Questions and answers

Radio Record, Volume III, Issue 18, 15 November 1929, Page 27

Questions and answers

Defective Batteries. "G A." (Invereargill) states that since ¢ shifting his factory-built set and driving his speaker by means of a connection through the wall he has not had success. One of the valves will not light. \AS} Although the set worked before this alteration we suspect that the batteries. are at fault, the greater lead to the speaker will occasion a greater voltage drop, and. unless the batteries. are well up they will not be able. to sustain it. The ‘fact that the valve does not light indicatés either that the valve itself is at fault or the batteries will not supply enough current to light the filament. It is possible that the new position of the speaker is causing a faulty eontact, making a high-resistance connection, ; Failing to Log 4YA. "J F.O, (Nelson), who has: just shifted * into the district, states that he cannot receive 4YA in his new locality. ‘A: It appears that he is in a bad position. Hach station has.a zone in which it is difficult for a receiver to pick it yp, and it appears that our corresponent has happened into one of these. Maybe the set has been damaged in transit and rendered less sensitive. Power Transformer Interference. BH are in an unfortunate position, writes "Transformer" (Waitotara), for a power transformer close by occasions a great deal of interference. Can we do anything to minimise this in the way of altering the direction of the aerial? A.: Yes. The aerial should point away from the transformer. If this does not minimise the interference, erect a counterpoise, which in effect is another aerial slung under the main aerial at a distance from the ground just sufficient to be out of the way. See our article on "Interference" published a short time ago. , , A.C. Adapter. CouLtp you send me the circuit of an all-electric adapter and also explain what you mean by saying "the use of battexies were required?" writes "T.N." (New Plymouth). . A: All-electric short-wave adapters are rarely recommended for amateur construction and it is usual to employ one of the battery models. So long as battery current is used on the filament, eliminator current may be used on the plate. For the filament dry cells would be sufficient. A suitable adapter was deseribed in the "Listeners’ Guide," but omit the fixed condenser between "P" of ithe valve sockef#and A plus. Oscillation Trouble. . I HAVE a new kit set, writes D.M.L. (Hawera), but find that I cannot turn down the filament current without causing oscillation. If I wish to control ‘yelume I must detune. .: There may be several reasons for oscillation, as it seems that this is the cause of the trouble. There may be an open neutralising system (if this is used with the screen grid valve), excessive plate voltage on the R.F. valve, excessive grid bias on the screen grid or open bypass condensers between the plate of the

radio valve and the filament circuit. Bxamine these points carefully and if necessary apply the continuity test with either a pair of telephones or a voltmeter and a small battery. Excessive Filament Voltage. "DHEOSTAT" (Oamaru) has _ constructed the "Radio Record" "B" eliminator, and has had considerable success. He used the filament winding to light a power valve, but in this part of the business was unsuccessful. One valve was burnt out, and he now wants to know how he can test the filament potential. A.: It is advisable to obtain a reliable A.C. meter. These are readily obtainable and will read low voltages from half a volt to» about seven. They are, however, rather expensive,, and it would not be advisable to purchase one for a single test, The easier method is to obtain two 30 ohm rheostats and insert one in each of the leads from the. filament winding to the valve. Turn these into the maximum position and try another valve. If an old one of which the emission is almost exhausted is not readily obtainable get a torch bulb registered at four volts and try it in the circuit. If it burns out or unduly bright increase the resist‘ance, but be quite certain that the resistance will carry the current. Units for Cone Speaker. HAVE made a large linen cone speaker, writes W.M.B. (Matamata), but ‘I am unable to obtain a suitable. balanced armature unit with a reed of necessary length. With both Blue Spot and Baldwin units the reeds are too short. A.: Use either and extend the reed by tapping it and threading on a collar and an extension reed. Suitable tapped rods may be obtained from almost any radio dealer or failing these from Fear and Co., Wellington. waar is the difference between 66A and 66K units? asks J.B. (Nelson). A.: 66A is not adjustable, whereas 66K is. 66A and 66 Anchor differ only slightly. 2. If the rheostat is turned full on there is less volume when it is about half on. Is this usual? A.: There may be two reasons for this. Either the valve is, with the rheostat half on, heated to its maximum and further heating causes chdking, or with the greater emission with the rheostat full on, either the valve or the speaker is overloaded. This would first manifest itself by distortion. 3. If I disconnect the detector or r.f. lead I can still hear weak signals. Ts this correct ?-Yes. . 4. I have a four-pole 6-volt car generator which I wish to use for a battery charger. It works all right in a motor but will not generate. What is the cause of this? : A. The question is not clear. Does the correspondent mean that it will -work all right with the motor, but will not function satisfactorily when charging an outside battery. If this is the case, or if it will not charge at all, it is a ease for a motor electrician. 2RF Browning Drake. | I HAVE just finished the 2RF Brown- ) ing Drake described in the Radio

"Listener’s Guide," writes R.A.B. (Kaiaua), but am in doubt as to whether the end of the secondary winding of the secondary radio frequency transformer should be connected with A-- or A-. A. A negative bias.is always applied to the radio frequency valves so that the connection (the grid return) will be joined, to A- as shown in the lay-out diagram. 2. I have used .00035 condenser in all four position; how many turns shall IJ put on the aerial coil for one of these condensers? A. Use 24 gauge double silk covered wire and wind on 68 turns without any space. This would be the specification for all grid coils tuned with these condensers. 38. The reaction condenser does not have any effect. With the tickler in position as per instructions, the set is uncontrollable. A. Reduce the number of turns on this coil and include a by-pass condenser between the plates of the radio frequency valves and A- or earth. Screen Grid Detector Two. (CONCERNING this receiver, K.A.K. (Oamaru) asks the number of turns for broadcast coils for this set. A. As the aerial is tuned with a .0001 condenser, two coils will be necessary. Using 24 gauge wire D.S.C. the number of turns is as follows :-150-300 metres, 80 turns; 300-500 metres, 160 turns. Note.-There was a slight confusion in the text, follow the diagram for connections. Improving Selectivity. | Hew ean I improve the selectivity of my set? asks "D.F.B." (Dunedin). The. tuning condenser has 21 movable plates, with a honeycomb coil. I find

the local station, even when very low: power, comes in all over the dial A: The simplest way to improve the selectivity of any set is to build a wavetrap like that described in the ‘Radio Listeners’ Guide." Other methods are reducing the coupling between the aerial and the grid coil of the first r.f. transformer, putting the condenser’ in- series with the aerial, and shortening the aerial. 2. I have added a two-valve amplifier, but when I heat the filament to anything like the: correct voltage the hiss becomes too loud to hear the distant stations. I hear 2BL very weakly, ne stronger than with the single detector. The voltages and grid bias are O.K, and the set oscillates properly. A: There appears to be either a mistake in the wiring or a defective com ponent. Taking it for granted that you have thoroughly checked up the wiring and compared it with an authentic diagram we should suspect first the audio transformers. Try reversing the connections to the primary and test these by the continuity method. This commonly known as the ‘phones and cell has been several times described in the "Radio Record" and the ‘"Listeners’ Guide." Beating the valve past its allotted temperature will not increase the emission but will shorten its life. , Converting a Set to AC. M.K. (Wellington) has raised some * interesting points relative to the conversion of a three-yalve set to AC. He has enclosed a copy of his circuit, in which we have marked the changes -necessary to make this an all-electric. (1) My valves will consume 1.95 amps. If I wire the transformer with 18-gauge will this be sufficient,? _meseeEenue

a A.: 18gauge is rated to carry 2 amps, but it will carry an overload quite easily. It will be safe to run an extra valve, but to be perfectly safe wind the transformer with 16-gauge or two "Parallel windings of 18-gauge. . (2) Where does the filament centre tap go to, or is it left out? / A.: Filament centre tap is used to provide grid bias or reduce hum. When an amateur is constructing his own transformer, the most successful method of obtaining the centre tap is to place a centre tap resistance of about 40 ohms across .the filament wiring. This provides the electrical centre, which is more accurate than a mechanical centre. Battery Charger Problem. K A.H. (Lower Hutt) constructed a « battery charger, to the specifications in the "Radio Listeners’ Guide," Th. rectifying valves in use (828) become very hot, as does the resistance lamp as the rate of charging of this valve is 1.8. (329), an ammeter in the circuit discloses the fact that the rate js 1.75 amps, whereJt would seem then that the filament is being overheated, causing a greater emission. . This. will in a short time ruin the valve, so that it is wise to take a few turns off the filament winding to reduce the voltage. This valve when functioning properly emits a dull green glow. It heats, but not very much. The resistance lamp becomes quite hot, but if excessive heat is noticed, reduce the secondary voltage associated therewith. A.C. Amplifier. I AM constructing the amplifier described in the "Radio Record" (and in #Al] About the All-electric"), and I have

to deviate slightly from the specifications. The secondary of the power transformer will deliver about 300 volts. I. Is a drop of 50 volts about the correct amount to allow for a Raytheon rectifier?-Yes... _. 2. Do. you recommend the use of a potentiometer, value 50,000 ohms, between B -+ and B- to obtain the voltage for the screening grid of the s-g valve? A. Yes, This can take the place of the fixed resistance (usually 10,000 ohms) between B -+- detector and . 8. Do you consider it worth while trying a stage of resistance capacity coupling between the first audio stage and the push-pull stage? A. Yes, this would. be a decided improvement as far as tone and quality were concerned, but it would slightly diminish the volume. Note: Both output transformer and choke output are unnecessary. Put a 1 mfd, condenser in each of the leads to the moving coil and leave out transformer. " Output Problems. A.S. (Khandallah) asks what would be the output of two B405.valves in pushpull A. These are good valves for this purpose, and with 180 volts in the plate and increased bias the output should be all that will be desired for domestic use, roughly about 15 watts. Q: What rectifying valve at present on the market will provide 250 volts smoothed D.C. at a load of 75 milliampres? A.: The 280 type of valve is made for this purpose. Q.: What would the secondary voltage of the transformer to supply the value require to be?

A,: The only loss that"has,to be provided for is the voltage drop across the choke. ‘The resistance of the valve is comparatively small, and the ‘practice is counterbalanced by the extra D.C. obtainable when th AC peak .voltages are rectified. The resistance of choke should not be such that more than 50 volts is dropped. The AC voltage can then be 250+. 50, or 800 volts. If biasis to be obtained fromm the B supply, this also must be added on to the transformer secondary, ,Q: What is the emission of a valve? A.: The emission is not to be confused with the total emission. The emission in the example you cite. is the normal anode current, passing in the plate circuit when anode voltage and bias are adjusted. This will automatically explain a few of the difficulties you mention. Adding a Screen Grid Valve. J S.K. (Wellington) has adapted his * yveceiver for use with a screen grid in the radio frequency stage. He finds, however, that it will not work. He has followed the theoretical diagram with one exception. The lead from the plate on the sereen grid is taken to the low potential end of the following secondary transformer. This should be taken to the grid end. BPxamining a typical radio frequency

stage such as is shown in the diagram, the following position presents itself. The high frequency: current from the plate of the screen grid valve travels to the radio frequency ,choke, through which it cannot. " pass, Radio frequency or alternating current upon encountering a coil of wire is blocked by the inductance and high frequency resistance of the coil (impedance). It is like placing a very high resistance in the path of DOC _ current. The DC current from the battery can, however, go through this choke, as induction with this type of current is not present. All that matters here is the DO resistance and this is very low, probably only a few ohms. It is, however, blocked from earthing by the 2 mfd condenser between it and earth ~~ | To return to the path of the radio frequency current this must find its way to the grid of the detector valve, and it has only one path left and that through the blocking condenser (which prevents DC current from the choke from passing on to the grid). If this blocking goes directly to the grid of the detector valve a transference of energy will take place and the set should function. If, however, this lead is taken to the low frequency end of the coil, it is found that a coil of very high inductance, which presents a high resistance through the path of alternating current prevents the impulses from’ reaching the grid of the detector valye. They can, however, find a very much easier path through A + through the battery to A- to earth. The resistance both DC and AC of the good storage battery is negligible. Short Wave Adapter. I WISH to construct the short wave adapter described in the Listener's Guide. I have on hand some spare parts which I wish to utilise. My conde + rs are .00004 and a..00035. Could you give me the necessary coil data to cover the short wave band?

J A. .00004 is a capacity rather too small to be of any value in short wave tunifg. .0001 is the optimum capacity, and it would pay you to-obtain a condenser of this capacity. 2. Would a 500 turn choke be suitable? A. No. Watch for special article on the design of radio frequency chokes appearing shortly. an Daniels Cell Charger.. B.W. (of Waikato) asks for information concerning a Daniel cell battery charger described in these pages.’ Q.: What amount of bluestone is required? A.: It is uSual to employ a saturated solution of bluestone in a 10 per cent. solution of ‘sulphuric acid. ° Q.: Is the eopper electrode used in place of the carbon usually employed. ‘What would the output in amperes be per cell. A.: In a Daniel cell the copper. electrode, which can conveniently be in the form of a copper container for zinc and porous pot, corresponds .to the crgmons in other cells, and is the positive cdnnection to the cell. The output in amperes per cell cannot be definitely fixed, but it depends upon the size of zine electrode and the amount of acid and solution. The

size of copper has effect when the internal resistance of the cell is of account, and has to be of good area when heavy current is expected from the cell. When used as a battery charger. (rather an expensive method the battery ¢an be left on charge until the zine or acid is used up. Qne of the advantages of the Daniel cell is that, it does nét require time to recuperate after a few hours’ use, but gives’ a steady output until "ther the zine or acid is used up.