Questions and Answers

Radio Record, Volume III, Issue 4, 9 August 1929, Page 26

Questions and Answers

The Super Set. " ¥." (New Plymouth) has asked for * the diagram of a set that will embody the following points: 1. Full range of A class stations (if posse, others) with rear!» undistorted reception on the loudspeaker. 2. Natural, undistorted tone. 3. Set to use screeagr... i. . Jt feasible, pentode and three electrode valves. 4, Fading, atmospherics, reduced to a minimum without sacrificing tohe, powér and range. «. dc. iving ‘the broadcast bands with ex ace. A.: A set embodying these points has been the aim ox raw euyuleess sylice wireless Was. It is utierly impossible for any set to include all the points enumerated. The. range of stations tp be reeeived is Very uncertain, and very few sets can rely on receiving overseas sta-. tions-at excellent strength under all conditions. Musical reproduction is proceeding at a pace, but it is impossible at the present time to get any apparatus that, will givé a natural undistorted tone. Distortion can in a good amplifier and speaker be reduced to a minimum, but it is ever-present, in some slight form or another, The nearest approach to perfect tro js obtained through the use of a push-pull amplifier with big power valves "w. . a good moving coil loudspeaker, The use of *screengrid and pentode valves is yet only in the experimental stage, and the fate of them is unknown. Atmospherie conditions can in no way be controlled by the radio receiver, When the set is made more sensitive these extraneous noises are increased, A shortwave set with coils to receive the broadcast band, cannot be more than a compromise for reasons that have been explained in past numbers. a ot ss ° P

Tf "HF." wishes to construct his own receiver, he will find the two R.F. Brown-ing-Drake with a push-pull amplifier to be one of the best ‘he can build. This, with a shortwave adapter, should suit his requirements. Both have been described in the last issue of the "Radio Listeners’ Guide." . , Direction of Aerial. Ts there any advantage in having the aerial east and west in preference to north and south? writes "J.H." (Manaia). ; A.: An aerial running north and south brings in the New Zealand stations well, but the strength of the Australians are sacrificed. An aerial running north-west to south-east will bring in the Australians at a slight sacrifice to New Zealand ones. An aerial east and west has little in its favour as far as direction is concerned. , Undirectional Loop Antennae. "DPI0GENES" (Cromwell) has ¢onstructed several loop antennae, but finds: although they are sensitive yet they are undirectional, He found that the set could not be neutralised when connected with the loop. , A.: A circuit diagram showing how the Aoop was connected to the set should have been sent, as very many errors are made in this direction. The rules for frame antennae are:-~ oy 1. Use it to replace the serial coil and tune as this coil would be tuned, by a variable condenser, 2. Screen the remaining coils, as the frame has a very large field. 8, The use of a separaté earth is advisable. : -_

Coils for the Peridyne. " H.B." (Auckland) asks for full de- * tails to make the coils for a Peridyne set. : A.: These coils are almost invariably purchased ready made, as they involve delicate work and are nearly. always shielded. If any reader has made them he might send détails. of his experiments to "H-H.E.," care of us, Eliminator Interference. ‘"TNLIMINATOR" (Dunedin) has complained of interference which he considers to arise from his eliminator. A continual buzzing which is barely per--ceptible when the aerial is disconnected is heard in the speaker. Artificial static bursts in at intervals of five or ten minutes. . A.: The fact that on removing the aerial the buzzing décreases in intensity seems to indicate that tle trouble lies not in the eliminator but in some local source close tothe correspondent’s aerial. A power leakage of some description would give symptoms similar to this. A defective eliminator would cause mild static, but this would be neither inereased nor decreased with the, removal of the aerial. 3 one winding was burnt out the eurrent arcing across this gap would set up a field which would influence the receiver. 2. What is the correct number of turns on a M.C, speaker for, two 171A’s in pushpull?-75. Bog a 3; What is.the purpose of a series condenser in.the aerial lead usually specified for the Browning Drake? wA.: A condenser in series with the aerial is equivalent to a condenser in series with the aerial. tuning condenser. The sum of condensers in series is equal to the sum of the reciprocals inverted. This gives.a final eapacity less than the capacity of the aerial tuning condenser, with the result that the wavelengths received are lower or the frequencies higher. This condenser usually has the effect of greatly sharpening tuning, but of reducing volume slightly. Adding Screen Grid to Reflex. C4N a screen grid valve be added to a reflex circuit? asks "H,U.M." (Whangarei). A.: No, the only form a screen grid valve can be connected to this receiver is. by the use of a booster, such as has been described previously in the "Radio Record’’-even then it is doubtful, if the result would be successful. The refiex has been made to handle relatively weak signals, and when they are stepped up, fresh difficulties are likely. to be encountered. , 2. Can a pentode valve be used in this cireuit? ~~ 8 A.: Yes, a pentode ean be used in any circuit by merely connecting the extra grid terminal to "B+." ; 8. Can. resistance: he used instead’ of transformers in this circuit? , A.: Yes, but all advantages of the reflex would by this be lost. Concerning. Push-Pull. "QUESTION=> rat c* ‘yolfington) in submitting a diagram for consideration asks:

1, Gan two pentodes be used in’ pushpull asa final stage ‘amplifier? : A.: Although the pentode is a power valve, it is not intendéd for ‘use in circuits where a great deal of volume has to be delivered, and as this is the main attribute‘ of push-pull amplification todes are not suitable: To obtain. ¢he best. tesults: from a stage. of push-pull power, valves should be of the 171A type. The semi-power valve is not so. suitable. _ 2. Would any advantage accrue by inan audio frequency choke after the output transformer?-No. .. . 8. Can a push-pull amplifier be. used immediately following the detector? A.: No, an intermediate audio stage is necessary. . et 4, How many A.C. valves can be used in conjunction with a Philips PPP power pack? In other words, what is the amperage that can be drawn from the "A" transformer and the milliamperdge from the "B’? , A.: From 4 to 5 valves can be used quite safely with this output, which delivers 30 to 35 milliamps at 180 volts. The "A" transformer will deliver about 2 amps. vie eae Request for a Circuit. } "L B." (Dunedin) has asked for the Ade diagram of a eircuit employing three stages of R.F., detector, and two AK, using reaction with single-dial or two-dial control. — / A.: The best we can recommend is © the. two R.F. Browning-Drake with a stage of push-pull. .Single-dial : control ean be used with balancing condensers. © Connect the moving plates of all the condensers to earth, remove the grid leak, and insert it between the grid and F plus terminal of the detector. By. employing balancing condensers a certain dégree of sensitivity is retained which would otherwise be lost. . Amplifier Trouble. "RévI0 FIEND" | (Epsom) ‘has had poor results conneciing an amplifier to. a short-wave set... He finds particularly that when he places his finger on the "B plus" of the first trensformer a high-pitched whistle re: ‘ts. When he first made the amplifier, volume .was splendid. A.: It is apparent that feed-back is occurring through the battery. The t that a high-pitched squeal results fon placing the finger on the battery terminal strengthens this view. * The cause is no doubt due to the high resistance of the "B" battery, which is probably in a semi-discharged state and requires renewing. A 1 mfd. condenser connected across the batteries would reduce the trouble. Linen Diaphragm: Speaker. M* linen diaphargm speaker does not come .up to expectations,. writes "J.N.N." (Te Mata). Generally speaking, voices come through as hough. spoken ‘into an empty tank. I used shellac as a dressing for the linen which was not of the ‘highest quality. . __A.: The wrong dressing has-been used. The article stated Duco or Collodin, and either of these should have been used. 2, Wishing to tone down the volume from 2YA, I am using a. power. vélve (256) in the first audio stage, and an-

ee other power valve (603) in the second \audio stage. There are 18 volts grid bias on the last valve. ‘ .. Avs: This is a very unsatisfactory. arrangement. A 256.valve without bias is consuming about 20 milliamps, and the 603 biased at 18% volts consumes 18 milliamps, resulting in a total draw for these two valves of 88 mas. This is far too great. A six-valve set using the correct valves rarely requires more than 30 milliamps altogether, so that under these circumstances, the: "B" batteries will run ‘down very rapidly. -The best method of controlling volume is to insert a high variable resistance between ‘aerial and earth.

+ Be Screen Grid B.D. ARTON" wants to add a screen grid’.stage to his Browning-Drake. A.: The correspondent would .be_ unwise to interfere with his two R.F. Browning-Drake, as selectivity and sensitivity’ would likely be impaired. A sereen grid booster could be tried, likewise the four-valve cireuit which appeared in "the "Listeners’ Guide." In this latter case, two stages of radio frequency might follow the screen’ grid stage, but it is doubtful if the original two R.F. B.D. could be excelled by this arrangement. A diagram submitted by the. correspondent is quite. unsuitable. In Brief. "FE FF." (Wanganui).-The "B" bat- * tery using test tubes was described in our issue July 29, and August 5, 1927. Earth for A. C. Set. J "A ¥.G." (Queenstown) has found * that his A.C, set worked equally well with the aerial removed. He has tried all approved types of earth, and none: are effective. A.: As the set appears to be giving satisfaction it is apparent. that it is earthing through the A.C. mains. Trouble with a Long Acrial. .B.P." (Blenheim) has an aerial 70 ¢ ft. high and 80ft. long which does not give as good reception as a smaller one 120ft. high and 20ft. Jong. ‘, Av: 150ft. being the total length of serial and lead-in is far téo great, and maximum signals are no longer received. Noise. is increased and selectivity spoiled. It. is possible, too, that the small aerial was directional, while the slanting leadin of the big aerial would offset any advantage: obtained from this:-L type aerial.

, " Sereened Aerial. "H 8." (Hunterville) states that he has id erected an aerial attached to a galvanised wire which stretches between a tree and a hill half a’ mile apart, £0 that it lies just under . the ‘suspending wire. It is well from it. He states that the results from this are not as good as the reunite from a smaller aerial of the usual A.: There are three distinct effects due to the aerial being suspended to the galvanised wire :- 1.: This supporting wire is earthed, consequently the effective height. of the aerial is a few inches.

2. The wire is acting as a screen and absorbing energy from the ether, leaving the aerial in a shadow. 8, There is a capacity effect between | Both writes decreasing the efficiency of the aerial.