Our Crystal Corner

Radio Record, Volume II, Issue 30, 8 February 1929, Page 31

Our Crystal Corner

By

Galena

Dy The Plate Circuit. {t is necessary, now, that the electrons in the. filament. be drawn to the plate of the valye. For this reason the plate must be heated by a flow of electricity, different from that in the filament, to attract the electrons, and to accomplish this a B battery is introduced in the circuit. The positive terminal will go to the other side of the. terminal, while the negative terminal to link the B battery up with the renidindér of the cireuit, will go to the A negative:or A positive. If it goes to the A positive, the B battery voltage will be raised by the amount of voltage on the A battery; thus the-A battery helps the B. This: how. completes the one-valve iplifier. All one-valve. audio ampli‘3 aré more or less the same ag this. These.are the general principles for audio amplification, and it will. be found that.eyen. the most complicated receivers will fall back on these fundamental ptinciples. A diagram showing such a cireuit was given. last week. The Double Grid and Péntode Valves. ITH these types of valves an extra grid or grids is provided. In the ease of the double grid valves the extra grid reduces the plate voltage if necessary and, in-the case of the pentode, helps to boost up the amplification. In the case of the pentode a high plate voltage is still required, but the amplification and. the capacity of the valve is greatly increased. ‘With both these valves an auxiliary terminal is provided on the side, and is "ii be connected diréctly to the B poaitive. This may be done by running straight on to the battery or going to the positive terminal of the speaker. The correspondent wrote in last weék’s issue asking for some details of the use of this valve. They siniply can’t bé given. All is plain and _ straightforward. ; The double grid valve requires only 224 volts on the plate, and is highly suitable for a single stage of amplification such as the average crystal user would like to add. "Galena" has been using such a circuit for some considerable time, and it has given nothing but satisfaction. The valve makes the set no more difficult to operate, and has a very low running cost. If the constructor is willing to provide a high B battery veltage, that fs, approximately 90 volts, he can do littie ’ better than employ the Pentode valve, whieh will give very fine amplification, but with a fairly high running cost. oS: diagram showing the connections sen there are two grids is shown below. The full descriptions-.of such a set appéared in our special issue, but the foregoing remarks will enable any constructor to use one. a

vv As far as the writer can ascertain the Philips A441 is the only valve of the double grid type obtainable in New Zealand. However, of the Pentode valves most of the makers now turn out some very fine products, and anyone constructing. should consult their dealer to find out which he con-. siders to be the. most suitable make for his requirements. , Notes on Assembly .and ‘Operation. TN choosing the amplifier for his erystal set, the beginner has first to take into considertion. his locality. To work an amplifier of too low power in a bad locality does not give

satisfaction, and great care has to be exercised in the selection of the right circuit for the right position. It is well to look around among the other radio enthusiasts in the district, and find out how reception is in that area. This varies from place to place in a most remarkable manner. Reception may be good in one point, while in another, just a few miles away,.it may be. particularly bad, and require another’ valve to bring the erystal output on. to the loudspeaker satisfactorily. Hills and bush have a screening effect, and should be taken into count by the constructor, and if he finds that the output from one valve is not sufficient, he should not be too ready to blame either the circuit or the valve. The Direction of the Aerial. : ANOTHER very important factor to be borne in mind by the constructor is his aerial. The output from a erystal is not particularly great, and everything has to be taken into consideration in order that he may get satisfactory loudspeaker strength from his amplifier. Not the least among these is the aerial. Sereeds have been written about this important part of the receiving apparatus, and reiteration here is both unnecessary and uncalled-for, Reférénce can be made to articles that. hare.an

peared previously in our paper, particularly in the special issue.: There is one outstanding point that the amateur must bear in mind, that igs, the direction of the aerial." One young constructor known to the writer had a very ingenious. aerial. After some effort and the co-operation of all the family, he had raised a conspicuous mast some 40 feet into the air. From this, he ran several wires to his house, which was much higher than the aerial. He had spaced the wires so as to get the maximum effect, but had the aerials so arrangéd that he could receive off one wire if he so desired. He then tested each separately, and found that the loudest reception was received from the one that was running in a direct line to the station. This points out the need for the obtaining of directional effect in erecting the aerial, which should be run, if possible, high in the air towards the station to be received, the lead-in coming from the station end. This. is not always possible, but should be kept in

view by . everyone erecting an aerial to receive from one station only, and crystal reception from any other but the local station is not worth * while, even with an amplifier. The Lay-Out. MANE young constructors with a. busitiess-Hke eye and a businéwslike method of construction, sé¢atch through, and find out "many ‘different. cirenits" of crystal sets and amplifiers, to turn down séveral as "nd good." This is a fatal mistake, as there is really no difference between crystal circuits. Some are more selective than others,. perhaps, but selectivity showta not be the aim of the crystal uéer. in New Zealand. In most cases, especially where an amplifier is being used, the, tréublé is caused thtough defe¢ts in the la¥-ont. There are sevéral important pditts te watch in this respect, and the cone structor should, whéte. possible, fé6lléq the lay-out diagram which dlmodt. ine: variably follows the theoretical dia gram of the sét which he is te cénstruct. In some cases, alteration. ig essential, and in altering, the fol-. lowing points should ‘be borne ‘in mind: 1, Keep the wires going te the: gid, of the valve, short; insulate them well; and keep them off ‘the basé-board, 2. The plate wires are to be képt well away from the grid wires, likewise well insulated, and cléar of any, conductor, 8. Filament wires, and battery leade, may be bunched. Do, not leave. them straggling about-the- set as-one. seés in . so many home-constructed jobs. This is not only unsightly, but is véry Hable to cause short-circuiting. 4. Keep your components wellgrouped together, and many of ‘the: troubles enumerated above will not be> encountered. Besides the set is given a néat and. business-like appearance. 5. Do not-let the coil get. into’ .con-: tact or into proximity ‘with either the condenser, the crystal, .or the wooden base. It should be well. insulated, otherwise the operator will find that his volume is sadly réduced.