Questions and answers

Radio Record, Volume III, Issue 11, 27 September 1929, Page 26

Questions and answers

"B" Eliminator Problems. ¥OW is one to earth the core of a transformer when ouly outside strips can. be connected to the earthing terminal, the remainder of the laminations are separated from one another? asks "J.W.C." (Napier). A.: It will be quite sufficient to earth only the outside strips in the manner suggested. 2. On bolting up the core the outside strips have buckled up. Should this be so? A.: Put a heavier piece of steel or other clamping material across these to prevent buckling. 3. Is a resistance of u.0 watts necessary for the output resistance? It appears to be that 6 or 7 would be suftficient. ‘A.: With an output voltage of 200 a 10 watt dissipation would be necessary to allow passage of 50 milliamps.

4, What is the value of the grid bias for PM254 so as to bias them when in push-pull to the middle of the curve? A.: Biasing should be to the left of the mid point of the curve, and this value would be 222 volts. 5. What is the impedance of Philips PCJJ speaker? A.: Impedance varies with frequency: at 50 cycles corresponding to.a low note, the impedance is 2000 ohms. Charging with Leclanche Cells. N our issue of September 6 we replied to a correspondent who had omitted both name and nom-de-plume, by stating that "2 volt cells could not be economically recharged with Leclanche cells." Further to this, a correspondent, C. A. Pope, 406 Townsend Street, Hastings, wishes to communicate with the inquirer re some charging cells he has for disposal.

Varying Capacity. ISHING to adapt the Round- theWorld Two to the broadeast band, "D.McW." (Westport) suggests placing a .0003 fixed condenser so that it may be switched in series with a .0005 to reduce its capacity to .0002 at will. He asks if this will work. A.: It should work quite well as a compromise, but would not, of coursé, be as efficient on a short wave as a .0002 variable condenser. However, for an allwave set the idea is quite a good one. A Tuned Anode Booster. COULD the 485 type of valve be used in an R.F. booster? asks "Backblocks" (Glen Murray). A.: Yes. A diagram will be published which. illustrates the point and answers your other questions. "Pentode’s" Crystal and Valve. "J. O." (Auckland) asks the following questions :- 1. Would you publish a circuit of this receiver incorporating the combination moving coil and condenser control _reaction? A.: The diagram will suggest the combination.

_ 2. How can a double grid valve with a plate voltage of only 22} possibly give the ow notes? , A.: It cannot. 3. Which will be the best type of speaker to use with a single valve with 90 volts on the plate? A.: Usually a medium size, but you should try out if possible different types. A sensitive cone would give you the best results. 4. Is it not undesirable to earth both the primary and the secondary of the transformer? A.: It is desirable to do so, unless results prove otherwise. 5. Is a 3/1 transformer sufficient for the set? A.: Yes, but a higher ratio would give higher amplification. Improving the Audio. "WADIO FIEND" (LHalcombe) asks the improvement of his audio stages: 1. Could I connect two power valves in parallel and the last stage of audio? A: Yes. With improvement, but push-pull would be much better. 2. Would it be worth the trouble of installing ? : No. Much better to carry out the first suggestion. . . 8. Could a screen grid valve be used in a one-valve set? A: Not without seriously «© ‘aplicating the circuit; even then it is doubtful if it would be a success,

4. How can a T-wire ¢able be connected so as to give four separate tappings for grid bias? It ecannot-three are the most it can take, unless the number of "B-}-"? tappings are sacrificed. Three different bias tappings are surely unnecessgy in an average set. . , Neutralisation Trouble. HEN trying. to turn on the volume signals become suddenly weak, states "J." (Napier). I have a good aerial and earth. A: It appears as though the set is not neutralised properly, though there may be other causes that could be detected only by inspection. The Tetrode Amplifier. I HAVE constructed the Tetrode Amplifier described in the "Listeners’ Guide." Would this be suitable for a 2-valve amplifier? asks "G.B.C." (Auckland). A: Yes, it should be quite a good circuit. If more volume is required, construct the tetrode set with reaction described a couple of months ago. . ¢ Carrier Waves Only. CAN get the carrier waves of about 8 stations, states "C.Q." (Auckland). but I am unable to get them at any strength. My set comprises detector and audio, and when I place my finger on the grid terminal of the detector valve, a howl results. I have altered the grid leak to no avail. A.: The fact that a howl results when the grid is touched indicates that the audio side of the set is in perfect working order. Examine carefully the aerial circuit for loose terminals, and dirty or poor contacts. See our reply to a correspondent in last week’s "Questions and Answers." 2. When I turn the dial on the reaction condenser with the aerial disconnected a howl results. What is the cause of this? A.: When the aerial is disconnected the load is moved from the grid circuit, and howl results. Interfering Morse Station. HAVHB been troubled for the last week or so with an interfering moyse station on about 1000 kilocycles./@ is so close that it is impossible to hear any station on the broadcast band except 2YA.-Newtown. A.: Probably H.M.S. Dunedin, and if so this trouble will have ceased by now. If still continuing communicate again and steps will be taken to have the trouble rectified. Fading on a Crystal. HAVE an all-wave crystal set, writes 7 "All-Wave" (Dunedin), and toDunedin’s silent night, I am listening to 2YA, but very bad fading makes listening difficult. Can anything be done? You are getting excellent results for a crystal set, and at that distance. fading will be accentuated. We take for granted that you have examined very carefully for loose or dirty connections in both antenna and earth circuits. A Suitable Portable. CAN you recommend the Rotorua port~ able as described in the "Radio Record" as a set for the summer months?

! -A,: Yes, we can give it the best reeommendations, but within a month or gix’ weeks we will be bringing out a special: portable number, in which we hope to describe a screen-grid portable set. An "A" Battery Eliminator. CAN I break down the voltage from the detector tapping of my "B" eliminator to 6 volts to supply .75 amps to the "A" circuit? asks "C.K.A." (Dunedin). , A.: You can break down the voltage, but if it were connected to the filaments of the valves, the resistances within the eliminator would be burnt out very rapidly. A good eliminator supplies 100 milliamps, or .1 of an amp. 2. Is it possible to convert a trickle charger into an "A" eliminator by the use of condenser blocks and at the same time operate a moving-coil loudspeaker? A,: Trickle chargers are made to supply .5.amp. Your valves require .75, and the speaker about .5, making a total of 1.25'amps. The particular charger under discussion, however, will supply current at two rates: .6 and 1.3. It can be seen that it will be able if it can be sufficiently smooth without chokes, that this will suffice. To smooth the "A" current, follow the instructions in last week’s special issue. Question of Selectivity. "QONORA" (Wellington) asks if he can shorten his aerial to eliminate 2YA. A.: This will have an effect in the direction of the desired effect, but, as he suggests, will weaken reception slightly. The aerial should not point to 2YA. wave-trap is your best solution. 2, They compare one with the other favourably. .