Tips and Jottings

Radio Record, Volume II, Issue 9, 14 September 1928, Page 28

Tips and Jottings

Crystal and Amplifier. RUNANGA (Greymouth) constructor, who is a beginner, has built the crystal receiver with amplifier described on July 6, using a Harlie crystal, and reports getting 2YA on several nights as his first station. This correspondent inquires as to the maximum capabilities of the receiver, but this cannot be given, as there are so many factors to take into consideration. A receiver such as this is quite capable of sometimes receiving Australian stations, conditions all being favourable, but no guarantee of any kind can be given. Several experiments showed that the specified number of aerial turns gave best results. The Last Valve. [Ff it is suspected that the power valve may be losing its emission, the point can easily be settled by any listener who owns a milliammetre reading up to 20 or 30 milliamps. A reading should be taken when the valve is new by placing the meter in the plate circuit, and if the indication agrees with the plate current stated on the manufacturer’s leaflet at the particular voltage, all is well. Readings taken at subsequent intervals should show the Same reading under the same conditions, and when a drop is recorded, if plate voltage is correct, the valve may be at fault. Drilling Ebonite. A DIFFICULT thing to do is to drill ebonite with a hole over 1-8in. in diameter, and finish with the hole in the exact spot required. The shape of a. morse twist drill at the point makes it very inclined to cut more to one side than the other, pulling the drill from centring on the punched

mark, and thus displacing the hole from its true position. After punching a mark with the centre-punch, a smalJl drill may be used to enlarge the dig’ pression, and give the larger drill’ 2 start. This depression should have a diameter not less than the straight portion on the point of the large drill to be used. A rose countersink can sometimes be used to advantage to enlarge the punched mark before drilling. Some such precaution is very necessary when drilling a set of holes such as may be required to take four contact pins for a short-wave secondary and tickler. Aerial Stays Not Insulated. {t is not necessary to use insulators in aerial stays unless the aerial is only a few yards from a transmitting aerial. In such a ease the insulators are inserted not for the benefit of the broadcast aerial or reception, but to prevent them from absorbing energy that should radiate further afield from the transmitting aerial. The stays of transmitting aerials are equipped with insulators for the same reason. There can be little gain by inserting an insulator in the centre of a stay, the top of which is connected to an iron mast whose lower extremity is buried in the ground. Amplification Factor of a Valve. HE amplification factor is the measure of the control exercised by the grid upon the plate current, and is very easily found from characteristic curves. If the readings obtained show that a rise of .5 milliampere. in the plate current is produced by) an increase of 10 volts in the plate voltage, and that with the plate voltagie_ steady a similar rise can be by making the grid 1 volt more positive, then changes in the grid voltage have ten times the effect of changes in the plate voltage and the amplification of the valve is 10.