Questions and Answers

Radio Record, Volume III, Issue 41, 24 April 1930, Page 28

Questions and Answers

"P J.K." (Palmerston North) encloses * a sketch of a push-pull amplifier, and asks if it would be suitable for a gramophone amplifier. A.: Yes, quite. The resistance in the anode of the detector should be about 100,000 ohms., and that in the grid circuit of the first amplifying valve about 50,000 ohms. The intervening condenser should have a capacity of about .01 mfds. You will probably find that a 201A valve zm the second audio will be quite sufcient, "W R.B." (Palmerston North) states * that he cannot get American stations on his five-valve receiver, although he can get the New Zealand ones at good strength. He has a new 45-volt B battery. and asks1. Would the set be damaged by connecting the two 45-volt batteries and running 90 volts through the set? A.: No, not under ordinary circumstances, but it is unwise to put the full $0 volts on any valve but the last, or at the most on the audio valves; 674 should be applied to the radio valves, and 45 on to the detector. If you intend to connect the old 45 battery with the new one, our advice is to make fresh resolutions. It is most unwise to connect the new battery and an old one together for the sake of obtaining the extra voltage. When the battery drops slightly in voltage the re-

sistance due to defective cells increases greatly. and the actual inefficiency of the battery is very much greater than it appears. . 2. Would reception be improved by the addition of another 43-volt C battery? A.: This depends upon your circuit. Trace out the grid bias negative wire, and see if it connects with one valve only. If it does, and you are using anything but a 201A in the last stage, you can safely inerease the bias to 9. If the grid circuit of the audio valve preceding the last valve is connected to the grid circuit of the last valve, you will have to disconnect these two wires, and put’ in a separate tapping for the first audio valve. This should be 3 or 43 volts, according to the plate voltage. . Would reception be improved if I fitted a fixed-condenser between the ' aerial and the set, and how much do these cost? A.: Reception would not be improved. The function of the condenser is to effectively shorten the aerial and is used usually to sharpen selectivity. Note: We cannot assure you that by increasing the plate voltage and putting extra bias on the last valve that you will get Ameriean stations. We can state, however, that the sensitivity and tone of your receiver will be improved, and if you have already had four American stations on the telephones you may be able to get them on the speaker now- " B." (Auckland) wishes to know how "e to excite the field coil from the A.C. mains. A.: "Pentode" is now revising the description of the dynamie cone which proved so successful, and when this is complete we shall publish with the details for transformers to work from the A.C. mains. "EB R.W." (Island Bay) proposes to * eonstruct the silk diaphragm speaker and wishes to make slight alterations in order to make a cabinet speaker of it. His questions are :- 1: Will substituting hard wood for the O.B. rimu be detrimental?-No. 2: Will the addition of a back of stout ply wood affect the performance? A.: Unless the back is left open or covered with gauze, it will result in a booming effect. At the least, bore many holes in the back, to allow the sound waves to escape. 8: Will the addition of feet affect performance ?-No. 4: It appears from the article that the silk is really tacked to the frame. Is this all the precautions that are necessary for silk is flimsy material? A.: Use fine tacks with wide heads and put them very close together. 5: What would be the effect of altering the size of the frame to 20in. by 20in.? . A.: It is very difficult to say ly, but we should imagine it would have practically no effect. " H.M." (Southland) states that he * cannot get good results from’ a 2valve transformer coupled amplifier when connected to a single valve set. Both amplifier and set use PM4 valves. Ax: You should be using PM4D or PM3 as detector and PM8 as first audio, Be quite certain you have adequate bias on the last stage valve, and that the transformers are of low ratio and good quality.

2: What is the corifect method of wuiring the set to the amplifier? A.: The plate lead of the detector is taken to the P terminal of the first transformer, while B-- of the transformer is taken to about 45 volts. A choke may be connected between P of the valve socket and P of the transformer, and if the one valve set uses reaction, there will be a condenser across the primary and P of the valve will be connected first to the rotating coil. If of the condenser control reaction type, the wiring will vary according to the circuit you are using. Use about 45 volts on the detector, 67 on the first audio, and 90 on the second audio.

"TOLUM" (Auckland) states that his four-valve set will work on three valves only. It distorts on distant stations when working on the four. The volume control does not make any difference on the local station, but makes distant stations bubble. A.: You have not stated exactly what you mean when you say "it will not work on four valves." It appears it does work on four valves, but poorly. Your transformer may not be a good one, or there may be some mistake in the wiring of the last stage. If you are using an eliminator, it appears that it cannot supply sufficient current for the set. Try reversing the connections to the primary of the last transformer, and increase the grid bais (or decrease it according to the amount of bias you have on); the particular valve you are using requires about 9 volts when using 90 volts, and about 12 volts when using 135 volts. Regarding the volume control, we cannot give -you any advice because you have not stated the type of control it’ is. "W B.A." (Auckland) has a crystal set *and two-valve amplifier, and when he turns on the electric light, a noise like an aeroplane is heard. A.: Probably the switch is defective, and you should call in an electrician to examine it. The contacts may not be making a good connection.

a C.C." (Pelorus Svund) asks where * Condor valves can be purchased. A.: Try John Chambers and Son, Wellington. "GPARKS" (Christchurch) asks for the a . particulars of valve-base coil for Round-the-World" Two. He is referred o the inquiry of the above correspon- ° ent. " P." (Norsewood) wishes to. know 635 the number of turns for A609 and A.: A609 requires 15 for a 38-inch for mer, about 20 for a 2-inch former. A635 requires about 30 for a 38-inch former and 36 for 87 or a 2-inch. We would advise ou to use A609 in preference to 2: I am using a Phasatrol. Have the primary turns to be adjusted because of this?-No. 8: I do not have a volume control, but have put two 380 ohm rheostats in the filament lead between the detector in high frequency valves. Could you suggest a better panel control? .: The best volume control consists of 500,000 ohms potentiometer shunted across the aerial coil. It is used as a rheostat, the third terminal being neg lected. -~ | 4: When I connect a horn speaker to my set there is no hum, but when I connect up a linen diaphragm speaker there is one. hy? A.: The hum is likely to be 60 cycle note, which cannot be reproduced by a horn speaker. 5: When I turn the audio rheostat on too far the set begins to bubble badly. A.: It seems that you are overloading the eliminator, as it is rated to give only 120 volts and about 20 milliamps. The only satisfactory cure is to use valves. with less plate consumption or to buy another eliminator. If the eliminator is not at fault reversing the connections to the primary of the last transformer, inserting a grid leak of about 4 megohm. in series with the last valve may have some effect. "MO WHARB" (Feilding) asks if the . unit 66R is a "Blue Spot" obtainable from Fear and Co. .: Yes. If Fear and Co. do not stock it, try the Rodger Importing Co., Christchurch. \ 2: Would the enclosed piece of material be suitable to surround the cone, to attach it to the baffle?-Yes. — 8. Would it be possible to organise a purehasing department for country constructors who cannot obtain parts they want? A.: It would involve more organisa- tion than it would be worth. The big. city firms are always prepared to deal with mail orders from the country, and the right components ¢an always be obtained from them. "QELECTIVITY" (Wellington) encloses a diagram of a super-hetrodyne and asks if it will be better than a neutrodyne of the same number of valves. A.: The super-hetrodyne has to be used with a loop antenna, and becaus of this it loses much of its sensitivity. The hetrodyne set submitted appears to be a good one, but we think: the neutrodyne would be better, 2. Could you advise ie of expert superhetrodyne builders in Wellington? A.: Try Collier and Beale,

"DYNAMIC" (Lower Hutt) asks the 3 following questions re "Round-the-‘te World" Two :- \y 1. Can two UX199 valves be used?- \ Yes. 2. If so, would the amperite be necessary? . ‘ A.: A 30-orm rheostate would be beter. 3. I am using non-insulated buz-bar for wiring purposes. Will this make any difference? A.: Not as far as the efficiency of the set is concerned, but it will not be as easy to work as glazite or a: soft wire of that nature. The fact that it is uninsulated makes it very prone to short circuit. Note: Re your 8.0.8. for a reply by post. We are sorry your letter found its way into the batch for this week’s issue. We hope you make the set up on the off-chance. (T J.C." (Dunédin South) wishes to -e make the four-valve screen-grid Browning-Drake described in last year’s "Radio Listeners’ Guide," and he finds that by connecting A-+-- to one condenser and A- to the other, he will be directly short-circuiting the battery. A.: The grid return of the detector valve should go to A- and not A+. The 5 megohm grid leak should be connected to A+ as shown. In this manner the positive bias is put on to the detector valve. " J." (King Country) complains that ¢ his reception is very poor. He states that he cannot use a 199 valve in the first stage as recommended by the makers, as it burns out too rapidly. He ean use a 201A and does so without a resistance. A.: A resistance of some description is essential in the filament circuit of 201A when using the 6-volt battery. An amperite or a rheostat of 10 ohms. should be placed in the filament circuit in place ‘\ of the resistance cartridge. The quality \may be effected by many things, but probably because you do not have sufficient bias on your last valve. The impeda--formers in your set are probably giving trouble, as they have seen long service. and they are a weak spot in your type of set. There is very little that can be done except by an expert, as you might go to a great expense in replacing the impedaformers with transformers and then not get results commensurate with your expenditure. 2. I live in a valley and run the aerial between the surrounding hills, so that in all I have very nearly 200ft- Is this a good or bad aerial? _ . A.: It is too long. be sharpened by using the lead-in only. "pYb" (Mangaweka) has a five-valve portable set, and has been unable to rocure an A battery to fit. The size is 5 x 4 x 4 inches; he wishes to know where he can obtain one. A.: We do not know of an agent, but the DTC battery should fit your receiver 4f you can obtain one. Try some of the Jending firms in Auckland, Wellington, or ‘aristchurch. "ROTARY" (Gisborne) states that he has tried to add a screen-grid booster to a 8-valve screen-grid receiver. He asks :- 1. How to add a scréen-grid booster to the set? . A.: You may have considerable trouble in placing a screen-grid before a set of this nature. You should have defined your troubles more clearly. 2. How can I get better results with a pentode in the last stage. The present results are poor. A.,: A pentode meeds fairly high voltage of the order of 135 to 180 volts, and _ grid bias between 12 and 18 volts. 8. Is it impossible to fit the screengrid detector? An effort I made resulted in splendid daylight reception, but the set was too unstable for ordinary use. A.: The set is designed for ordinary valves, and when you commence making alteration of this description trouble is invariably encountered.

"puUzzLED" (Gisborne) asks the following questions relative to his 5valve factory-operated receiver. — 1. What is the best of valves for the R.F. and A.F. circuits? A.: You should use a medium power valve in the last stage. The detector should be a special valve, not a 605, which is a last stage valve. Try changing over the 201A in the last stage with the 605 now in the detector socket. 2. Will a moving coil speaker work from a set operated on 90 volts? A.: Not usually satisfactorily. 8. Can microphonic valves be remedied apart from the cap. A.: Very rarely. 4, When the plates of an accumulator lose their coating is it worth while applying a new coating, and how? _ A.: This is a job for a battery specialand we would not advise you to attempt it yourself. By examining your battery he can tell whether it is worth while. "['HIOK-HEAD" (Lower Hutt( wishes to construct "Round-the-World" Three, and using valve base coils. He asks the number of turns for the coil. A.: The approximate number of turns is as follows :- .

The figures for the wave-band are approximate only. A diagram accompanying shows how to avoid the tapping on

the secondary: coil which would be fairly awkward on coils of this type. 2. What gauge of wire would be required? A.: The secondary coil is wound with 26 DSC wire, and the reaction coils with 380 DSC. 8. Does this circuit lose any efficiency insofar as short-waves are concerned as compared with a straight out short-wave set? A.: The valve base coils used in this circuit will slightly decrease the efficiency. As far as the all-wave properties of the circuit are concerned, it is the long waves that will suffer. 4, Will any alteration be required in the wiring if valve base coils are used? A.: The alterations are shown in the diagram. "R T.D." (Rununga) has had difficulty * with his all-wave receiver. It suddenly bursts into oscillation with the reaction condenser right out. On other occasions, the volume builds until the set bursts into oscillation. On short wave the set does not oscillate freely. A.: Give the screen-grid valve a slight negative bias by connecting the .0005 condenser now connected to earth to the

paceenanaens negative terminal of a bias battery. Check over the connection to the grid of the detector and be quite certain the grid-leak is quite all right. Keep the lead to the grid of the detector as short as possible. We are returning your diagram with some suggested alterations. ‘ "ANNOYED" (Taumarunui) gets a shock every time he touches’ the phones of his battery set. A.: This can be prevented by incorporating a choke and condenser shown on page 38 of our issue of April 11. Disregard to connections to speaker 2. This will stop all A.C. current getting into the phones. It does not necessarily signify a short-circuit or a leakage.

Secondary Reaction: Wave-band: 9-16 5 8 15-30 8 10 2 28-55 15 50-110