Questions and Answers

Radio Record, Volume II, Issue 41, 26 April 1929, Page 12

Questions and Answers

In Brief. -P." (Oamaru).-Before we can answer your questions, we would greed to see a diagram of the circuit. Defective Power Unit. "J S.C." (Nelson) writes: "I built the e 2-R.F. Browning Drake, but it was not altogether successful. I had difficulty in ‘neutralising it to my satisfaction, so I changed it, making the following alterations: The aerial coil comprises 58 turns tapped at the 18th from the ground end. The radio frequency transformers, primary 82 turns centre tapped, 80 gauge D.S.C., and the secondary 85 turns of 22 wire on a 38-inch former. The plates of the radio frequency valves are connected to the top end of the primaries, the neutralising condensers to the bottom, and the centre tap to the "B-+-" R.F. Between "A-" each of the detector and the first audio valve and the "B’s" of the first and second transformer respectively I have connected a 2 mfd. condenser. In the final stage I have incorporated push-pull. "T am now getting excellent results, especially regarding tone, selectivity, volume, and distance, but I am getting an excessive milliampere drain, As I place the valves in their sockets, I notice that the first valve draws 14 milliamps. The two or three following make little or no difference, the meter reading 14 until I come to the two power valves in pushpull. These four valves are all connected to the same tap of my eliminator. .: The particular eliminator supplies taps. for "B-," "B+223," "B+4+-45," "B490, " and op 4 136, t Between "B135" and "B-" is a resistance, and the tappings for the other valves are taken off along this resistance, so that it would appear "that towards the negative end of this a break has occurred, and full power eannot be supplied. The first valve should read 5 milliamps. only. but as it draws 14, the following valves are not getting their share. The correspondent’s best plan would be to take his eliminator to a radiotrician and have it tested. Failing this, the set would need to be overhauled by an expert. A Case of Overloading. T appears to be a case of overain the last valve has occurred to

"J.L.S." (Waihi).. He has a five-valve neutrodyne, and is using five 201A valves, with 43 volts bias on the last valve. When handling volume, the set distorts. He has recently brought the anode voltage to 135, adding 9 volts grid bias. This arrangement, he adds, has made a wonderful im-

provement in tone, and now great volume is handled without distortion. We are asked if this would be as effective as a 112 valve. The 90-yolt connections and the 44-volt grid bias is common to both audio valves. Is this satisfactory, or should the first valve have less grid bias? What change should be made in circuit connections to bias the second last valve? A.: No doubt increasing the anode voltage and applying correspondingly greater grid bias to the last valve would improve tone, but if the correspondent were to use say a 171 with from 20 to 30 volts bias, he would see the remarkable difference in tone. A 5-valve set will take such a power valve quite easily, and it would be bet-

ter than the 112, while this would be an improvement on the 201A for the last stage. second last valve could be a 201A and bias should be added in the following manner: On examining the set it will be seen that the G.B. negative goes to the

audio transformers. Disconnect the lead to the second last valve and take it to -44; connect the positive to the C. positive on the binding post. . What increase of plate current there be with 135 volts and 9 volt G.B. over the 90 volt and 43 volt? A.: The difference will be almost negligible. Will I need an output filter with 135 volts and 9 volts G.B.? A.: It is not essential, though it would improve tonal quality. : I have a 1-valve reflex which I believe is radiating, for reaction acts very yA.: Reduce the number of turns on the tickler, and when there are no stations broadcasting, try advancing the reaction until the set oscillates. A plot will indicate this. Find the oscillation point for different parts of the dial, and make a note of these, so that. when listening in there is no need to make the set oscillate and cause a howl. Details For a Charger. "DECTIFINR" (Hitham), in asking for details for an "A" and "B" charger also makes the following inquiries :- 1. Would a 201A valve be as good a rectifier as the chemical rectifier, viz., aluminium and lead? A.: It would have a smaller percentage of efficiency, but chemical rectifiers are not as satisfactory as valve rectification, though they are considerably cheaper. The 201A valve is not suitable where any amount of current is to be handled. 2. What current in amperes would the 201A pass? A.: Approximately .01 amperes. 3. Could it be used in an A and B charger ?-No. ; Several requests have come in for an A and B battery charger, so ‘"Pentode" will describe one in a future issue. Valve rectification will be employed. Applying the Screen Grid. T sending us his subscription for the new Listeners’ Guide, "H.A." (Waiuku) writes: I have made up the shortwave receiver described in your paper, ‘but as it was not a great success, I changed it into an all-wave receiver, and am getting splendid results, both for tone and quality. I am now looking forward to making it a whole-world all-wave model. Could you supply particulars of .guch a set?

A.: Yes. As many correspondents have been asking for details of such a receiver, "Pentode" has agreed. to: describe onethe "Wonder All-Wave" receiver, comprising screen grid, detector, and ‘audio, which may be a pentode. ws Could a screen grid valve be substituted for one of the radio valves in the. shielded-5 Neutrodyne. If so, what alierations would be necessary? .: Yes. The screen grid could be quite well applied to the "shielded-5," the alterations necessary being described .in a previous issue of the "Radio Record." The screen grid would have to go in the first stage. The last word in the applieation of screen grid to the broadeast receiver has been incorporated in the. "Adapted S.G. Browning-Drake," described in the Radio Listeners’ Guide. Sizes of Coils. "R G." (Denniston) wants to know * the tuning range of his shortwave coils, but he has not given us sufficient data. We can work out the range of any, coils provided the following details are provided: The size of wire, \its’ covering, the size of former, the distance between the first and last turn on the secondary coil, and the tuning condenser which operates with the coil.

Concerning Accumulators. I HAVE a new accumulator which I charge at 2 amps., but cannot get it high enough to read 1300, although after gassing freely it reads 1250 and each ceil 2.5 volts. When the battery has been taken off the charger and placed in the set, the panel meter reads 6.5, but after the set has been working for about five minutes, volume decreases until the meter shows about 5.5 volts. On adjustment of the filament rheostats, volume remains steady. Should this be so? Occasionally I notice while the set is working that the meter flickers -‘"A.W." (Auckland). A.: 1250 specific gravity is the normal for accumulators, and the rapid fallingoff in voltage until a steady value is attained is quite normal. The flickering of the needle would seem to indicate ~ a loose connection in the filament wiring. At the same time, look over the set for a short circuit. A Short-Wave Adaptor. I HAVE a factory-made set, and I wish to know whether it is advisable to instal a shortwave adaptor or to purchase a shortwave set.-‘"N.W.A." (Wairoa). A.: It is rather difficult to say off-hand. Some sets suit an adaptor more than others. The best plan would be, if possible, to try an adaptor on the set before purchasing it. Usually, a special shortwave set works better than an adaptor to a broadcast set. "Pentode’s" Dynamie Cone. " D.C." (Waipawa) writes: Could two good magnets be used instead of the field pot specified? A.: No. An electro-magnet such aj created when a coil of wire passes round, a soft iron core is very much more powerful than any bar magnet, and a very powerful magnet is essential to a moving coil loud-speaker. I am thinking of making the electrolytie trickle charger for charging "B" batteries. Would it be suitable? The trickle charger was not intended to supply heavy current and is not so efficient as a valve rectifier such as "Pentode’"’ will describe. It was a good cheap charger. The same correspondent has described a method of neutralisation which will be elaborated in the "Beginner Corner." "Pentode’s" Trickle Charger. " C.M." (Auckland) has made the . trickle charger but has not found to work satisfactorily. He is using a step-down transformer which delivers 10 volts, and it appears as though he is not getting sufficient eurrent. The _ transormer is rated at 1 amp. at 10 volts. Would it improve matters if I removed

CEC (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 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. PUTTIN ITTV TT TT ATT TTT TTUTTTTTTATATTTAT ATE TU TTA AT ATA gE

the secondary winding and put on more turns, or should I procure a larger one? A: is unnecessary to purchase a new * one, simply double the number of turns on thé secondary. The transformer will then deliver about 20 volts, which will give a. higher charging rate. To Prevent Distortion. "RACK BLOCKS" asks the following ; questions :- My three-valve set distorts when it has to handle any volume; would a pentode in the last stage prevent this? A.: A power valve would be the better; see the answer to J.L.S. and the article on the pentode in the latter pages of this issue. What is the price of a pentode?Usually 37/6. Would you recommend an alteration in the value of the grid leak? | A.: A variable grid leak is recommended only in the case of "all wave" receivers, and where distance is to be ° etined, It could be mounted on the ’. * Mounting a Milliameter. A CORRESPONDENT from Norfolk Island asks: "In what position could @ panel mounting milliameter be mounte F add A.: Mount it on the panel so as to give balance as far as the other components thereon are concerned, and connect one leg to B-- power and the other to the positive terminal of the speaker: "Should a one mfd. condenser be placed across the B battery whether this be wet or dry?" A.: Yes, it is advisable. An All-Wave Set. "W M." (Hawke’s Bay) asks the following questions to the combination one-valve receiver described by "Pentode" recently. First, could it be made into an all-wave set. If so, how? . ‘ A.: It would be rather complicated, 4 but "Pentode" will describe a very simple one shortly. Could a "B’’ eliminator be used ?-Yes. Would "C" battery improve reception. If so, where will it be applied? A.: Yes, slightly. Break the connection between the transformer and the rheostat, and put the battery in here, the positive to the rheostat and the negative to the transformer. How much wire is needed for wiring in feet, and what is meant by insulating sleeving? A.: Insulating sleeving (or spaghetti) is composed of an insulating material built up in the form of a narrow tube (like household spaghetti) and this can be slipped over the wire to insulate it properly. About 64 feet of wire would be necessary to wind the aerial and the reaction coils. Are the measurements for the panel correct, or should they be 12 x 6? A.: A printer’s error. The size should have been 12 x 6 x 3/16ths, but the mistake is, fortunately, obvious. -~What are the measurements for the bpseboard ? . »A.: 12 x 9 should be sufficient. ‘What gauge wire should be used for the coils and how much? A.: A quarter of a pound of double cotton covered wire should be ample to ecomplete the job. Screen-Grid Booster. "CON STANTINOPLE (Matamata), has : constructed "Megohm’s" screen booster, but cannot get it to work. He has altered the coil to a squirrel cage type, and wonders if this has anything to do with it. Signals are no louder than without the booster, and at the same time has become very unstable. The valve lights brilliantly for :a while and then glows to normal again; ‘if the booster is shaken the same thing /will happen. "I think the filament must come into contact with the control grid." A.: It certainly appears as though the valve is at fault. Have it tested and at

the same time check over all the filament wiring. It may be that the electrodes of the valve are unsteady, and there may be a bad connection in the filament wiring. ‘The correspondent has not stated how the booster is applied to his set. It may be that it is incorrectly coupled. The different coil should not affect this. Changing the Value of a Condenser. "W B." (Hastbourne) asks how he * might convert a .0005 condenser into a .001. A.: Connect a .00025 fixed condenser in series with a .0005 variable, or connect a .001 fixed condenser in parallel with a .00025 variable condenser. There TT TTUNAT TIT MOAT TL TTATUT I TUTTANHATT UT GTGTTTAT TNCHOATITAUATATATUUGEHEAT Ge La CSTV UT

SUUUININO.UEIUUTUEELAGRANULOSENSU ULERY is the alternative of removing plates on a .00025. The number of plates for different values was published in the "Beginner Corner" some time ago, and will be republished. An Eliminator Transformer. "7 RT," (Hataitai) has built a bat- ¢ tery eliminator, and wonders why it will not work. He includes a diagram which is all right except -for the fact that the transformer has no secondary, and the valve is an ordinary receiving valve instead of a rectifying valve. He states that if he places his fingers across the terminals marked "high" he can "get a good kick," and yet it won’t work the set. A.: We are not surprised that the correspondent should get a "good kick," seeing that he was taking 230 volts unrectified. A secondary should be wound on the transformer and this secondar) winding centre tapped and connected to a rectifying valve. Full details of a good eliminator will be published in the "Listeners’ Guide." . Sundry Points. "W N." (Auckland) asks for an all- * wave circuit. One will be described by ‘"Pentode." Would such a set give louder signals than a detector and one stage audio on both broadcast and short waves? A.: It should do. Would a wave-trap be necessary? A.: On the short waves, no broad tuning with a screen grid employed in a broadeast receiver is the great. difficulty of that valve, so that a wave-trap would be almost essential, unless devices were incorporated in the set to overcome this broad tuning. 6 ALL-WAVER" (Wellington) wants particulars of an all-wave set. He is referred to previous inquiries. Incorporating the Screen Grid. ‘ C.D.T." (Katikati) asks for details * to apply a screen grid to the Browning Drake. Details were given in Vol. 12, No. 37, but the full details will ye given in the "Radio Listeners’ Guide" n the "Adapted Screen Grid BrowningDrake." . New Zealand Broadcast Stations. ‘Pp AH" (Timaru) writes: Can you tell me where the following stations yroadecast from? :- 4ZO (Barnett’s Radio Supplies, Dunedin). 4ZL (Radio Service, Dunedin). 4ZM (J. B. MacEwan, Dunedin). 2BH (Sydney). 9XL (Schenectady, New York). ~ WGNR (probably WENR, Chicago, TIilinois). ‘ Detector Tapping. W" (Runangat) writes: I have an eliminator, but am not quite cer- —

tain where to put the brown covered wire from the set. A,: This is usually: the detector tapping and should be placed on the 224-45 tapping. Details of a Horn. "A.D." (Christchurch) writes: Could you supply the details for a horn for a Baldwin unit? A.: The best plan woud be to build an exponential horn as described by "Switch" some considerable time ago and recently described in the issue of January 11. ULAUUELGNUNQIOUUENSUALQUUOSDUGSUGUALIEEULUA LAST AHA

IUNQSDLARDAGDOM AOE UES NSRP A Noisy Rheostat. A RHECISTAT is one of the most simple yet one of the most important pieces of radio apparatus. It can act as a switch, and it is indeed advisable that it should be used as a switch, and it can be used to dim the filaments of the valves, to leugthen their life, and to reduce volume, although this latter is not particularly wise, as a certain amount of distortion is introduced when

the filaments are turned back yery much below their stipulated voltage. A defective rheostat will cause an ennoying crackling in the receiver when adjustments to fit are made. There is nothing more annoying in operating a receiver than to find the filament control is "wobbly" or hand to be "nursed" ia order to prevent loud crackling noises. There is no need to endure this annoyance, for a new rheostat can be purchased for a very trifling sum, and fully repays such an expense. But it is not always necessary to go to this expense. Adequate pressure of the spring upon the resistance element-but not undue pres-sure-are essential points in the smooth operation of the rheostat. Wxamine this piece of apparatus if it is giving trouble, and see that the moving arm makes a good solid contact with the resistance. Press it down into position, or bend it slightly to obtain smooth but adequate pressure and operation. .

ENQUIRIES HELD OVER.

Many inquiries and suggestions relative to the Screen-Grid Valve have been advanced during the week. These have been held over and will form the basis of a future special article.