Questions and Answers

Radio Record, Volume V, Issue 43, 6 May 1932, Page 30

Questions and Answers

N D. (Auckland): I wish to make a * crystal set capable of separating 1ZR and 1YA, but the "Rejecta Two" is not suitable. "G.O." in July 31st issue, 1931, appears to be to receive most of the stations. Will his circuit.do for me? A.: You are setting up a fair problem to discriminate between 1ZR and 1YA, and yet maintain the strength of both. A crystal set can be made to separate any station by decreasing the coupling between the primary and secondary coils. If the "Rejecta Two" will not separate them, we do not know what will, if you wish to keep your circuit simple. Of course you can make it complicated, but then by doing so you will probably lose volume. If you write in in the usual way, "G.O.’s" address will be sent you. 2. Is there such a thing as a shortwave crystal set? A.:. Some crystal sets with a shortwave: coil will receive very powerful nearly S.W. stations, but as there are very few sufficiently close, it is not much good experimenting in that direction. 3. Where can I obtain the diagram of a full-wave crystal set? A,: In the 1929 "Radio Guide." D154 (Matamata): I have a sevenvalve super het. and below 850 k.e, the set is insensitive. I am using a 600foot aerial, 18 feet high. . ‘A.: Your aerial is neither one thing nor the other-it is not a Beverage, and it is not the ordinary aerial. Before you can get satisfaction you had better get it either one or the other. Try this: a 30-40 foot aerial, 80 feet long, and see how results are then. This is your problem: Every aerial has its natural wavelength, below which it will not tune satisfactorily. Yours evidently will not go below 850 k.c. satisfactorily, so by shortening it you would decrease its natural wavelength and so bring in the station you require. ‘The alternative would be to increase the length to 850 feet and earth the distant end through a 750 ohms resistance. ° 2. Why is it necessary to ground a Beverage through a resistance? A.: Explaining why is beyond the scope of "Q. & A." It is sufficient to say that it is necessary for one end to be grounded through a resistance approximately equal to the surge impedance o the aerial, which, for most Beverage aerials is between 200-800 ohms. K H. (Gisborne): What is the correct * value of bias resistance for a 224? A.: We are rather unfortunate about the 224, There has been a series of printer’s errors. Let us hope they get it right this time. The correct value of this resistance is 400 or 450 ohms. 2 Is $242 an American valve?-An exact American equivalent. Which way should "B-+-" and "P" be connected with the primary? A.: It is usual, as shown in the theoretical diagram to connect "B+-" to the bottom, and "P" to the top, but it pays to change them round sometimes, as is shown in the layout sketch. ' -4. What resistance would I need to make a four-volt transformer deliver 2.5 volts? The total current passing being 5.75 amps. A.: £5 of an ohm. Your best plan would be to obtain three yards of No. 16 Pureka wire and wind this round a nonconductor. Place an a.c. voltmeter across the valve sockets, put all your valves in, and connect all the resistors in the circuit, Then take off sufficient of the wire to get the exact 2.5 volts, but remember, always have all your valves in the sockets. If you pull one out the voltage will jump and probably ruin your set of valves. Really the best plan would be to get a new transformer. If you are not

=Ty using all the valves at 2.5 volts, add together the current taken by the filament of those you are,.and then divide this into 1.5. This will give you the number of ohms you require in the circuit, and from the table of resistance wire, published in the 1980 "Radio Guide,’ you will find out the quantity of wire needed. 5. Where is the centre tap of the potentiometer connected in the "Radio Gram" circuit? A.: You need not worry about the centre tap if you wish to use the exact circuit as is shown in the Call Book. However, you can connect one side of this to earth and connect the moving arm to the screen of the valve. If you ‘have not yet bought your parts, you had better make the value 250,000 ohms if you wish to use this slightly more modern circuit. 6. I find my pushpull input transformes has broken down. Could I use a 3-1) audio transformer with resistances across the primary? What ohm resistor should I use? Yes, you could use a 3-1 audio transformer, but the resistances, which should be about + megohm, should be exactly the same. Ask your dealer to match them carefully before you put them in the circuit. 7. I have two condensers-first a seventeen plate, and next, a forty-three. 'rom both condensers I-have removed all but seven plates. What capacity are they now ? A.: It all depends upon the size of the condenser. You have not told us the size of the plates, but presuming they are fairly large, as they appear to be, the eapacity should be about .00015, L D.Y. (Auckland) : I wish to build a * four-valve batiery set, but cannot get the details I. want. Can you supply diagrams? A.: We think you will find the very circuit you want in the current issue of the "Radio Times," a four-valve battery set which is not only cheap to build, but economical to operate. C BH. (Auckland): My set just fails to * bring in KFI if there is any static about. Would it be possible to add another stage? A.: No, unless your set is an exceptional one, and it is built to take an extra stage. Some are. — (Wellington): We have not tested the set you mention. E B. (Clive): What is the best book * to give a list of all the stations that are to be heard-both long and shortwave? oo A.: A very comprehensive is published in the "Radio Guide," but fuller lists will be published from month to month in the "Radio Times." Our next list in the "Times" will be a complete ct of the American stations. } THIRD GRID (Greymouth): Why is push-pull so widely used for receiver output stages and for transmitting while parallel is hardly ever used? A.: The push-pull circuit gives a greater output. You can double the output by paralleling values, but you can practically treble it by using the valves in push-pull. . 2. When a stage ‘of push-pull is double biased how will the output be as compared with a single valve with the normal bias? A.: About three times as great. Yes, push-pull increases the percentage of anode watts consumption, which may be utilised as undistorted output. Would changing from 90 volts anode potential'in a s.g. detector to 120125 make a noticeable improvement in either s.w. or b.c.? A.: This is rather hard to say. According to theory, yes, and we have found that to be the, case.