Questions and answers

Radio Record, Volume III, Issue 40, 17 April 1930, Page 30

Questions and answers

"TY B." (Kilbirnie) eriticises us for not * recommending the one-valve set, to a correspondent who asked for a circuit a couple of weeks ago. He states he has had wonderful results on both New Zealand and Australan stations. Our reply is brief: "But have your neighbours had equal enjoyment from their sets?" C.B." (Christchurch) asks for 2 * transformer to step down 230 A.C, to 110. A.: The specifications are being prepared for the 1930 Radio Listeners’ Guide, which will be on sale shortly. "L W.C." (Blenheim) asks the follow- ° ing: 1. Will a Philips B and C eliminator deliver sufficient current to operate two 171A’s in push-pull? A.: We understand not. It supplies about 30 milliamps, at 150 volts. By taking more from it the voltage drops considerably:

2. Tone is at present edgy and reedy, on the high notes, yet good in the bass. ‘A.: It is difficult to explain why this should be, particularly as you assure us everything is O.K. Have you tried another speaker? Are you using push-pull? If so, are your valves balanced? Have you tried another valve in the last stage? We suggest you use a valve with a smaller amplification factor in the first audio. 8. Can I supply bias to the last valve by centre-tapping the filament. winding and taking the centre tap to C+ of the eliminator and the negative tap to GB of the transformer.-Yes. 4, Can I use a 200 ohm. potentiometer across the filament? * A.: Yes, if it will pass the amount of current required by the valve. The usual value is 20 ohms. ""7ALVH" (Papakura) wishes to know something of valve combinations. He submits three possible combinations and asks us for the best. A.: No. 2 is the nearest, but not correct. The following will be ideal: Detec‘tor 4D, first audio PM8, second audio PM4. We regret we cannot discriminate between two makes of valves. 2. I have been using PM3 and PM4 since 1928. Is this a good performance? -Yes, quite. " J.J." (Wanganui) asks :- ¢1. Which is the most efficient aerial? _ A.: Aerials were described very fully in an article a couple of weeks ago. We there pointed out that the best aerial was a single wire from 60 to 80ft, long, and as high as possible. 2. The number of turns required to tune over the broadcast band with a .0005 tuning condenser. A.: There are a large number of different coils that may be used, the number of turns depending upon the diameter of the former and the gauge of the wire. A convenient one may be built up on a 23-in. former, 20 gauge, D.C.C. wire un--spaced-~ 78 turns. ; 8. Which is the better combination for’ a 2-valve set-radio frequency and detector or detector and audio? A.: Radio frequency and detector, e.g., the radio and detector stages of either the 4-valve Browning-Drake or Hammar lund-Roberts. You may obtain a diagram for either of these in the 1930 edition of the "Radio Listeners’ Guide," now in preparation. "DROWNING-DRAKE" (Havelock 4+" North) cannot get good tone from set. He has renewed almost all his parts, and has good valves with their correct voltage and bias. . A.: You may suspect two thinks. The position of the 6/1 transformer, which is far too high, 84/1 is now very much more common than 6/1, and it has been shown that the lower ratio transformer should go first. Try changing them round, and if this does not imprvve tone try a 34/1 in place of the 6/1, The next point is the number of turns on the primary of the R.F. coil. Frequently, commercially made coils have far too many turns on this coil. This factor, however, would not interfere with tone so much as the former. "YPPCOHM" (Auckland) cannot. tune _ in the local private stations in his crystal set, although he has tried differ-

ent tappings on the soil and _ shorted out some of the turns. » A.: Shorting out turns is really unsatisfactory. ‘The best plan is to construct another coil of about three-quar-ters of the number of turns. This will probably tune in the local station at one end and the private stations at the other. "Tr W.W." (Bilerslie) asks where he * may procure a Philips valve for the R.F. stage for a four-valve set. A.: From any good dealer. Probably A415 will be as good as any, but it will not be any improvement on the valve you are already using in that stage unless the latter is old. s P.H." (Christchurch) asks the fol- * lowing in connection with the allelectric Browning-Drake :-- 1. Can I use a .0005 and a .0003 condenser ‘with separate dials ?-Yes. 2. If the balancing condenser is not required can the neutralising condenser be placed on the panel A.: It is not wise, for the leads will have to be lengthened and this may cause trouble. Shift the regenaformer into a central position between the two dials, if you wish for balance on the panel. , 8. Would parallel feed to the radio — valve be an improvement. If so, what kind of radio choke would I use? .: Parallel-feed is an improvement. A suitable choke has been described by "Megohm" and will be redescribed in the 1930 "Listeners’ Guide." 4, If grid bias is obtained from a resistance in the centre tap of the filament windings can the bias terminals be left out, and "KE" of the transformers joined — to the sub-panel?-Yes. "BEDUNATOR" (South Invercargill) "has had trouble with an electrolytic eliminator. He asks where he might obtain advice regarding same. A.: Thomas Ballinger and Co., Victoria Street, Wellington, specialise in electrolytic eliminators, and will probably help you. . . I have renewed my valves but do not get results as good as I did with my old set. A.: Have you had the set neutralised for the new valves, and are you supplying adequate grid bias for the last valve? The pentode valve does not always give satisfaction if it is not supplied with a fairly high voltage. 8. What causes my speaker to howl when I stand it on top of the set. It did not do so before. A.: It sounds like a microphonic detector. Try a lead cap on the top of the detector valve. "ATOVICE" (Epsom) states that he is using a dynamic cone speaker which obtains its eurrent from the mains.. After it has been running a while the transformer and the rectifying unit become quite warm. . A.: Unless they become hot, there is no cause for anxiety. If they do heat up communicate with the dealer who sold you the unit. "TPUNING DIAL" (Opunake) has changed a four-valve set over to A.©., but the first dial is hopelessly broad and he cannot find: the centre of the whistle. A.: The valve you are using has a very high amplification factor, and is

to be unstable. Check over the grid connection to make certain the return is quite O.K., and, most important, put a negative grid bias of 1 volts {on the screen grid valve. " " A J." (Bokeno) has "a five-valw?: fac- * tory-made receiver, and he wishes to dismantle it and build a two or threevalve short-wave receiver. He asks for information on how to go about it, and where he might obtain a copy of the circuit of a short-wave set. A.: You are ambitious and you may not be altogether successfu,l as quite frequently components that are used in a broadcast receiver are not suitable for a short-wave.. However, if you wish to make the change, you will find the deseription of "Round-the-World" Two published in a recent issue of the "Radio Record" all you require for building a shortwave set. ° Odd Points. "JAUSPANKA" (Hawke’s Bay) ask ‘ the following questions :-- ' ps ee

mens aii The ratio of Ferranti transformer."WW 2. How much of the enclosed resistance Wire should be necessary to break down volts to 4 volts passing .26 amps. A.: About an ineh, but as the wire will not pass much current, take four inches and double it, 3. I. get better quality when I. place my fingers on the plate and grid terminals of the last valve. ‘ A.: Try adjustment to the grid bias, and if this makes: no difference, a grid leak across the secondary. Have the valves tested. ; 4. The speaker howls when I touch the plate terminal of the last valve. .: This is not an ill omen. ~>. 5. The set oscillates more easily when the detector rheostat is full on, yet I get more volume on the loud signals than when the rheostat is turned on only slightly. .: By turning up the filament of the tector, you are approaching very near the reaction point, and when strong signals have to be handled, this is a detriment to both quality and volume. 6. If resistance is waste, does a valve burn as much current with the rheostat full on_as it does half on? A.: No; a resistance prevents eurrent from passing. 7: Are the Time Signals broadcast from 2YA yet?-No. . "(LRID" (Timaru) has been using wet "B" batteries with his set and finds when he changes over to dry "B" batteries there is a continuous whistle. A.: Try earthing A~ and shunting the battery with a 1 mfd, condenser. "D W." (Masterton): We shall pubme © lish the ciyeuit referred to in’ our article "Clear as a Bell" as soon as we can obtain it. We are at present awaiting a reply. "B 8." (Matahurie) complains that =? there is a continuous noise like static. All connections have . been checked over and cleaned. A.: Take off the aerial and earth and turn on the set and see if the noise still

pouste. If it does, it is more than ikely due to a loose connection. A frequent cause of trouble of this nature is a break within the battery cable. The cable can be tested either by tugging at each wire to see if the set goes completely out of operation, by flexing it rapidly and noting the effect or: by substitution. Broken down by-pass condensers, internal valve trouble, dust between the condenser vanes, defective resistances, can all cause this trouble, besides points you have already examined. Turn on the set and one by one remove the valves. If the noise stops when one valve is removed the trouble is either in the valve or the current associated with it. 2: Could this battery set be converted into a short-wave set? How, and what would be the cost? A.: The best plan would be to build a short-wave adaptor, which would cost about £8. Such an adaptor has been described in the 1929 Radio Listeners’ Guide, and another will be described in the 1980 edition. "(\UERY" (Te Awamutu) asks how he might ascertain the capacity of a new 2 volt accumulator. A.: Capacity is obtained by the following formula :-~ N x A’x 20 ampere hours . 144 ‘Where N==No. of positive plates. _ A=Area of one side of plate in square inches, ; 2: I already own a broadcast receiver, but I wish to use a short-wave set as well. Must I pay another 30/- license? A.: No. It is unnecessary to take out a separate license no matter how many sets are employed, as long as they are used by the same family. The fact of your having two separate aerials does not matter. " C.F." (Seatoun) asks the following: *°1, At what distance does inductance cease between two wires as in the case of a twin wire aerial? A.: To be electrically clear of one another they. have to be-spaced by at least 2. Would it be of advantage to use low loss coils in the full-wave crystal set deserihed hv "Pentode"’? —

| A.: Yes,. but very little. ; . 8. If so, what are the for the coils? ‘ 8. Which is the most suitable wire, d.e.c., d.s.¢c., or enamel? A.: There is very little difference, probably double silk covered. The correct spacing for each of the above wires? A.: Double silk, and cotton are spaced by their coverings, that is, they are wound as closé as possible, while enamel is usually spaced by half its own width. These are approximations only, and competent designers will have different ideas on the subject. The total winding length determines’ the inductance, and this must be borne in mind when:a circuit is designed. 5, What is the meaning of SWG and B. & 8.? A.: SWG is an Wnglish term meaning standard wire gauge, while B. & S. refers to the American system, the letters standing for Brown and Sharp. The numbers indicate the size of the wire, the smaller the number the coarser. B. & S. and SWG do not entirely coincide. Reference to a table is necessary to-convert one to the other. ; 6. What is the capacity of a 23 plate variable condenser ? ' ' 7 epends on size of the plates, but probably a .000385. " A BH." (Waverley) has heard music * from his set when the speaker is not connected to it. : This is due to vibration, probably of the laminations of the transformer. There is ho cause for concern, but if it is annoying try tightening up bolts on the transformer. "(ONSTRUCTOR" (Wellington) asks if the details of an "A" eliminator have been described. ,

A.: Not fully; they are being given in the 1930 edition of the "Radio Listener’s Guide." 2. Will a 1, 3amp. battery charger as described in the "Radio Listener’s Guide," shunted with a shunted electrolytic condenser, and with a choke in series, run four 201A. type valves? A.: Yes, providing,the choke has a sufficiently small resistance. If the resistance is greater than 1 ohm., then voltage from the battery charger will have to be increased. If you are building up the charger with a view to using as a battery eliminator use more than 6 volts and break down the current with a résistance, , 2. Can the electrolytic condenser be made of 80 plates approximately 66 x 8, 16 gauge aluminium, spaced with aluminium washers and 1-16in. apart so as to form parallel connection of. all plates, that is, it is assembled ‘similar to accumulator plates of one polarity and using a glass accumulator jar, and formed as described in the "Radio Record." A.: It is difficult to say without being tried out, but- it appears as though it would be quite OK. 3. Will a choke of .1 henry be suf- ficient? A.: Better make up the choke that will be deseribed in the 1930 edition of the "Radio Listener’s Guide."