Questions and Answers

Radio Record, Volume V, Issue 5, 14 August 1931, Page 13

Questions and Answers

W H, (Gisborne); I desire to employ * the "Kestrel Three" cspecially for short-wave work, Would I gain in efficiency by using a tuning condenser of the usual short-wave type instead of the .00085? A.: Yas, use the .0001, preferably double spaced, condensers, and do not alter the specifications given for the coils, 2. What valves are most suitable? I have a six-volt accumulator. A.: Use six-volt valves of the special screcn-grid, special detector and highgain power types. 3. In the schematic diagram I see no connections for "B-" and "C+." Where do these go? Avi "A," "B-," "C.-L." are connected together outside the set. 4, What are the voltages for the differcnt "B" and "O" leads? A.: The detector voltage is, roughly, 224 to 45 volts. The plate voltage on the power and s.g. valves is 90 to 185, and on the screen 45-674. The "C" battery will depend upon the valves used in the last stage and the number of volts on the plate of this valve, ~ Ld I, understand that in a short-wave set it’ is advantageous to have the "O"

battery within the set and keep the leads short. Can this be done in the "Kestrel Three’? A.: Yes; you could probably find room for it when you are laying out your set. 6, Why is the 4in. armoured cable mentioned in the parts list? A.: The ecable, as explained in the text, connects the s.g. valve to the coil in the audio apartment. 7. As to the screening boxes-two open sides and open top only leaves two ends and bottom-hardly a box. What position is it mounted in? : It is mounted with one side screening the Fadio stage e from the audio stages. One end of the box is against the panel and the other along the floor of the baseboard in the radio stage. Neither of your sketches is correct so far as we can see, "A" is nearly correct, but instead of the piece being at the back of the set, it divides the compartments. A JS. (Oamaru): In writing with ‘Ae reference to queries asked previously in the ‘column it is not a bad plan to include the query to which reference is made. In your case, although we have gone to some trouble to look wp your previous query, we can find no reference

to a choke, Please get in touch with us again, . [STERESTED (Southland): I have an eight-valvye set being only 80 yards from a 66,000-volt power-line; am troubled with crackling noises. What do you advise? A.: Communicate with the radio inspector_ in your district and state your case. It is possible that something may be done to alleviate the trouble caused by the power lines. Failing this, get your aerial ag high as possible. If it is practicable, get it up another 20 feet with a very short top, and insulate your leadin by using armoured cable and earth ++@ armouring., See that your line runs at right-angles to the power-line, Change your earth and do not confuse it with the earth on the electric light or your water mains. Keep a separate earth altogether for radio work. }W2CH4NIO (Wellington): What would be the specifications for a radio frequency tuning coil iin. diameter tuned with a .00035 condenser with a closely-coupled primary of 800,000 impedance? A.: We suggest that on the secondary you use 87 turns of 26 s.w.g. wire, while,

for the primary, use 40 turns of No. 36 d.s.c. or enamelled wire. That should be about right. If you are troubled with oscillation, remove some of the turns because, although it is easy enough to feed into an optimum impedance, the transfer of energy may be too high to ma tain stability, and you will have lessen the coupling. D. (Bay of Plenty): I have a ® yalve all-electric set, and can get very little volume. I have it turned up until the oscillation point igs reached, but the change in volume is hardly noticeable. Would the valves be at the bottom of the trouble? My other two sets bring im the American stations by the dozen, Should the speaker be adjusted? A.: Quite possibly your condensers are out of alignment. Decreased sensitivity may result from almost any defect in the set, so it is very difficult to tell you where to look for it, especially as you are ope ating a commercial set. However, we very much suspect the condensers, or ond of the valves. H L. (Nelson): I use a 3-amp, charge? * and a B eliminator, which suppligs 13m. amps. to my set. How much ro cost me to operate both of these?

A.: Your charger can be operated approximately 500 hours for one unit, while the eliminator will use slightly more electricity running 400 hours on one unit. BERG PATE (Morrinsville) : Do you consider four 221, and one 171 a good combination for a five-valve American set? ; A.: Yes; instead of the 171 you could use one of the high gain power valves of the B605 type. 2. What are the optimum grid-bias voltages to work on the power valve with 180 volts on the plate? A.: The 171 requires 42 volts gridbias, B605 18 volts. 8. Which is the more suitable’valve for the' last stage, the 171A or the 112? A.: In your case the 112. We certainly recommend the use of the 221 instead of the 201A’s. ai G. (Invercargill) : I propose building Fe the "Sparrow Hawk" one described in the 1931 "Guide." Would Browning Drake eoils be suitable with a .0005 condenser? -A.: Yes, You could use the Browning Drake coil, but it would not altogether be the Sparrow Hawk One, would it?

WIRELESS. (Wanganui): Could you tell: me the number of turns for coils for a one-valve broadcast set? The wire used is 26 d.s.c. for all coils tuned with a .0005 condenser. ‘A.: Questions such as this have been answered repeatedly in the "R.R." Information was given in the "Radio Guide" for the secondaries of all coils, and you eould look this up. You will find that you need 85 turns on the secondary, 20-25 on the primary, and 25 on the tickler. N Z.D.X. (Auckland): What is_ the ¢ simplest means of tone control? A.: A variable resistance of 50,000 ohms in series with a .002 fixed condenser between the grid of the second audio and earth. 2. What is the right value of a variable resistance acting between’ the aerial and the earth in the volume control? A.: 100,000 ohms will be quite satisfactory. Mo WHARD (Feilding): Could I obtain the parts for the "Outspan Five’ cheaply by buying up an old set and using the parts from that? A.: There is no reason why you should not do this, though there would be a big

waste. Just check it over and see how many parts you ean use. You will find that the tuning condensers will be all right, so should the valve sockets and transformers, and you may be able to find an efficient r.f. choke, 2. Would the "Outspan" using pushpull in the last stage give me better results than a certain commercial set? A.: We are inclined to think the "Outspan" would be the better set, as not only is it a more modern circuit, but it uses sereen-grid valves. TH B.I, (Rotorua): My speaker is ex- * cellent, but should I derive any advantage by using one of the newer types? A.: One of. the newer types: will give good results, especially as you have a modern set. We think one of the 66R type speaker would be much better than the one you are using. However, if you like your speaker, do not change. An inductor dynamic speaker would be even better than the 66R. 2. My set calls for 40 volts bias and it uses two 171 valves. I use their equivalent in other makes, but found this combination ‘quite unsatisfactory. Two 201A valves with 9 volts bias were much better. A.: You will get greater strength by using two 201A in pushpull than two 71’s, and it-is possible that the improvement.of tone by using the 71’s will not be apparent when you are using a speaker such as yours. For this type use a P605 valve. These should be biased by from 12 to 15 volts, depending upon your "B" voltage. This, by the way, may be at the base of your trouble with the 71’s. If you are not using 6 "B" batteries the 40 volts bias is far too much. With three batteries it should be reduced to about 20 volts. For ordinary operation of course, you could use 45 volts by the double biassing. A really quite satisfactory scheme. ; 3. With the set came two pairs of for short-wave, but I have had no success with either the power valve or the 201A. A.: The last valve could have no effect ttpon the operaion of the short-wave coils. Try varying the detector voltage and the grid leak and if these do not bring about any improvement, communicate with the agents who sold you the set. 4, The set seems extravagant on current, both "A" and "B.". Would an alteration to other than 9 volts improve this? The set uses 140 vy. "B." A.: We do not quite get the strength of "the alteration to other than 9 v." The, best valve to use in the set would be the 221 type of valve instead of the 201A. They are very much lighter on current. and we recommend your double biassing the last stage valve,-that is use say two PB 605’s and 40 volts grid bias. This should make your set much more econom ical, 5. With regard to interference from electric noises, machinery, ete., I understand that owners can be compelled to instal some inexpensive gadgets to neutralise this. What stens can be teken? A.: It ean eertainly be neutralised quite easily by the use of h.f. chokes and by-pass condensers. The District Radio Inspector if called upon by you. will endeavour to assist in locating the snecific cause of interference. anil when this hes been discovered will communicate with the owner of such apparatus. intimating that the general laws governing the use of electric nower necessitate the installa- tion of such apnaratus as will neutralise the trouble. It is really a moral rather than a settled law. ¢6WIAKO." (Hauriki Plains): Your auestions in respect of the "Diagnosis Series" are very interesting and raise some fundamental points. We think the better plan would be to discuss them in the "Diagnosis" column. However, we can answer your number 3 question here, that is concerning the text-book that goes into theory in an elementary manner and Is not exnensive. This book undoubtedly, is "Wireless, the Modern Magic Carpet," and if you are at all interested

in the whys and wherefores, you thould _not be without a copy. V A.T. (Kaponga): I have constructed * the Beverage aerial with very poor results, I would like to get into touch with anyone who has constructed such an aerial, Will they please communicate with V. A. Tait, Kaponga, Taranaki, 667 IGGER" (Hamilton): We are very pleased to note the good results you are getting now that you refer to our: radio time-table. We hope you continue: to get good results. Now, about the selectivity problem, we think a wave-trap would not have the desired result. A wave-trap will cut out one station, bnt cannot be made to improve all stations, The best plan is to use a .0003 condenser in the aerial. That would slightly weaken the stations, but will certainly sharpen selectivity. Thanks for your concluding remark, but forget about it; but we do appreciate your sentiment very much. ¢*PENTODE" (Opunake) : I have constructed with success, the "Night Hawk" receiver. ‘Could a pentode be used to increase yolume? A.: Yes, you ‘could use a pentode, but it would be preferable to’-make an output transformer or an output choke, such as that described by Mr. Dawson in his series of articles a few months back. Only by so doing would you get really improved results, §¢¢NONCRETH" (Auckland): Recently my a.c. set has developed an intermnittent distortion on both music and speech. It is particularly noticeable on the upper notes. A.: Your trouble should really he located by a serviceman, as with an ae. set itis very difficult to tell you what to do. We suspect a bias resistance, or a punetured condenser on the audio side. When you had your valves tested, -vas the rectifier O.K.? er / "PADDY" (Christchurch) : Is jit possible to have a stage of s.g.r.f. followed by a three electrode stage of r.f,? Could I obtain a cireuit? A.: We cannot call to mind a circuit except the "Outspan Five." For the ordinary r.f. stage you could refer to the four valve Browning Drake. 2. Could you give me the connections for an eight-point switch to be used for a gramophone pick-up? A.: If you send us 4 more complete description of your set, we will sketch out the connections for you. ‘Ve could not, however, undertake to do it through the "R.R." It would have to ba dona through the post. subject of course, to- the conditions imposed upon replies by pust. ° 8. Would a 112A in the first audio stage and a 171A in the last stage be suitable for amplifying a gramophoue pick-up? . A.: No. You do not need a 112A in the first audio stage. Use a 221. CURIOS (Te Aroha) : Is there, ny way of cutting out overlapping oxN ations? Would the wave-trap describet’ in the "Radio Guide" be suitable? (Concluded on page 30.)

Questions and Answers

{Continued from page 14.) A.: Yes, but it would be preferable to use two formodensers, instead of the fixed condenser shown. A wave-trap will cut out only one station at one time, and you will have to alter it if you want it to ‘sharpen up the other station. ~~ — -- -~ ---__- -- -_

W J.H. (Wellington): It would * preferable to raise the lead-in en of your set. The experiment of cutting gut the insulators would be worth trying. ou may, however, pick up more nolse, and the set would be less selective. Yet there are occasions when long aerials are satisfactory. . , 2. Would an earth attached to a waterpie with a 25ft. lead be better than a in. copper tube wound with copper wire, buried to a depth of 5ft. with an insulated lead of 15ft. to the set? A.: We are inclined to think the latter would be the better, 8. The aerial and earth terminals on the set are ahout an inch apart. Can any leakage of waves occur through the earth instead of getting the full power to the set? Would binding with insulation tape be of any assistance? A.: Theoretically a little power does escape through the aerial and earth terminals being close together. It would be better if you used armoured cable where thev approached one another and grounded the armoury. However, we do not think, taking everything into consideration, that this alteration would be worth while, Af G.M. (Gisborne): Will I have to buy * a license if I get a erystal set? A.: Certainly, all radio-receiving gear from the humble crystal set to the mighty super-heterodyne must be licensed. However, if you already have a set, you do not need to get another license. J.A.H. (Napier): Could you give me * thg values of the following condensers :-Variable condenser with 17 plates? A.: It would depend upon the size of each plate. It is probably a .00025. 2. With 28 plates?-Probably a .0005. 8. Differential condenser, four moving and three fixed plates? A.: You do not state whether they are separated by air or mica di-electric. If they are mica di-electric it is in all probability a .0002. AvpIo (Blenheim) : Owing to my 40foot aerial being well screened by trees, my set is: not proving yery efficient on DX work. Would the addition of a resonator make any appreciable difference to long-distance receptio's? A.: It would help you slightly, but would not overcome the screening effect of the trees, E B.C. (Auckland): Could you supply * the circuit of a screen-grid super het, adapter using battery valves in place of a.c.? . A.: We cannot make any promise ip _ respect, but will see what we can 0.

IRMAN (Paerata}: Could you give me the qualifications necessary for (a) a "B" class station; (b) an experimental station? . A.; A "B" class operates on the broadcast band, and the license for such is issued to private firmy or persons who guarantee to operate for a certain number of hours, and adhere to those hours, An experimental station is issued only in special cases, such as to electrical firms or advanced students in radio tele- , graphy, colleges, ete. It is rarely granted to private individuals, and only when it is considered the applicant is sufficiently advanced in radio engineering to experiment profitably. . PETER (Pahiatua): Could I put a battery-operated r.f. to my set and use the parts of a commercial one-valve et? A.?: It will not be practicable to do so.