Questions and Answers

Radio Record, Volume VI, Issue 42, 28 April 1933, Page 21

Questions and Answers

f "J ESTER" (Dunedin): What do you consider to be the best one-valve cireuit to receive the N.Z. stations on headphones? A: The revised version of the "Sparrow Hawk One," which will appear in the 1938 "Radio Guide," which will be out in a month’s time. The approximate cost with batteries should be less than £4, DW. (Hamilton): Can you recommend the.fitting of a power-line filter as a means of noise suppression? A: Power-line interference was discussed fairly fully in the "Radio Record" of February 21, 1930. It will be dealt with again in the "Radio Times." It is quite feasible to cut out a great deal of the noise with one of these filters. 2: It has been suggested to me that if I used an armoured lead-in ani earthed the cover, it would cut out a gr@it deal of the noise. A: It is often successful, 3:.Is it possible to pick up Morse from the main telepgraphic wires which pass the front gate? A: It is unlikely that this would bappen, though it is possible that radiution may take place. [-4-4. (Wanganui): Is it possible ro use a pick-up for making records by connecting it to the speaker terminals ?-No. "JKATHODE" (Westport) : Will 2244 Valves do for the "Cathode Su- _+ per ?"-Yes, \ 2.: What is the plate and screen currents of these valves? A: At 250 volts, as an amplifier, with -l meg. load: resistance, the current is .5 mas, and the screen current negligible. Used as an amplifier in an ordinary circuit the plate current is 4 mils. and the screen current 1 milliamp. 2 87W: Your best plan will be to take a piece of the wire and send it to a dealer and ask him to replace it with sufficient length to rewind. The iron we described was a low voltage soldering iron and would not be suitable for your use. It was published in the 1930 "Radio Guide." R. X (Wellington): Pitch is run‘ning out from underneath my transformer. A: There may be a short circuit, possibyy in the primary. If you do not hate sufficient skill to do so yourself, take it into a dealer, and have it pulled

down, and the defective wiring attended to. 2: Does a circuit which has silent tuning have any effect upon dx? A: Generally these sets are less sensitive than the average, although some of them have a control by which the silent tuning feature can be cut out. 8: Below 1400 k.c. stations can only be tuned in to a whisper with the volume full out. Above 1400 k.c. the yolume is much greater. A: It seems as though your set has been lined up for a frequency above 1400 k.c. You could get over this difficulty by having it relined at the lower frequencies, but you may lose sensitivity on the upper band as.a result. 18 HB: I have had hand-capacity-trouble with the "Outspan Two." A: Surely you are making a mistake? The "OutSpan Two" was a broadcast set. 2: Please supply me with the address of a reliable firm of Patent agents. A: Henry. Hughes, Ltd., 2nd. Floor, D.I.C. Building, Wellington. } "PUZZLED" (Livingstone): The amperage of a car dynamo remains constant at and after a certain engine speed. Does the voltage also remain constant? A: Yes, providing the load remains constant. 2: When switching off a dynamo while running is it necessary to have the switch complete in the second circuit, before the first circuit is broken? A: It is not necessary to have a coinplete circuit for the dynamo, but it must be switched to the "off" position before the battery is removed. 3: If I switch the dynamo off into another circuit would it be necessary to include a resistance in the circuit, or would it do to short-circuit the dynamo A: You cannot short-circuit the dynamo without burning out the winding. Leave it on open circuit if you wish. FrM. (Palmerston North) : R1 must be connected to a tapping on R38. R2 is unnecessary. Values: Ri, 500,000 ohms; R3 depends on the plate current circut you are requiring. This is how to work it out: Add together the plate currents of all the valves in use. We cannot do this for you because we :do not know what battery voltage you are using, and you are not certain of the valves. However, you could find this out quite easily.. You know the amount of bias you want to put on from the second valve, so, by Ohm's H , Law R = -- 1 | you can compute the value of R383, R2 will be tapped in at R8 in a position to | give-appropriate bias for the audio valve. You can use the same formula, I remaining constant, but EB in this case will be the bias of the audio valve. This will give the number of ohms between A- and the tapping. . 1884: I wish to construct a ghortwave adaptor to plug into one of the valve sockets of my commercial set, using a detector valve in the adaptor circuit. Has a suitable circuit been

published in the "R.R." or ‘Times?’ A: A suitable circuit was published in the 1980 "Radio Guide," but this has not been brought up-to-date. Generally a.c. adaptors, unless very carefully made, are unsatisfactory. Your~ best plan would be to build a battery set such as that described in the last issue of the "Radio Times." You would be certain of results, and the battery cost would be very small indeed.