Questions and Answers

Radio Record, Volume VI, Issue 1, 15 July 1932, Page 10

Questions and Answers

"QPARKS" (Rotorua)-Would yon supply primary, secondary and tickJer for coils wound on 2}in. former, tunel by a .00025 condenser, to suit A442 and A415? A.: Using 30-gauge d.s.c, wire, 90 turns on both secondaries, primary of intervalve transformer, 40-45 turns. Reaction, 40 turns. 2. Reaction is smooth with the leak shunted across the condenser. When it is connected to "F-+-" regeneration is violent. HWven the voltage is reduced to 224. A.: With the leak shunted across the condenser does the grid return go to "W497? Tf it does, there is fundamentally no difference in the circuit, The best plan would be to take the grid return. to a potentiometer between "F-+-" and "WNADA" (Napier): I have a three-valve set, Could I gang the condensers? lf so, what value trimmers would I use? A.: You ean obtain modern three-gang tandem condensers, with trimmers already in position. You could quite easily gang them, You could use connectors with balancing condensers, about .00005 mfd. capacity or less, shunted across each. 2. When I am tuning in a distant stution my set is inclined to whistle. Does: this go out on the air to be picked up by other sets? A.: In all probability, yes. The set should be very carefully balanced. Apparently it.is slightly out. ~ 8. If I use PM 6 in the first stage of the r.f. I get much better results, but the set whistles more. I am using an AB. | eliminator. A.: It whistles.more because it is not balanced for the PM 6, which by the way should not. be -used in that stage, as it is a power valve and it draws more current than a more suitable valve would do. Balance or have your set balanced. "QHORT-WAVH" (Temuka): During the summer 2ZX came in at great volume, I have not been able to pick up the earrier lately. Is. this merely a vagary. of radio?-Yes. 2. Will the set give better results with a large reaction coil and small movement of the reaction condenser, or a small reaetion coil, with a eorrespondingly large movement of the condenser? A.: The smaller the coi] the better Generally there is optimum value. " AIL" (Christchurch): What sixyolt valves should I use in the commercial battery set mentioned? A.: Vhe set was designed for 201A valyes, but the modern 221 could be used in their stead. However, use A415 detector, A409 in the first and second stages, and in the first audio. If you do not put grid bias on the last valve, use A409, but the best plan is to break the connection between the grid return of the last valve and take the return instead to a suitable negative tapping. F W. (Auckland): Full particulars of ¢ the Browning Drake appeared in the 1929 "Radio Guide." copies of which are still obtainable. ANGI (Palmerston North): I recently fitted phones to my a.c. set, but reception is not nearly so loud as it is on the speaker. ; A.: Make quite certain that you have connected the phones in the plate circuit of the pushpull valves. and not in the grid circuit. It is quite probable this is what you are doing You must get louder signals if you use the phones in the proper circuit and have made all the other connections correctly. Do not alter the capacity of the blocking condensers; ’

2. What eapacity variable condenser is needed for 2 tone control across 1714 valves in pushpull? : : You can. use a 50,000 ohms resistance in series with a .06. mfd, condenser. ’ M.T.P, (Gisborne) : How can one find * the frequency in kilocycles from the wavelength in metres? A.: Divide the metres into 300,000. 2. Would an eight-valve super-het. be: workable in a city such as London or Birmingham? : A.: It would be very noisy, but only when you were reaching out for distant stations, It woulkl be quite satisfactory on stations-that come in strongly. 3. When a set that is adjusted: to 230 volts is taken to a place where the power is higher or lower, what part of the set should. be altered? A.: The primary of the power transformer. It is not easy to make an adjustment to this, however. "NFUSIC LOVER" (Pahiatua): I notice that all: new radio sets‘ are equipped with dynamic speakers, I-have a good set equipped with a magnetic speaker, the tone of which, on some occasions, is exquisite and. remarkably free from background noises, Has this type of speaker many serious disadvantages? A.: The dynamic speaker has a wider range of frequency and is capable of handling much greater volume than the magnetic speaker, such ‘as the type you are using. Again, the field of the dynamic speaker makes a very. convenient choke in the smoothing system of the rectifier. These factors account for its universal adoption. in modern receivers. > "TYAMS" (Auckland): What is the size’ of wire and number of turns for the 80-metre band for the Cossor Melodymaker? A.: In series with the .0005 mfd. condensers place .00025 mfd. fixed con- . densers, and then on a 3in, former, with 26 gauge enamelled wire, wind 19 turns for the secondary, 24 for the reaction (of 30 d.s.c.), and 7 turns for the primary, We are not certain if your partieylar set employs a primary coil for the detector stage. 1f not, use only the secondary and the tickler. For the first stage use the primary and the secondary. J M.S. (Morrinsyille): Your best plan, 7+ if you are studying for an amateur transmitter’s certificate, would be to obtain Wandy’s Handbook, in which is published. a great deal of information of use (Continued on page 22.) ~

Information Coupon (To be used with all requests for information.) Name of set Model Name ) SPP OSHSESESAAOSHESESOEHEDES Address ear temwsasreneerees PeCOeeeoecorosegeeosessor: PISS ATSHASSOOSS HS SSOSSESESSOOOHSEOROOLS SPT EPHH HS EETESTESOSSSESSESSHSSSEBOESEL® Nom de plume .... se To be kept In Please Note:(1) Be specific and brief, tabu lating, if possible. (2) Write legibly. and on one side of the paper. (3) We do not design circuits (3) We do not design circuits, (4) Limit three questions. unless 1/- is enclosed. (5) Postal queries limit three questions. Enclose stamped envelope and 1/- fee. ~

Questions and Answers

{Continued from page 10.) to the amateur transmitter. Included is the code and the amateur abbreviations. We cannot send information by post unless the rules respecting this are complied with. "TAOXY" (Hokitika): Can a erystal set be converted so that it can be used on the West Coast to pick up 2YA? A.: No, but some of the parts may be used to make a valve set which would be able to pick up 2YA. 2. What time is considered necessary for daylight reception of the Australian stations for the DX Competition? A ruling on that point was given recently in the "Radio Times." 38. A certificate is awarded to the best log in each district. At the present time Westland and Nelson are in one, but it is probable that in the very near / future Westland will ‘have a separate — distriet, | EK D. (Hastings): The intermediata * transformers will tune to the correct. frequencies. 2. We are sorry that short article was left out, We are reproducing the diagram and an account in this issue. "PULWwoob" (Papakura) : You cannot go on the broadcast band without making special application to the Post and Telegraph Department. You will have to considerably modify your tuning arrangements, and your best plan would be to get in touch with the secretary of | _the N.Z, Amateur Transmitters’ Associa-_ tion, who will give you directions as to how to go about it. [-X1884 (Te Kuiti): I wish to construct an all-electric local receiver, published in the "Radio Guide, 1931." Gould you give me further details? A.: It would be necessary to re-design the circuit in order to incorporate the amendments you wish, and giving you such particulars are beyond the scope of Q. and A. Why not build the Loftin White, or a somewhat similar receiver, which has been described fully? "RJOVICE" (Grey Lynn): How can 1 improve my earth? I have a solid piece of copper half an inch thick and oft. Gin. long, driven into the ground. which is very wet. The lead-in is wound ground for about three inches and sweated on. With the earth connected to the set reception is not us loud) as without the earth.. ; A.: In the ease of an electric set is sometimes the case. You have a good earth, and would not get better results by trying to improve it.

CORRESPONDENT asks how grid-bias might be obtained without the use: of separate grid-bias batteries. It is possible by utilising a system similar to that employed in a.c. cir. cuits and of course B+ supply is robbed of the grid bias exactly as it is in the ae. circuit, so after all it cannot be termed free grid bias, Refer to the cireuit in figure 1, Notice that if 120 volts is used across B+ and B-, and 9 across C+ C-, this really amounts to a total of 129 across B-+- C-. with A- tapped in _at a convenient point. If the two batteries are done away with, and one complete battery of 129 volts is used, the same thing is accomplished. the grid bias depending upon the position of the "A" battery tap. thus we are obtaining what is wrongly called "free" grid-bias. It is wrong, because while there is no separate battery for the grid-bias, we still have bias on the valves, and it is at the expense of the potential on the plate. Figure 1C illustrates the conventional method of connecting the circuit so as to do away with the grid-bias battery. The points indicated as Tap 1, 2, 3, ete, may either be points on B+ batieries

or an eliminator. The grid-bias is always the voltage between the point where the filament circuit is taken into V

the B supply and B-. Observe that there is no connection between A- and B-, as is usual, B- becoming GB-.