Questions and Answers

Radio Record, Volume III, Issue 27, 17 January 1930, Page 26

Questions and Answers

. Amplifier Problem. "A R.H." (Ohristchurch), asks the s following ‘questions relative to the construction. of an amplifier: 1-Is it. possible to build a 2-valve amplifier using one transformer? A.: Yes, the first stage is resistance capacity coupled. to the crystal A diagram has been forwarded. you. 2-Would 4 2-valve set halve the life of the "B" battery? A.: That is difficult to say. It depends on the types of valves used, and the length of time they are used, and the grid bias. It would probably about halve the life. 8-I am using.a 90-volt "B" battery with 3 volts on the "O’. According to the valve makers I should be using from 6 to 9 volts, but if I attempt to use this the signals are greatly decreased. Why? | A.: The application of grid bias acts ‘in the same manner as a decrease in plate voltage, but with this important difference, that while the plate current is diminished by the application of bias, the quality is greatly improved. The recommendations of the manufacturers no doubt refer to the bias required when 150. volts is applied to the plate. When the volume decreases, the quality should improve if the application of the greater bias is in accordance with the manufacturer’s recommendations. . The "Combination" Set. (oULD you supply me with particulars of thé "Combination" set published in the "Radio Listeners’ Guide?-"A.G.G." (Wellington). A.: Full details were published ‘in the "Radio Record" of April .12, 1929. The Two R.F. Browning-Drake. IN building the 2-R.F. BrowningDrake, must the coils necessarily be 2% inches in diameter?-"Raydio," (St. Kilda). A.: No, any convenient size from 2 in. to 3in. will be satisfactory, providing adjustment in the number of turns is made. See "All About the All-Dlec-trie," and the table of comparative inductances in the reference section of the "Radio Listeners’ Guide." A.C. Transformers. HAVE seen on the market lately small transformers to step down 230 volts A.O, to 6 volts D.C. Is this a miniature power pack? How is the A.C. stepped down to D.C. without a separate rectifler?-"R.G.H." (Wellington). A.: Alternating current cannot be stepped down to D.C. of a lower voltage without some rectifying device, This may be a metalic rectifier which takes only a few inches of space. AS we do not know the product in ques- tion, we cannot give any details, Robert’s Reflex Circuit. WHERE could I obtain a circuit for ’ the Robert’s: circuit reflex?-~ "T.C.W." (Pt. .Ohevaliér). ; A.: See the "Radio Broadcast" data sheets which should be obtainable from a bookseller who stocks wireless literature. Try Te Aro Book Depot, Wellington. ogi

. ° ? Coil Specifications, a | OULD you give me the number of | turns for a 2}in. former tuned with a .0005 condenser 24 "Y.c.c. wire? -‘Kauspanka" (Hawke’s. Bay). A.t 65 turns unspaced onthe secondary and 15 on. the reaction, Voltage Divider: Problem. I HAVE built an eliminator from the instructions in the "Listeners’ Guide," and can supply 250 volts for a 245 valve. My trouble has been to get suitable resistances to break down the voltage to 150 volts for a 227 type valve in the first audio stage, 90° volts for the R.F., and 50 volts for. the detector. Can you advise me. whe + can obtain a suitable voltage | ; -'P.W.H.,, (Palmerston North). A: The best plan is to put in variable resistances as shown in the "All About thé All-Hlectric," These resistances need only be 0-6,000 ohms. If, however, a voltage "divider is wanted, the following -resistunces should be. placed in order... Between the 250 and the 150 tap, 4,000. ohms; ‘between this and the 90 "tap, _3,500 ohms, followed by 6,000 ohms to the detector, and a "bleeder" of 2,000 ohms. The total consumption. of this résistance will be about 30 milliamps, and as the 245 will require about 30, the eliminator should be capable of supplying 60. ‘ 2.-The power transformer appears to affect the set in that when it is placed near it a decided hum-is heard. I presume the power transformer will require shielding. o A: Yes, and earth the shield. Obtaining More Volume, AN you tell me how I can obtain greater undistorted volume from my 7-valve all-electric, . factory-made set. . Would it be an expensive operation to have the set altered to take 245 type of valve in the last stage?"H.W." (Wellington), © -. A: It is not advisable to interfere with a factory-made receiver. It will supply a definite output when used with the valve recommendéd by the makers, or their exact equivalents. We should not advise any attenipt to adapt this set for use with the 245, as this would require great alterations to the power pack. ‘The use of the valves suggested by the correspondent to replace those already in ‘his set, would have little, if any, beneficial, effect. : ‘ }