Questions and Answers

Radio Record, Volume IV, Issue 40, 17 April 1931, Page 15

Questions and Answers

C 8. (Wellington) : See the series of * articles which ran in the "Radio Re- ! cord" a short time back. They are eall"Breaking into the Amateur Game," and deal very fully with the questions you raise, I HAVE a five-valve Ameriepan set. What is the best combination of valves? A.: Four 221 type and one 605 type high gain power valve. . 2. LI enclose a diagram, which is the most likely to be the detector? A.: The one in the right rear of the ganged condensers. Tap the glass and the detector will produce a ring in the speaker. OOLSTON (Ohristehureh) I have a \ six-valve American set which works excellently when I am at home, but when away, although I appear to have a good installation, I can neither get BYA nor 4YA satisfactorily. A.: This appears to be either the effect -Q2 the locality or the installation, ‘Try ipereased height and -see that all the joins ste in good order, CURIOUS (Ormondyille): I have an aerial 70 feet long. Will it be improved if I increase it to. 200 feet, and how would this affect the reception of Wellington? A.: It would certainly increase signal strength and incidentally noise by lengthening the aerial. ‘This would impair selectivity, but as you are some distance from 2YA the fact that it spreads. that station over an extra degree or so would affect you very little. 2. Should a lightning arrester be on | ‘the inside of a wall "It is not necessary unless the conductor is not waterproof. ACBAR (Parnell): How many turns are required on a short-wave coil to ‘cover the 20 to 30-metre bands, reaction Pr as being controlled by .a differential condenser? I am using a 3in. former. A.: Use 18 gauge wire d.c.c. or d.s.c., ‘and wind three turns through for the sece ondary and four for the tickler. 2. Could I use a rheostat for controlling: the filament current?-Yes. . PUZZLED (Christchurch) : See QR Tee eent "Radio Record," in which the specifications you ask for were published, ROSLYN (Dunedin): Will a Sparrow Hawk pdapter be suitable for my 4: valve 2. I use 90 volts B battery,. Will it be. an advantage’ to increase it; if so, how much? A.: Yes, 138 volts on your last two valves should have a beneficial effect both to volume and tone. 8. I use two 221A’s and two 201A’s, ©an this combination be improved? A.: You could use a high gain power ‘valve in the last stage. It will improve tre, but it may slightly reduce volume. : I G.A: (Wanganui): How can I con- * nect phones to my a.c. set which has provision for an extra speaker? A.: Follow the makers’ instructions and fit phones in the two points provided. You must use a resistance of 500, ohms in. any one of the leads to control the volume. 2.: Is the makers’ combination of valve correct? : Most decidedly yes. The distortion is either to fading or overloading. 3. Is my make of set a good one? A.: Yes, a laboratory report appeared some time back, and you will see from it the test was entirely satisfactory.

GTEP UP (Greymouth): Is it possible to set-up an electric current by means of a Ford coil ; A.: It is possible to do so, but ordinary battery current is transformed into a pulsating eurrent that is.of little use to radio. ‘The scheme is not. practicable." The set to which you refer has not been: tested against the Outspan Five, but we: are sufficiently confident to assert that the "Outspan Five" would be the better: ; 2. I have a National Union 280 valve. Can you give me some data? * = { A.: It is the same as an R.C.A. 280 rectifier. 3. Can I use a telephone type microphone in a low-power transmitter? ' Av: Yes, but it will not be very satisfactory. You need more power than -you contemplate using for ‘phone: work. QGHAKY CITY (Hastings): Will you tell me a suitable method of connecting a microphone to a set? sae. Se

A.: Yes, You need a special microphone transformer and connect as we have shown in the diagram. " (CHARCOAL (Wellington): I have _ 100ft. aerial 25 to 80ft, high, the are only 12ft. from a bank in their centre. How could I improve the aerial? A,: It is certainly. a poor one. could be improved by moving the top mast down nearer the crest of the bank or putting another mast there to keep the aerial well away. You could increase it to a three-wire serial, but this would not be ag effective as the plan just elaborat2 ° rie "oe ‘ F. (Dunedin): I get better tone by *using the first stage audio only. am using two PM6 valves. You should not use two power valves in a circuit such as this. The

first PM6 is saturating your second transformer.and spoiling the quality. Use a general purpose valve. We hene you are biasing the last PM6 correctly. i will require about 9 volts with 90 volts B. Would I get better tone if I changed the 33-1 transformer to a 5-1? A.: No; you -would get a greater stepup but poorer quality. Corns (Dunedin) : Instead of the two moving condensers, both .00035, in the enclosed sketch. can I substitute by a .0005 and a .00025 condenser? A.: It is unwise to depart from the specifications unless you. make adjustments to the number of turns to thé coils eoncerned. You can reduce the capacity of the .0005 to a .00085 by the addition of a .001 fixed: condenser in series with it. For regeneration you .could wind on a few more turns and use .00025. Your circuit is wrong. If you-wish to use a loop antenna, coils B and © (the secondary and reaction) must be the loop itself. You will not get any pick-up by carrying out the plans suggested by your sketch. A two-valve portable set was described in the "Radio Record" last year, and this shows the correct method of obtaining reaction from a loop antenna. H F. (Waikato): I am. having diffi- * culty with the neutralisation of my set. When I reduce the voltage on the rf. valve the set blasts... When it is inereased I cannot carry out the neutralisation. A.: You may find it easier by using a 1 mfd. condenser between B+ hf. and: earth. One of your h.f. valves may be at fault. Try substitution. have my h.f. and 1.f. battery supply ‘feeding through one rheostat.. Would & separate one on the hf. side help neutratisation? : In all probability, yes, but there is need to have a rheostat on the Lf. valves. If they are American type using 5 volts the difficulty can be overcome -by using a fixed resistance of about 10 ohms newer each valve and the negative supDly.: DUNCE (Hawera): Is the enclosed diagram of the L. W: with parallel: lvalves OK.? A.: Yes, but you have omitted the ground connection from -the centre tap of the high voltage winding. hat type of resistance do you ade lyise? | A.: 7 watts dissipation. 3. Will the 280 rectifier be -satisfactory for two 245’s. in. parallel? : Yes, we used the 281 in the sketch only "to show the connections for anyone who wanted particularly to use, that ‘valve. 4, What is the value of the first resist"ance after the centre tap?-2500 ohms. 5. Is a centre tap choke necessary for a full-wave rectifier? : No, although it will give a more silent background than the ordinary, . Are the resistances in the bias chain of grid leak type?-Yes. Can the h.t. voltages be put on_to the valves at the same time as the filament current? As: It would be better to put a- switch in the h.t. supply in order to allow the valves-to heat up first. ote.-Your attention is drawn to the fact that correspondents are limited to three questions except upon payment of one shilling fee. , J. C.8.. (Christchurch) : Could the knife edge ejector described in the 1931 "Guide" be attached to an a.c. set with three tuned circuits and give good results? A.: Yes, but it should not be necessary. A set with three tuned circuits should be quite selective. REGENT (Methven): Oscillation is fierce and ploppy. ‘Tf I turn the de tector back from full the set .suddenly Roes dead, .: Probably the resistande is defective, for there is evidently poor regulation.

You should borrow a voltmeter and try to find out what happens when you turn back: the knob. 2. The set consists of a detector and two audio: stages.. I am using two 201A’s and a Condor PR54. The Condor valve is an ordinary gp. You should: use a high gain power valve of the B605 class. This will give far better tone. You will need more biasing with this valve. probably 15 volts. You might increase the voltage os Condor valve to .9 with good results,