The Experimenter

Radio Record, Volume IV, Issue 21, 5 December 1930, Page 15

The Experimenter

In response to several requests, we are opening a section devoted to the interests of the experimenting amateur. Tested ideas received from enthusiasts will be published and acknowledged. Experiences of listeners relating to points raised in our Questions and Answers column will be welcomed. We, however, cannot undertake responsibility for any of the ideas published.

Connecting a Pick-up HEN using a radio set as a . gramophone amplifier, the pick-up may be brought directly into the grid circuit of the detector of the radio set. A volume control is necessary in practically every gramophone amplifier, and this is best included between the pick-up and the grid. If, however, the pick-up is not connected in the right way to the potentiometer (which forms the volume control), the tone may vary as the volume is regulated. The simplest and one of the best ways of ~ including a volume control is to connect

one end of the latter through a fixed condenser (of about .0005 mfd. capacity) to one terminal of the pick-up, the other end going to grid-bias negative, while the glider is connected to the grid of the valve. Earphones as Substitute on L.S. HE following extract from "Dun’s International Review" for September, 1930, may be worth a paragraph: "Disconnect one lead going to the loudspeaker (one of the leads to the voice coil in the case of the dynamic) so as to insert a table type clarostat in the circuit. The connector block of this device then takes the tips of the usual earphone cords. By turning the clarostat knob to decrease resistance the loudspeaker plays full volume and the earphones are inoperative. By turning the knob to increase the resistance, however, the earphones become operative while the loudspeaker vyolume is materially reduced to inaudibility." I have not as yet tried this method of providing earphone reception in combination with a set and if you consider it worthy of a test I would be interested to learn the re-

sults.

H. E.

Chapman

(Wellington).

Blocking Condensers WHERD condensers of small capacities, but very high voltages, are required, such as "plate blocking" conGensers in high-power transmitters are required, procure those condensers to be found in magnetos. They have to stand very high voltages and back e.m.f. Another "kink." Where a resistance is required to dissipate some considerable heat, procure from one’s local electrician the porcelain element "formers" as to be found in

heaters and radiators, and wind the required resistance on the same. One could have several to "plug-in" or screw-in bases, thus obtaining several values as required by various valves."Microamp." Eliminator on Short-Wave ME. SELLENS, our short-wave correspondent, writes:-For short waves "B" batteries are a thing of the past. I have a commercial eliminator but experienced a certain amount of hum which has been overcome. It was: (1) An audio transformer which has.since gone "West" and been replaced by a first-grade article; (2) set wiring quite O.K. on battery, but attention required for mains working and, lastly, a B38 choke in detector lead bypassed with a 4 mfd. condenser. Result: Phones, after first audio, no sign of hum or background, with almost as much "kick" as two of audio before. Phones, second audio, some slight hum, but not safe for tuning by as morse stations come in at deafening strength, so I do all searching on first stage and plug in speaker on secondif signals strong enough. An Improvised Choke . N excellent home-made s.w. choke may be made by winding about a hundred turns of fine insulated wire around an ordinary test-tube. A dab of sealing-wax or of battery compound on the windings at each end will keep the winding in position. The precise number of turns to be included in the winding depends, of course, on the con-_ ditions under which the choke is to be used, but is, generally, not critical. Drilling Precautions DEFORE using a centre punch on an ebonite panel, care should be taken to ensure that there are no small screws or other obstacles on the bench beneath. If this precaution is omitted, it is quite possible that the panel will be split in half. When performing the actual drilling, it is a good plan -to place several thicknesses of tissue paper underneath the panel to protect its surface,