TIPS AND JOTTINGS

Radio Record, Volume I, Issue 42, 4 May 1928, Page 13

TIPS AND JOTTINGS

FILAMENT VOLUME. CONTROL. S has been mentioned in past issues, dimming of filaments, either R.F. or detector, must not be carried to excess, or tone will ‘suffer. Dimming must not be carried too near the point where the valve ceases to. function, because distortion and bad -tone are liable ‘to be introduced. Detuning with the dials is satisfactory where a station. spreads over a: wide space on the dials, so that the detuned position is a number of degrees from the maximum. Rules cannot be laid down to exactly suit all receivers and all conditions, and ideas are given so that operators may find out which method best suits their case. "HARNESS" FOR A.C. SUPPLY. AMERICAN firms are now manufacturing what is called "adap--‘tex harness," by means of which an ordinary battery-operated set may be changed in a few minutes into on A.C. operated set without any struc-

tural or wiring changes. There is a transformer unit from which proceed several flexible cables, one for connection to the mains socket. To the other cables are attached several adapters, which are plugged into the respective valve sockets in the set for filament operation. Necessary | grid-bias is also provided. | D.C. GENERATOR LIGHTS FILAMENTS. "THE Day-Fan Electric Company, of Dayton, Ohio, has made a new departure in batteryless receivers. This firm is now turning out sets in which a small motor generator giving direct current is driven by the alternating mains supply. For silent running the generator is suspended by straps and springs in thé cupboard below the receiver cabinet. NEW A.C. VALVES. IVE types of Cossor mains valves will soon be available. The chief point of interest about these valves from a listeners’ point of view is that they can be used to replace the valves in any existing receiver. The only alteration in the receiver is the short-circuiting of the A positive and A negative terminals with a piece of wire. A pair of terminals at the top of each valve is then connected in parallel with the six-volt winding of a special transformer ‘which supplies the heating power. B current is supplied in the same way as previously, from eliminator or battery. BURNING THE SOLDERING IRON you will find that the soldering iron soon becomes burnt if left too long in the gas flame, and all the tinning becomes oxidised. I£ this has happened badly, it is necessary te remove the coating with a rough owe

file, and this soon destroys the iron. ‘If, however, a "tin can" be used as a small "furnace," the burning is, to a large extent, avoided. The tin can is placed over the gas flame, and becomes red hot, the "iron" being inside. Two iron wires can be placed across the can for the purpose of supporting the soldering iron. The tin can should not be too large, or the iron will not get sufficiently hot. It should be, roughly, a very loose fit’ for the iron. CONCERNING MICA. HERE is no other uatural substance, or artificial substitute, that possesses the same perfect cleav age, transparency, lack of colour when in thin sheets, flexibility, toughness, and resistance to temperature changes, or chemical decomposition. Finally it has exceptionally high electrical insulating properties. Commercial mica is usually either of the potash or magnesian varieties, known respectively as muscovite or phlogopite. Indian ruby mica is found to be the best dielectric for condensers, whilst silver amber is widely used for separating the commutator segments on D.C. dynamos and motors, The Loudspeaker. When the impedance of the foudspeaker is twice that of the last valve, the greatest amount of undistorted volume will be obtained from the valve. Thus the loudspeaker shonld at least ave an impedance higher than that of the last valve, An.output transformer of suitable number of turns will suitably increase the impedance of tlie tpeaker circuit, }