Questions and answers

Radio Record, Volume III, Issue 21, 6 December 1929, Page 29

Questions and answers

Aerial Efficiency. AM using a crystal set and a bar amplifier with a 100ft. aerial, approximately 80 feet high, Would reception be proved if I .erected another aerial, so that the two formed the sides of a _triangle haying a 50ft. base, the leads-in being at the apex. ‘The height would in both cases be the same.--A.P.G. (Silverstream ). . __A.: When using a crystal set every addition to aerial efficiency affects materially the oytput of the set so that the improvement should have a beneficial effect upon the strength of your set. Sereen Grid Detector Two. Worn you supply me with the name and issue of the overseas magazine containing the description of the screen grid detector two?--L.H. (Christchurch). A.: It was published in "Popular Radio and Aviation," a journal which is now not in existence. The magazine has been returned to its owner, and as far "as we ean recall the description appear- _ ed in the August, 1928, number. Selecting a Dynamic Speaker. ISHING to purchase a dynamic speaker, F.H. (Westport) asks if one operated directly from the mains can be used with a battery set, and if this is better than the 6-volt type. A.: So long as you have the A.C. power available it matters not how your speaker is energised. The A.C. type of speaker is far the better iinvestment for anyone who has not an accumulator and facilities for charging it. Output Filter Choke. T HAVD two transformer cores, one an ™ _ shape and the other a shell. Which is the better for making an output filter choke? What gauge of wiré and how -much would be required?-A.G.C. (Te Aroha). A.: The EB type marked in your diagram A is the most suitable as it con"tains more iron. Wind on as much 36 gauge wire as possible. If double-cotton covered there is no need for insulation between. the layers. Shortwave Adapter. Wworrp the adapter described in the "Listeners’ Guide" be suitable for use with a screen grid three-valve set, and would it give as good results as "Round the: World two’? A.: Our experience has been that the "Round the World" two has been the most successful short-wave receiver (excluding the screen grids) that yet we have published. The "Round the World" two would give you excellent results, but as a start try -the adapter described in the Guide. .Omit the fixed condenser _between the plate and A+. — . 2.; Can. I use a .0005 and a .0001 condenser instead of .0002 and a .00085?-~ C.B. (Nelson). A.: A .0005 is far too large for shortwave work. The .0001 condenser is to be preferred for tuning, and for this the table on page 108 has been prepared. Nothing larger than a .00025 should be employed for reaction. a

Transformer Design. GOAN you give me the details for building a transformer to deliver 20,000 yolts at 20 milliampres? asks A.J.M. (Kelburn). : A.: We are afraid: that the electric light authorities would. not greet with pleasure any project such as this, When it is understood that a difference of poten- | tial of 20,000 volts will cause-a spark 2in. long some idea of the difficulties in building and the danger attached to such a transformer it can be understood why ‘it would not be permittéd. There is no apparent reason why a_ transformer of these dimensions should be required. Has the correspondent committed an error? 2.: In the AC hand-book there is an article dealing with the construction of a crystal set and amplifier. Two condensers are specified for smoothing. Would not one electrolytic suffice? A.: The usual electrolytic, condenser is suitable only where small differences of potential are employed They can be used successfully only in "A" battery circuits. . A Would-be Experimenter, I HAVE been experimenting to find some other form of detector than valyes are crystals. I happened to stumble across the discovery that iron filings become a conductor of electricity when a magnet is brought near to them. The iron filings are in a glass tube and care must be taken that they are not too tightly or too loosely packed. I have lit lamps by holding a strong magnet a foot away from the tubes. Could this idea be embodied in an adjustable grid leak? The resistance can be controlled by the distance of the magnet from the _ tube. Could this idea be used for an adjustable grid leak?-A.C. (Ohakune). A.: You have not made clear the results of your. experiments. When you speak of iron filings and a system of detection it seems that you have happened across the coherer type of detector which preceded the crystal. We think that you have merely happened across well-known electric and magnetic facts in unusual circumstances. A grid leak of this nature would be very noisy. Silk Diaphragm Speaker. HAVE made up the silk diaphragm speaker without success, writes H.L.D. (Christchurch). In nailing the silk to the frame should it be drawn drum-tight and then painted? I have been doing this and then painting it, but the silk has been slipping. A.: You are right in drawing it drum- tight, before painting, but unless the silk is of good quality "and nailed very evenly all round you will get trouble by splitting. : ‘2QRE Browning Drake. I INTEND building a 2REF Browning Drake, but would like to be cleared up on the following points, writes G.C. (Dunedin) :-- 1. What is the size of former and gauge of wire for chokes for the parallel feed? A.: "Cathode" has dealt very fully with the theory and construction of choke eoils in an article which will in all probability be published in next week’s issue -at the latest it will be published within a fortnight. 2. The receiver has four condenser controls and three rheostats. Could I make

the set one-dial control or lessen the control without losing a great deal of effficiency ? ; A.: The first and second’ RF condensers can very conveniently be ganged, the moving plates in each case being connected together and to earth. One srheostat can be made to control both RF valves, and if valves of the: same voltage as the accumulator are used, there is no need for the vheostat controlling the audio stages. 8. To control the RF stages a variable high resistance of 20,000 ohms should be included in the RF plate supply with a by-pass condenser to B-. Is this a good method of controlling volume? A.: Yes, but the simplest method is to take out all rheostats except the detector and control volume by a variable resistance 0 to 500,000 ohms shunted .across the aerial and earth terminals. ---$---$~$._ yo: --

One-Valve Sct Roguired. LTHOUGH I have been a readsr of your paper for close.on 12 monihs, I have never seen a description of a one valye set. Could you publish one?-_. "New Chum" (Hastings). 7 A.: The combination set was described in the "Radio Record," volume 2, No. 39 (April 12), Small regencrative rece-yers are not looked upon with’ any great favour by the Post and Telegraph Department. Although small, these receive ers are capable of creating a great deal. of interference, and there is always a temptation for the user to endeavour to bring in a station that is really beyond his power. A great deal of annoyance to listeners’ with larger sets who cu, under normal circumstancese, enjoy these stations, is the result. Battcry Charging. WAVE a battery charger with which I charge a 6-volt 30 amp. battery, but hina ---- oo a aenes --

when I connect it to an 80 amp. battery the liquid boils and the charger gets too hot to touch after a few minutes. A.:: There appears to be a short circuit in the 80 amp. hour battery." Treat it as described in the "Radio Listeners’ Guide" 5 | 3 A Legal Point. CAS a shopkeeper sell a second-hand two-valye set which belongs to a private person. He does not have a dealer's license -I'.J.H. (Wellington). _A. No. ‘here is no objection to a man selling his own wireless set, but when anyone sells another man’s set he must have a radio dealer's license, 2. Is there any remedy for a paralysed valve which will oscillate on the local station but not on a distant one? . A. This is an unusual case, and it appears that a new valve is required. Various Points. HAVE been troubled with a dead spot, writes K.A.D. (Wellington), and nave found that when I watered the earth is shifted. 1s this usual ?- 0. 2> How can one tell for certain that the set is not: oscillating? A. Damp the finger and place it. on the aerial terminal. If a."plop" is heard in the speaker the set is oscillating. 2. Is it natural for stations to come He as follow: TZL on 80 degrees, 1YA on 1. A. Your dial is back to front. Hither turn the condenser up the other way, get another dial or put up with it as it is.. 3. I wish to go away for Christmas, taking a*set with me, but leaving one to be operated at home. Do I take out another license for 30/-? A. No. ‘Take out one for the period you will be away. . Getting America. HOULD I be able to get America with a six-valve battery. set?-R.A.S. (Mamakura). . A. Getting America depends very wuch on location and the type of aerial. Th writer has heard several American stations on this type of set using a Beverage aerial, such as was described in the "Radio Listeners’ Guide."

Audio Transformer. HICH transformer should go first, a 5-l-or 34? A. The lower ratio transformer should always go next to the detector valve, the reason being that better quality and sensitivity occurs through matching the impedance of this valve to the transformer. The valve impedance is high, so a high impedance transformer must be used. The 5-1 having a‘lower impedance takes the second place. 2..I am using two shielded transform: ers and when I place my finger on the shielding of. the -first there is a loud howl] in the speaker. What is wrong? Feed-back is- occurring, so earth the core and the case of the transformer. 3. Do two transformers of the same ratio work well in. an amplifier?-Yes. Do you suggest the use of a variable grid leak in a short-wave set ?- "Grid Leak" (Blenheim). A. No, they are inclined to be noisy. Valves Wrongly Placed. I AM using B 605, A 615, © 603, in my radio set. Can you identify the ‘sockets and tell me where the valves should go? A.: It is hard to identify the sockets when the receiver is not a usual one. The radio frequency valve is always agsociated with the coil and condenser. A lead from the coil goes to the grid terminal of the RF socket. ‘The detector is always recognisable by the grid leak and condenser attached to its grid socket. The power valve has its plate connected to the speaker terminal or out put jack. The RF valve (in Philips series) should be A 609. The detector A 615, the power valve B 605, or C 603, with nine volts grid bias. When using the 171 A with 90 volts about 20 volts bias should be applied. 2. I have to constantly adjust the screw of diaphragm of .my speaker to prevent rattle. A.: It appears you are saturating the speaker winding with too much anode current. Increase the grid bias. and try the use of A 609 or its equivalent in the place of B 605 in RF. Stations Mushy. ¥ HAVE an American set, and am using x Continental valves. I have altered the coils and now find that‘the reception from overseas stations is mushy. Can this be altered?

A.: You have made a rather unfortunate mistake of using anything but Ameriean valves in an American set, This affects the neutralising- and quite frequent, ly series filament resisters are used in the circuit to break down a 6-volt battery potential to 5-volt for the American series. You should.revert to American valves and re-neutralise your set. 2. Could I use three blocks of "B" batteries, and two "C" batteries with this set? A.: Yes. The highest potential of 135 volts should be applied only to the last valve. Likewise the highest grid volts 4should go to the grid: return of the power valve. A small.grid potential of about 4% volts should be applied to the second fast valve. " Turns on a Voice ¢ Coil. I HAVE an: English 6-yolt dynamic speaker equippped with a high: resistance speech coil. © This coil has apparently developed a short-circuit and. L am contemplating removing it and winding on an American type ‘low resistance coil for use with a 25-1 step-down trans former. Can you supply me with data re guage of, wire and number of turns? Valves used, Osram P625 A’s in pushpull. Will ‘you also supply data of turns and guage with, P625’s and Phillips B403’s R.K. (Hastings). A: All voice coils wound with 380 S.W.G. enamelled wire. P625A, 65 turns; P625, 105 turns; B408, 75 turns. .