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WIRELESS NEWS.

EXPERIMENTERS' NOTES.

A FEW HINTS FOR THOSE WHO

BUILD THEIR OWN

(SrKCJALLT WRITTEN" FOR "THK TRESS. )

'.By "Ekctra.")

Actual measurements show that the plate current roquirctl for the operation of a five-tubo neutrodyne. receiver varies from a minimum of 10 milliamperes in tha best sets to a maximum of about 30 milliamperes, ami that the. average fivetube neutrodyne requires nfbout 22 milliamperes. The secret of reducing the B battery current is in the C battery. The insertion of a C battery in the audio-frequency circuit of a neutrodyne will not only reduce the pla«e current, but also improve the quality of reproduction. This is easily accomplished by connecting the two filament terminals of the amplifying transformers together and connecting the negative terminal of a C battery to the wire joining the transformers. The positive terminal of the C battery is connected with the negative of the A battery. If it is desired to use a C battery on the Tadiofrequene.y tube, this may also be accomplished by connecting the filament terminals of the radio-frequency transformer to the negative of thte C battery. The use of a C battery in a radio-fre-quency circuit will merely reduce tho plate current required by these tubes. It will not affect either the distance that the set is capable of receiving or the quality of reproduction. It will be found, however, that a neutrodyne with a C battery in the radio-frequency circuit is very much moro difficult to balance than without this G battery.

i Covering Short Wave Bands. Some experimenters are desirous of having a receiving set which will respond only to a very narrow band of wave-lengths, sueh as 75 to 85 meters, or 100 to 110 meters. Such a receiver may easily be constructed by building a coil and shunting a fixed condenser across it, and then connecting a variable condenser across only a portion of the coil. Tho fixed condenser and the coil should bo tuned to the minimum wave-length the set is to receive. Then, by connecting tho variable condenser across a fractional part of the coil, tho desired wave band will be spread over tho entire condenser dial. Protecting Filaments. Many fans insert a ten-watt lamp in series with their negativo B battery lead to prevent tubes from being burnt out when tho B battery is accidentally connected across their filament terminal. This idea works out to perfection, provided a condenser having a capacity of 1 m.f.d. is connected across the lamp. If the condenser is omitted, an audiofrequency howl is apt to-be heard. A Variable Eesistance. Those who are in need of a variablo resistance of about 15,000 ohms, may easily construct one from the following parts: One two-ounce wide-mouthed bottle, a rubber cork, two ounces of distilled water, pinch of salt, two needles about three inches long. To construct the resistance, drill two holes in the rubber cork about one-half inch apart, and force the needles through these holes. Fill the bpttle with distilled water and insert the cork. The needles serve as the terminals of the resistance, and the value of resistance may be varied 'by adding salt to reduce the resistance, or by removing part of the water to increase the resistance. The variable resistance described in< the above paragraph will be found very useful in a B battery eliminatdr, where it may be employed to reduce the voltage y for the detector tube. Measuring Voltages. Voltmeters used for the purpose of measuring the voltage obtainable from a B battery are usually of no practical value, unless they are of special design. The current which the average B battery eliminator is capable of supplying- is so limited that the average voltmeter; which i consumes about ten milliamperes at full scale

reading, would cause a voltaic drop of from 10 to oO volts. Readings of this kind would be very misleading, to say the least. If it is'desired to measure the voltage of a B battery eliminator, a very high resistance voltmeter, which should not consume much more than one millianipere at full scale reading, should be used. Use a Battery Switch. In sets which employ dry cells as their source of filament voltage, even a very high resistance connected in shunt with the filament terminals will cause the batteries to run down much faster than usual. Because of this fact, it is advisable to connect a switch in series with tho battery wires running to tho set and to open this switch when the set is not in use. The value of this suggestion can be better appreciated when it is known that actual tests show that in many cases when the tubes are turned off with the rheostats, tho resislance of the rheostats'- insulation is so low that an appreciable amount of current can pass through tho filament circuit. Measurements made with a home-con-structed set showed that, in one instance at least, the current which was lost in this way was as high as two milliamperes per tube. B-BATTERY SUBSTITUTE. A B battery eliminator with sufficient output to supply the plate current for a two or three* tube receiver may easily bo constructed from parts that will bo found in the average experimenter's laboratory. An ordinary audio frequency amplifying transformor may be used for the step-up transformer to supply the high voltage to the rectifier tube. A bell-ringing* transformer is used to supply the filament current. Tho secondary winding of another amplifying transformer is used.as a choke coil and four 1 mfd filter condensers are connected across the lino on either side of the choke coil, to complete tho filter. A standard TV '2OIA tube is used as the rectifier, with a !20olun resistance in series with the filament to regulate tho voltage. A variable resistance, such as is described elsewhere in this column is used to regulate the detector voltage, and the standard wiring diagram for a B battery eliminator is employed. A doTico such as this will give an output of approximately 100 volts, with sufficient current, to supply a three tube receiver. It is not recommended for a five-tubo set, however, as tho parts will become overheated and probably burn out. NOTES ON BROADCASTING. The bed-timo stories transmitted from 3AC last Tuesday were certainly a success, tho modulation being good and tho voice reproducing very clearly. If I may suggest it, I think tho stories could be told just a little moro slowly for tho benefit of distant listeners-m and> those using one-valve sets Th<?se bed-time stories aro given every Tuesday and .Friday, 6.45 to 7.15 p.m. Other items of interest transmitted from this station are: Weather report, T>algety's market report, and tho correct time. Thoso items, of-course, are mainly for out-of-town listeners-in. Mr J. W. Wobb. of Colombo street, Sydenham, has undertaken to arrange programmes, and anyone who is willing to supply musical items should ge£ in touch with him.

Verv shortly owners of radio sets who are licensed will have to voto for the representative they favour for the New Zealand Broadcasting Board. Hie three nominees are Mr Salt, of Auckland, Mr Hollands, of "Wellington, and Mr L. Slade, of Christehurch.

One still hears all sorts of rumours from Wellington regarding; the New Zealand Broadcasting Company, the last being that an agreement with the Government had been reached. Let us hope it is so, for everybody has waited long enough. CORRESPONDENCE. "F.L."—'lt seems almost impossible for me to obtain a good ground connexion. Almost everything in sigh* has been tried with no avail. Tba oarth in the vicinity of my house i» chiefly sand. Is it possible to obtain a good ground connexion in soil of this land? If not, what is a person to do? Answer—Sandy soil offers a high resistance to the passage of an electrical current and for this reason it is difficult to obtain a good connexion to the earth. The best remedy is the use of a counterpoise. A counterpoise is nothing more than a number of wins suspended directly under the receiving aerial. It should consist of at least as many wires as are used in the aerial and may be supported from five to ten feet above the earth.

Permanent link to this item
Hononga pūmau ki tēnei tūemi

https://paperspast.natlib.govt.nz/newspapers/CHP19250704.2.64

Bibliographic details
Ngā taipitopito pukapuka

Press, Volume LXI, Issue 18425, 4 July 1925, Page 11

Word count
Tapeke kupu
1,369

WIRELESS NEWS. Press, Volume LXI, Issue 18425, 4 July 1925, Page 11

WIRELESS NEWS. Press, Volume LXI, Issue 18425, 4 July 1925, Page 11

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