A.C. Detectors

Radio Record, Volume III, Issue 25, 3 January 1930, Page 30

A.C. Detectors

--- -w a 2 Avoiding Hum NE of the difficulties éngountered when an alternating eurrent powerpack is employed and also valves having heaters supplied with alternat-.-ing current) is of elimingting the last traces of hum. It is a comparatively ‘easy matter so to arrange the circuit and the parts used that there is very little hum, and it is only when efforts are being made to-reduce or cut out. this last trace of hum that every part of, the circuit must be carefully examined; Particular care is necessary wh the detector, and the question: as¥ to whether leaky-grid or anode-bend is to be used is sure to crop up, for the reason that when a leaky-grid detector is employed there is almost bound to be a certain difficulty in cutting out the hum. This is beca.se the wire joining the grid condenser and leak with the grid of the valve is extraordinarily sensitive to stray fields, and the valve itself may collect hum or noise. It is easy to prove this by placing . the fingers near the detector valves. I have found that no effect is pro-duced-by placing the fingers on the top of the bulb of a certain type of mains valve, but that a loud hum is set up as the fingers are moved along the bulb towards the base of the valve. . The reason for the leaky-grid type t" of detector being so liable to pick up hum is because between the grid wire (and the grid of the valve itself) and the filament is a_ relatively high impedance, in the form of the grid condenser. When this condenser is removed the grid is in direct connection with the filament through the tuning: coil and low-frequency voltages, therefore, cannot be set up on the grid. It follows, then, that the anode-bend ° type of detector is much less sensitive to hum than the leaky grid type. It may not be convenient to employ anode-bend detection, however; then one has very carefully to arrange -the position of the detector valve itself, and also of its grid condenser and leak. The grid leak should be joined directly» between the grid terminal Mf the valve holder and the __ filamjent circuit, and the grid condenser s connected. with as short a wirt # possible. Different valves of leak resistance should always be trted, of course.