Questions and answers

Radio Record, Volume III, Issue 28, 24 January 1930, Page 26

Questions and answers

Aerial Dimensions. waar should be the height and direc- \ tion of an aerial to get the best results. from all quarters? asks ‘"C.T.G." (Blenheim). A.: The height should be 40ft., although 30 gives ‘good .results. The length should be from 80 to 100 feet long, including the lead-in and ground wire. The direction depends upon the stations ou wish to receive. If it is the New stations an aerial running north and south with a lead-in in the centre; if the Australian stations are preferred, one from north-west to south-east; while the American stations come in well with an aerial running from north-east to south‘west. Crystal and Two-Valve Set, I WISH to construct the crystal. and two-valve amplifier described in "‘AllAbout the All-Hlectric," and would like to know how this compares with a tlireevalve set, . A.: As you near a broadcasting station, you would find that unless you use a wave-trap the three-valve set would be less efficient.than. 2 crystal and twovalve amplifier. If a wave-trap ‘were used, the best results for distance would be obtained from a three-valve set, as -a erystal set is rarely of any use for more than the local stations. _' A.C, Short-wave Problems. | " LG." (Frankton Junction), who * has constructed an A.C. shortwave adaptor, and is not getting the best results, wishes to know the following :- 1. A variable condenser, .00014, could not be procured, would a .0001 do instead. There is very little difference be~tween these. two condensers. The tuning will be slightly altered, but not enough to notice, . 2. A resistance is shown shunted across the tickler. The specifieation states that this must be 0-8,000 ohms. I have been unable to procure one of this value, so am using 0-2,000 ohms., but the results are not satisfactory. We have returned your diagram with an alteration, and for the reaction eontrol a 0-50,000 ohm. resistance should be employed. Smooth reaction cannot be obtained with much less value than this. 8. Could I use a fixed -resistance of 6000 ohms. in series with the variable 2000 ohms. in order te bring the two up to 8000? : A.: This arrangement would not give sufficient variation to allow of smooth regeneration control.

4, For the coils I am following the details given in "Radio Listeners’ Guide.’ Are these alright?-Yes," 7 ‘Another Short-wave Circuit. Hew can I alter the adapter described "in the "Listeners’ Guide" for A.C.? asks "A.H.A." (Avondale). . A.: Connect the cathode of the valveholder’ in the adapter to earth where Ais shown in the diagram. Connect this to the grid and cathode-prongs of a valve base. ‘Che filament wires will run from the valve base directly to the valve, and will in no way be connected to the remainder of the circuit. They should be run in twin-flex. The A.C. plate. connections are as shown for D.O. (A dia--gram of an A.C. valve socket with the connections clearly marked was published on September 20). 2: Can an adapter be prevented from howling? It is commercially made. A.: This is probably no fault of your receiver. ‘Take the adapter back to the ‘dealer who sold it to you, and ask that it be examined or tried out on another receiver. ‘ ‘ . 3. Could the detector valve be neutralised to prevent it howling? A,: A plain detector cannot be neutralised, for this process can take place only when one or more stages of high frequency are used. To. get Greater Volume. I FAVE a screen grid 4-valve receiver, ‘and-I wish to get greater volume. Could I add a stage of untuned radio frequency either before the screen grid or’ following ?-"M.L." (Hastings). A.: The simplest way to improve the sensitivity would be the addition of another audio stage. At present you have the maximum number of transformer coupled stages, so that it would be necessary to use a resistance coupled stage immediately following the detector. Another method would be to use an untuned stage infront of the screen grid valve. This might conveniently be the R.i. booster -deseribed in the "Radio Record’ some time ago. It is possible that trouble would be thcurred by instability. 2. What grid bias would be required ‘for two B405 valves used in push-pull, with. 150- volts on the -plate. Ai: 18 volts is necessary to bias to the mid-point of the curves. _ 2. What is the output of these valves used in push-pull? A.: About 2 watts.

8. Will I get better results if I took the unit out of a commercial speaker and fitted it to a linen diaphragm? A.: It is most unlikely. We do not advise you to tamper with commercial products. : Building the Browning-Drake. I PROPOSE to build a battery model Browning-Drake, and before doing so, would. like to be cleared up on the following points, asks "B. D. Maximum" (Masterton) :- ; 1. Do you the incorporation of the -screen-grid? _A.: No. Many amateurs experience difficulty with this valve, so that if you have not experimented before we: should advise you not to start now. °

2. Do you recommend the use of pushpull audio? If so, would two Pentodes give high amplification? : We advise push-pull amplification, but with two Pentodes. Such.an amplifier can ‘give good results, but rarely in the hands of amateurs. 8. Would it be possible to convert this set later to A.C. operation per medium of A.C. harness, A.C. eliminators, or a power pack, and which would you recommend? A.: Yes, it would be quite simple to convert, as described in "All About the " The use of. the power pack and rewiring would give the best results. *Phones. with an A.C. Set. Fi x4CrLY how can ’phones be used with: an: AG, set? asks "H.D.H." (Nelson). A.: There pret two methods. Remove the 226 valve put one ’phone tip

into the grid terminal and the other to earth or the metal-work of the receiver. The other method giving slightly louder signals is to connect each tip to, the primary of the second audio t#mtformer. This is the side of the transformer nearest the valve. * oe _.. An Eliminator ‘Problem. | AM building the "B" eliminator described in the "Radio. Listeners? Guide," writes "R.H.H." (Lower Hutt), but do not know the values of each vari- | able output resistance. — A.: You would have to give us more particulars before we could state the exact resistances. Your best plan would be to build up the voltage divider as pictured in "All About the All-Electric," page 46. A fixed voltage divider may be . obtained commercially ‘madé.° To obtain grid bias a potentiometer’ 0-3000 -ohms will give ample bias for ‘any valve. 2. How can I wind the filament to ob« tain the centre tap? A.: If you are making up the eliminator as described in the "Listeners’ Guit,?~ you will require 31 turns with a tap Yat 154 turns. If this arrangement causes humming, put a potentiometer of. about 30 ohms across the filament supnly and -ase the sliding contact as_a céntre tap. The centre point would. be that to give minimum hum. Set Unbalanced... FoR some unknown reason, my factorybuilt receiver became unbalanced without any apparent, reason, writes "LC." (Waverley). It. all happened between switching off one day and switching on the next. When I turned the set up to inyestigate, I could find nothing shorting, except a flower petal lying between the aerial and C-. After this the set went well again. Could the petal have been the culprit? . A.: In all probability. There was a certain dampness in the petal and a leakage was occurring, resulting in instability. It is possible, that a small body such as a blob of solder may hay mbecome loose and have short-circuited fy Jf the neutralising system. This wetild ‘have slipped out unnoticed, Audio Instability. I AM using a 5-valve. Neutrodyne with two speakers, writes "B.T." (Green Island). I find the set works well from four valves, but the fifth valve causes the set to howl. A.: Reverse the connections to the last audio transformer, and if this does not »- cure the trouble, try a resistance of about + megohm across the secondary of the last transformer, or a large by-pass condenser (1 mfd. or more) across the speaker terminals. See.that the grid bias. is adjusted properly, It is possible that your eliminator will not deliver sufficie pit eurrent for five valves. If this is the caée, it will be necessary to bias the second audio valve, and if the trouble then does not cease, the radio valves will need biasing, for by this method more current is made available for the last valve. You should have stated the make and the size of your eliminator. © ~'~

Questions and Answers DBADHRS of the "Radio Record". who. are in difficulties about reception or set. construction. are ‘invited to write to our "Questions and Answers" department for help. We particularly wish to assist. those’ who know Tittle about radio, as very often there is some very slight trouble -which spoils completely ‘one’s enjoyment of the programmes. © Correspondents are asked to observe the following courtesies: 1, Write legibly. 2. Make your questions brief and to the point; do not make apologies for writing, and, where possible, tabulate. Do not ask for a reply by pout unless a stamped and addressed envelope is enclosed. Even in these circumstances, we reserve the right to answer any question through our columns. 4, Do not ask us to design’ circuits or send detailed lay-out diagrams; but we can offer advice regarding circuits. 5. Address all technical correspondence: "The Technical Hditor, P.O. Box 1032, Wellington." ~

"RB" Eliminator Trouble. HAVE a "B" battery eliminator which has burned out several valves, writes "S.P." (Wellington). I returned it to the dealer and received another, which did the same thing. A.: If your set worked all right with batteries you may take it for granted that you have either been very unfortunate in securing two defective units or are connecting. it up wrongly. The next time take your amplifier to the dealer and get him to connect it up and get it working before you take the unit away. Aerial Troubles. "BROWNING-DRAKE" (Miramar) . asks the following questions regard~ ing aerials :- 1, cls, there any advantage in having onat the aerial lower than the A.: No, it is the average height above the ground that counts. 2. Is it advisable to use insulated or plain wire? A.: It is immaterial, though enameled wire prevents corroding. 3: What should be the direction of the aerial for outside stations? A.: It depends upon the stations to be received, For the Australian stations use an aerial running north-west to southeast; for the American stations use one running north-east to south-west. c 4, I am troubled with tramcar interferenee, and although I have erected my aerial directly north and south and the lines run east and west I. cannot lessen the noise. A.: You should communicate with the Power Board, as it is clearly a case for them. If they cannot do anything for you, then use the usual remedies: a coun_terpoise earth, a shorter aerial, and even an indoor antenna. © Lessening the coup- _ ling in the antenna circuit will quite frequently increase the selectivity, but will reduce the volume. A loosely-coupled aerial coil consists of a separate coil between the aerial and the earth, which is mechanically separated from the grid coil. Wind a piece of insulation tape around the existing aerial coil and wind as much 24 gauge D.C.C. wire on this as the width of the tape will permit. Secure the ends and carry one to the aerial and one to the earth. Leave the existing earth connection, but disregard the tapping for the aerial now provided. 5. I am using Mullard and Radiotron valves. Do you think Philips would improve the set? — . A.: No. There is practically no difference ‘between first-grade valves such as these. 6. We regret we eannot decipher your writing in this question. The wording appears somewhat vague. Another Aeria] Problem. VE a six-valve commercially-built set, writes ‘Dead-Spot" (Blenheim), and although I have a good aerial running north and south I cannot pick up the American stations. A.: That is nothing unusual. The writer has a still better set, a still better aerial, and yet cannot pick up the Ameriean stations. Very much depends on " your locality and your patience.