Questions and Answers

Radio Record, Volume II, Issue 47, 7 June 1929, Page 11

Questions and Answers

(CORRESPONDENTS are requested to observe carefully the following points. Failure to observe these may mean delay in answering and even the loss of the communication. 1. Addresses.-All technical correspondence, whether inviting reply or not, must be addressed: "The Technical Editor,’" "N.Z. Radio Record,’"’ Box 1032, Wellington. 2. Each letter to bear (inside) the department to which it re-fers:-Construction, Crystal Corner, Questions and Answers, or General. 3. Letters inviting individual reply must be accompanied by a stamped and addressed envelope, but the right to answer any letter through the columns is _ reserved. Correspondents should watch the columns carefully as one letter may be answered in more than one section. 4. Advice requiring discrimination between factory-made sets or between makes of components cannot be given. . 5. All letters to be signed, but a nom-de-plume may be added. 6. Reports for the DX Club to be addressed: "DX;" Box 1032, Wellington.

In. Brief, "C H." (Karori) states that he intends * to purchase a "B" and "C" eliminator, and asks which we consider the best of a list of manufacturers’ names submitted. If he will but refer to our conditions, he will notice that discrimination between factory products cannot be given. He should consult a reliable dealer who carries a wide stock. Speaker Units. " P." (Petone) is unable to locate * an agency for "Blue Spot" loudspeaker units. He should try the Rodger Importing Company, Worcester Street, Christchurch, who are New Zealand agents. Hooking Up Speakers. "AM P.S." (Richmond) asks for the * necessary diagram to connect two speakers, so that when one plug is out, the other speaker will yet work. One is x at a considerable distance from the t. : A diagram is appended showing the necessary connections. It will be noted that an audio choke and by-pass condenser is incorporated. This is necessary only where an output transformer is not

employed. As few receivers employ this it should be fitted to- any set where the lead to the speaker is to be taken any ‘considerable distance. By this attachment, high potential battery current is not passed through the speaker leads, thus rendering a short circuit due to dampness or wearing of the insulation impossible. Furthermore, the speaker need not be connected in any particular "way. (See ‘"Megohm’s" article on "Chokes.’’) Improving the Set. "¢\HRISTCHURCH" asks how he might improve his set. It is a five-valve factory-made model using five 201A valves with 90 volts on the plate. He asks if the 185 volts could be used, and asks for suggestions regarding a power valve. A.: A set of this type can be improved by the addition of a power valve. If signals from the stations to be relied on are relatively strong, it would pay to introduce a valve of the 171A type. This valve with 135 volts on the plate, which is not the maximum, would require about 224 volts grid bias, but would improve the tone very much. However, if signal strength is relatively weak, and the slight reduction caused through the introduction of a 171 detrimental, the best valve would be the 112A type, which, with 135 volts on the plate, would require about 9 volts grid bias. As the set is an American one, it would be wise to adhere to American valves or Amerin type valves, as many of the Hnglish manufacturers, e.g., Mullard, are makg American type valves which ean replace the ordinary American valves without interfering with the balance of the set. Usually the warning that we have given not to change from American to English valves applies to R.F. and detector valves, and rarely to the audio stages, though it is possible that the change has some effect, but it is unely. . 2. Could I use the 6-volt moving-coil loudspeaker with my set? A.: It is difficult to say authoritatively, but in all probability yes. (See the Beginner Corner.) Charging a "B" Accumulator. J.W., of Greymouth, is intending to * purchase or construct a "B" aceumulator, and asks whether a 6-volt dynamo which is used for charging the "A" battery would be sufficient if the "B" battery were split up into 6-volt sections,

and then these connected together to form the 120 volts. A.: The method is hardly suitable, as it would be difficult to get the charging rate constant. However, here are the details: Generally a "B" accumulator has about 2 amp. hours capacity, and if 120 volts were split up into 6 volt sections, it would give a battery having a capacity of 120/6 x 2/1 ampere hours, equal to 40 amphours. As the charging rate should be one-tenth of the capacity this works out at 4 amperes. Charge at 4 amperes for 12 hours or until the cells gas freely. What is a Jack? "E J." (Ohotu) asks: "What is an oute put jack?" A.: A jack is a device employed for making various changes in the circuit of radio receivers. Among the more common uses of these are cutting in or out additional stages of audio or radio frequency amplification, operating different combinations of loudspeakers and headphones, changing from one antenna to another, control of power supply units or filaments, ete. An output jack is one that is used in the last stage or in the stage connected with the loudspeaker. It may have two or four connections, according to which it fulfils of the purposes mentioned above. The Trickle Charger. " R.W." (Island Bay) has construct- * ed the "A" and "B" trickle charger described by ‘"‘Pentode"’ and asks the following questions :- 1: Should I keep "RB plus" and "B-" shorted while charging the "A" battery. and the "A plus" and "A-" shorted while charging the "B" battery?-WNo. 2. When using the polarity, indicator, ‘the deep colour appears to show on the wrong wire with the "B" terminals. It appears on the one nearest the charger. A.: Exactly what is meant as nearest the charger is not certain. If the correspondent means nearest the rectifier, this is quite in order, as the "A-" and the "B-" are one and the same. 8. My transformer has an output of 8' volts. Is this high enough? A.: Not quite. If possible it should be brought up to within the region of 20 in order to allow for a drop in the rectifier. According to the functioning of Ohm’s law the provision of ample voltage means the supplying of the re quisite voltage at a higher amperage. 4, Could I have some turns taken off my transformer to obtain a greater output? A.: No, at least double them; reducing the number lessens the voltage delivered. 5. Could a rectifying valve be used in place of the jar in the trickle charger?

A.: Yes, but it means converting the charger from electrolytic rectifier to a valee rectifier, which entails changing the circuit completely. A trickle charger using a valve has been described in the "Radio Listeners’ Guide." "Q" Coils. "T’ H.C." (Tokaanu) asks regarding 4e "QQ" coils: (a) their form, (b) their function as different from other coils. A.: "Q" coils are inductances mentioned by one or two English journals some time ago. They have the advantage of being able to receive either high or low wavelengths, by the medium of a simple switching device. Actually, they were made up of two coils which when connected together in series would tune down to a higher wavelength, but if connected in parallel, one half partly cancels out the other half, leaving a small portion of inductance for the lower wavelength. Taken all round, they are not as efficient as when two separate coils are used.

The Exponential Horn. "WM XPONENTIAL" (Palmerston North) wishes to construct a "eurly" exponential horn, and asks the fdllowing questions :-- 1. Could 8-ply be used, and would this bend satisfactorily for the top and the bottom? A.: Yes, but beaver board would be better. It would bend easier, and layers would not be separated by the use of hot water or steam as may be the case with 3-ply. 2. Could you supply the measurements? A.: The safest way is to work out the measurements for oneself. Make a rough sketch of the shape required and compare this with the specifications given for the straight exponential. horn. Now draw one to scale (preferably a large scale) and then set these markings off on the beaver board. Lightning Arrester me A CORRESPONDENT from Nort Auckland requests a diagram showing the connections between a lightning arrester and the set using a single. pole double throw switch to connect the aerial to earth when the set is not in» use. This is given below.

[THE difficulty of running a movingeoil loudspeaker from an eliminator providing only two taps has been encountered by "W.C." (Waikato), who is using a five-valve set and operating it from an eliminator with two tappings. At the present time, high and low frequency current is being taken from the one tapping. He now wishes to use a moving-coil loudspeaker and requires all the power possible on the last valve. This means that the voltage on the R.F. valve must be reduced. A lead is taken

from B-- amplifier through a resistance to the new tapping. This is connected through a condenser to B-. A condensér of 2 mfds. with a high-voltage test is essential. The resistance should range. between a 25,000 and 50,000 ohm. wire wound or carborundum. If a variable resistance is inserted here the voltage may be adjusted. In addition, biasing the radio frequency valves with, say, 14 volts, would further cut down the current, making still more available for the moving coil. . The correspondent states that he is using 605 and 608 valves in the first audio and power sockets respectively. This is a wrong combination-605 is a power-valve and being used in the first audio, is consuming far too much current and thus depriving the last valve. Replace oo a higher impedance valve, say He goes on to state that he constructed the moving coil described by "‘Pentode," but finished it before the article on the matching of impedances appeared, He now uses 605 with 105 turns, and asks if this is correct? A.: With 605 the impedance of which is 2800, 50 turns are required with a step-down transformer, but 603 would be the better valve to use with such a speaker, It will slightly lessen volume but will make up in tone. Using this valve, 30 turns on the moving coil are sufficient. It would pay our correspondent to dismantle his speaker and take off some turns to give the number specified. Oscillation Troubles. . "QHIELD" (Waitara) inquires regarding his super-hetrodyne, which uses

three X222 valves. He has observed that he cannot increase the filament voltage beyond 2.8 without the set bursting into oscillation. As the normal current is 3.8 volts, he presumes he is not getting the maximum from his set. _ A.: So long as the set is he is getting the maximum, though in this case, he should try reduction of the plate voltages. This should bring the oscillation point lower, and allow a higher voltage to be applied on the filament. "Round the World Two." REGARDIN G this popular receiver, "we." (Lower Hutt) asks the following questions :- ; Would a glass tube of half ‘an diameter be suitable ou which to wind the choke. i . A.: Yes, if by half-inch the meter is understood. 2. Would a fixed condenser .0005 replace .0002. I cannot purchase the latter ?-Yes, 8. Would a midget condenser be suitable for aerial coupling ?-Yes. 4, What wavlengths could my five coils eover? Do I use the same coil for reaction all the time? A.: Regarding the coils: Before the wavelengths these will cover can be calculated the following data is essential. The distance between the first and the last turn, the number of turns, the size of the wire, the size of the former, and capacity of the tuning condenser. The tickler has to be altered for each coil. The correspondent states: A friend has constructed one, but when tuning with the reaction plates all in, all he can get is a very loud squeal all round ais dials. What is the cause? A.: Too much reaction. Don’t use the plates all in if this is the case, and try reducing the "B" voltage. *"Megohm’s" Speakers. (CouLD you please tell me which speaker will give the best results on the high and low notes, writes "G.F." (Dunedin). The one described in the "Radio Record" of May 8 or the 2it. cone described in the "Radio Listeners’ Guide"? A.: There is very little difference between them. The one described in the "Record" gives a slightly better overall amplification, but the difference is very small, 2. Will any four pole balanced armature type of speaker unit do for either speaker ?7-Yes. Coils for the Browning-Drake. "DHEHOSTAT" (Alexandra) has done some experimenting with the 2R.F. Browning-Drake and has found that replacing his 3in. coils by those wound on a 2in. former has made a great improvement in the layout. The particulars of these coils are: Aerial coil used in series with a .0001 fixed condenser tuned with a .0005 variable condenser, 65 turns wound 82 turns to the inch, regenaform tuned with a .00025 condenser 122 turns wound 32 turns to the inch. To neutralise a tapping has been made on the twenty-second turn of the regenaformer. He asks if this 1s correct. A.: It is rarely necessary to shift the _ tapping past the eighteenth turn, al- _ though the twenty-second turn should give quite good results. ould you give me the correct number of primary turns to suit the following valves, the regenaformer being wound on ‘a 2in. former :- A609 impedance 6,000 ohms, turns 16 PM8 impedance 16,000 ohms, turns 20 PM5 impedance 18,000 ohms, turns 22 A630 impedance 20,000 ohms, turns 28 A425 impedance 21,000 ohms, turns 30 The correspondent has noticed a peculiar phenomenon. With the set switched on, but with the speaker not connected to the jack, reception may be heard, although it is sometimes of a "buzzy" nature. At times it reaches: such yolume and intensity that speech may be folowed.

A.: One of the transformers is slightly defective, resulting in one of the laminations vibrating in sympathy with the varying density of the magnetic field resulting in a conyersion of electro-mag-netic impulses info sound waves acting on the same principle as does a speaker uni