filter when using high voltages? A.:. Yes, use either this or an output transformer, . . Capacity of Battery. OULD you give me the capacity of . the B battery accumulator described in the "Radio Record" of April 13, 19287 4