Questions and answers

Radio Record, Volume III, Issue 25, 3 January 1930, Page 27

Questions and answers

A.C. Mains 4s Aerials. OULD you tell me if devices plugged into electric light sockets, so as to wh the wires as aerials, are of any prac‘al utility? asks "Aerial" (‘Temuka). A.: By blocking the mains current with two by-pass condensers of the capacity of imfd, or more, the A.C. lines may be used as an aerial. It is preferable, and, in fact, it is almost essential, to use the specially constructed "Ducon" adaptors, for this purpose. If one makes this kind -o£ apparatus for himself, he may encounter: trouble both with the actual apparatus and with the power board authorities. ‘The "Ducon" type of aerial is by no means as sensitive as a good aerial. urrent is not drawn from the mains. Short-wave Radio. BENG interested in short-wave radio and wishing to construct a small set, I should like*to know if any. improvement could be added to the two-valve receiver, published in the "Radio Record,"

Apri 26. 1929.

H.E.

R.

(Wellincton).

, A.: No. This is one of the most successful sets we have ever described. It has given satisfaction wherever it has been constructed, and we have .not received any complaints, and difficulties are rare. This is. an excellent set for an ‘enthusiast’ to commence with. The -diagram of. the Reinartz circuit is quite a good one, but we consider that "Round the World Two" is better. Encountering an Harmonic. T CAN tune in our local station on: two distinct readiness. Te: thie nnuenal?

W.H.

H.

(Wanganui

A.: You have encountered the station’s harmonic on double the frequency of the transmitting station. BXvery note, with "a few exceptions, has a harmonic or a second note of double its own frequency. More than one of these may exist, but each one of these will be a multiple of the original frequency or fundamental. The stronger the harmonics the weaker the fundamental. Various Requests. OULD you tell me the following? asks B.G.F.: (Paekakariki) :-- > : The most suitable kind of earth? fA.: See our article published Novembgr 11, 1928, for a full account of various exurth systems. . : ee 2: Can ’phones be used with an A:C. set? . A.: Yes, but they should not be plugged in following a big power-valve or a stage of push-pull. In this case plug’ the *phones in to the plate of the first audiovalve and B plus. : 3: In a recent "Radio Record" there appeared a small paragraph headed "How ean I:slow down my set?’; following this was.a line "see page 32." What does this mean? A.: The line does not refer to the preceding text; it was a misconception on the part of the printer. Reaction Trouble. .° i EN I bring up the reaction conY denser the set oscillates strongly and signal is strongest in that howl. Bringing it past this signals are weak, but on moving the condenser to either ‘side the set howls again. The set will oscillate with the volume control shut off. . , ‘

A.: It appears that réaction is too fierce, Try reversing the connections. to the reaction condenser: and taking off some of the turns on the. reaction coil. Reduce the voltage on the detector, and, if possible, try another detector. 2: My aerial is approximately 30 feet hich. If I inerease the height, should

there be any difference ?-

Screen Grid

(Dunedin).

A.: After about 30 feet the increased signal is not proportional to the increased height. There should be an improvement, but not very great, ~ . Valves for Short-wave Receiver. A®# two 415’s suitable for detector and first audio in a short-wave re-

-eeiver? asks

J.W.

P.

(Wellington). —

A.: Yes. If troubled with feed back and other similar noises due to overamplification, try a 409 as first audio, Screen-grid Short-Wave. I HAYVB had difficulty with the screengrid short-wave receiver in the ‘‘Listeners' Guide,’ write "G.P." (Berhampore). When the A+ is connected to the screen the filament switch will not turn out. the filaments, and the set will not oscillate. A.: You have made a mistake in the filament wiring in that the switch is being short-circuited. There should not be any connection between A-- and the set other than through the switch. The only connection. to the shield is made after the wiring has passed the switch. In other words, the connections are these, Ay to switch, switch: to shield. . How much grid-bias should I use? A.: This depends on the valve and the "B" voltage, but probably three te four and a half volts will be ample. Note: If the set will not oscillate try a few more turns on the tickler, higher "B" voltage, and if possible another R.F.C. (See an article by "Cathode" in a recent issue for the construction of an efficient R.F.C.) Loudspeaker Strength from a. Crystal. [s it possible, to operate a loudspeaker from a erystal set. without an amplifier? asks "A.W.L." .( Wellington). A.: In some localities, yes, but usually no. See "Notes by Switch." .- ‘s Adding Another Stage. I have the components of a three-valve receiver, and I wish to add another valve. What procedure would you ad-

vise 2"

S.W.

Gisborne) _

A.: Build the four-valve bBrowningDrake described in the "Radio Listeners’ Guide." . This ‘may, necessitate the construction of new coils, but the trouble is well worth it. Set Oscillating. I > HAVE a six-valve factory receiver which was equipped with American valves. I have now changed -to nonAmerican valves and find that. the set whistles on certain stations. Has the neutralisation been affected? "W.H.B." (Feilding). ~ wc ae A.: Probably when a set is’ equipped with American valves it is not wise to change to other makes, especially for detector and radio-frequency. stages. There are equivalents in most.other. makes, but

our experience has shown that the best results can be obtained with the original valves. The audio stages are not so critical; in fact, it is sometimes to advantage to use a semi-power valve of another make in the last. stage. 2. How can one convert kilocycles to metres? ©. A.: Divide the frequency in kilocycles into 800,000. Audio Troubles, I HAVE recently added another stage of audio amplifications to my. threevalve set, but when I apply 3 to 43 volts negative grid-bias to the first audio a noise like a sawmill is heard, By applying a positive: 14 volts bias to the first audio the signals are clear, but equal only to. three valves. I have tried all the usual methods of tracking the trouble. and checked the wiring, and am sure

everything is correct.

A.C.

A.

(Wanga-

nui).

A.: This seems like audio instabuity and the symptoms regarding the grid-bias seem the reverse to what you have stated. In other words, when you apply the 43 volts negative bias you get the symptoms that you should get. with 14 volts positive bias. Are you quite certain that this is not the case? Have you tried bringing the grid return of the audio valve to A-? You might try the following. A better transformer for the. primary winding does not appear to match the high impedance of the proceeding valve and choke in the B+- lead to the audio valye by a by-pass condenser between B-- of the transformer and the filament negtive. The by-pass condenser should be from 1 to 4 mfds.,-and capable of withstanding the voltage of the "B" bat-

tery. Tt possible try another valve in the last stage to replace the Pentode. Factory Set Gives Trouble. HAVE a six-valve factory receiver equipped with 201A’s and lately a howl has developed which can. be stopped only by increasing the grid voltage to nine and .spoiling tone. My batteries are running down at a very rapid rate. A: A by-pass condenser between B--R.F. and earth has broken down, causing instability’ and short-cire cuiting part of the "B" battery. This replacement is very simply made if you know the receiver you are dealing with, but in the event of your not know‘ing the ‘set we should advise you to send the set to the local agents for that receiver. (G. G. MacQuarrie).° Better tone would result through using a valve of the 112 type in the last stage. This could take nine volts grid bias. 171 valye would be even’ better, but at least 135 volts would be required and 20 volts ias. Improving Reception. I HAVE a 2-valve receiver which by turning out the detector acts as a crystal and one-valye amplifier for 2YA. I am intending to use an eliminagor, and: wish to obtain. foremost good qualitye. ‘Could. you supply the following informa

tion

R.

R.

(Wellington).

il: Would you advise replacing the A409 with a power-valve or with: a pentode? A.: Real quality can not be obtained with only one valve; we should advise the use of another stage with resistance capacity coupling between the detector and the first audio, The first valve could be your A.409 and the last a semipower valve of the 112 type. (See "All

About the All Hlectric," page 65, for the valves in this class.) If you intend to buy a semi-power valve which will certainly give better results than the general purpose. A.409 you might just as well purehase the resistances (grid leaks) and condenser for the extra stage, ‘The resistances in the plate circuit should be 100,000 ohms, and in the grid circuit half to one megohm, The by-pass condenser should have a capacity of about .imfd. . Do you agree with the statement that after about three months’ use a power-valve is equal only to a general purpose valve? A.: Certainly not. The life of a general purpose valve during which time it functions as a power-valve is equal to that of an ordinary valve, roughly 1000 hours. 8. Would an eliminator delivering 8 to 10 milliamps at 100 volts be sufficient? Would one delivering 15 milliamps at 135 volts be better? ; A.: The latter would be the better; the first would be almost unsatisfactory. Very good quality cannot be obtained with much less than 150 volts. Sereen Grid Detector Two. COouLDn you supply details regarding _ the screen grid detector two described in your paper of November 1? A.: As was stated at the time, this was merely a reprint from another magazine for the sake of experimenters, and as we have not built the receiver ourselves, we cannot give any details. They will be found in "Popular Radio and Aviation" of May 1, 1929, Perhaps some reader can lend you a copy as it is now out of print. A short time back Mr. G. Patchett (264 Rintoul Street Wellington) offered a copy to any reader receiving same. 2: I have a .0005 condenser. Could this be used instead of one of the others specified? . . A.: The will have to be reduced by means of a series fixed condenser of .0005 mfds, to .00025. NOTHDR correspondent, H.L.M. (Whangarei), has asked a similar question and he ig referred to the above answer.

Pentode’s Crystal and Valve. ypocrp you supply information on the following points, concerning this receiver, asks "Carborundum" (Wellington) :- 1. Could 34 gauge D.O.C. be used for the tickler instead of 28 gauge? A.: Yes, use approximately the’ same number of .- 2. What voltage should be used for "A" and "B" batteries? A.: The "A" battery depends on the valve used. A four volt. valve is the best proposition. This will require three dry cells with a resistance to break the voltage down to four. The "B" battery should be as high as possible, preferably two or even three blocks of 45 volts each. 8. Could..a number of 43 volts flashlight batteries be coupled in series to make a "B* battery? A.: They could, but this is not recommended owing to the large number of connections necessary. The battery would be very expensive. 4, What valve is recommended and what voltage? A.: Use a 4-volt’ general purpose valve. 5. Would a switch to cut out the "A" and "B" batteries at the point of entry

be an advantage? A.: The rheostat is all that is necessary. "A" Battery Short-circuited. M* "A" battery is running down in about a week and I have traced out ali the wiring and can’t find a short cireuit. All stations are very weak. "Puzzled" (Epsom). , A.: Despite the fact that the wiring has been checked we still suspect a short circyit.- It may be in the valve socket and such that the two filament prongs are connecting. Look for such a fault as A- and A-- both connected to earth or B-, or it seems that the short cireuit is place in an unusual manner.

i, it cannot be located, take the set to a radio specialist, who can easily chec the circuit with a. meter. » Amplifier Problems. pice of Philips’ 6-volt valves would you recommend me to use in the amplifier and detector staves or fonr-

valve Browning-Drake?

V.L.

C.

. (North

Otago) _

A.: It is not stated whether the com-¢ bination is A.C. or D.O. For A.C., D415 detector and first audio; C603, power- . valve. For D.C. detector A415, first audio A409, power-valve B405. 2. In the first stage of the amplifier describing "All About the All-Dlectric," would 1424 be a suitable valve in place of 2272?-Yes. 8. I have provided a variable resistance in conjunction with the 1 mfd. condenser. Instead of earthing the grid \ returns could I take them to suitable ¢¥ tappings with advantage? . A.: If the centre tap across the filamexits is earthed the grid return can be taken to a suitable grid-bias tapping and the condenser and resistance omitted. Tf the condenser and resistance are to be used grid-bias is automatically provided. In the diagram submitted resistance 010,000 ohms are used as grid resisters, but these are too high, they will not pass the current taken by the plates of the valves; 0-3000 ohms will be ample. When two 171 are used in push-pull the correct resistance is 1250 ohms. Linen Diaphragm Speaker. I HAVE found, writes "Tank" (King Country), that speech on. my linen diaphragm speaker is. very unnatural; music seems all right. A.: There is probably a tension on the unit. Have the reed extended by means of a thread and coupling. If you cannot get this done locally, Fear and Co., Wellington, can do it for you. Generator Disturbs Reception. I have tried to work a five-valve battery set where there is a-220 voltage generator used for lighting, ete., but am

troubled with interference. . A.: The trouble can be lessened by the use of by-pass condensers, (See our article, "Noises and-Their Hlimination," Vol. 3, Nos. 9 and 10.) A.C. Browning-Drake. TS the A.C. Browning-Drake worth make

ing up? askg

R.

F.

(Te Kuiti).

tou state it is unselected. A.: The A.C. Browning-Drake is an excellent set, especially the four-valve model described in.the "All About the All-Hlectriec." Like all four-valve receivers it is unselective, but in the country this is not a disadvantage. 2. Could you refer me to the "Radio Record" containing information concern; ing this receiver? Le’, A.: It has been very fully dealt with in "All About the All-Hlectric," whieh publication every constructor of Browning-Drakes should own. P 8. What will be the capacities of thvo condensers, one 25 plates and the-other A.: If the plates are small the capacities will probably be .00085 and .00015, (or 50002) 5 if they are large, .0005 and