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Marconi and Osram H11 and L11.
The photograph speaks for itself when the subject is Midget Valves.
Two valves of particular interest to constructors of midget apparatus have been announced. They are Marconi and Osram triodes with filaments rated for a consumption of 0.1 A at 1 Volt. This unusual rating is brought about through an endeavour to obtain extreme economy and yet retain a robust filament. It is intended that a pair of valves be used with series-connected filaments and operated from an accumulator, or, alternatively, that the filaments be paralleled for heating from a single dry cell.
A marked absence of microphony is claimed to be a feature of these valves, and this is partly due to the robust filament which is used, and partly to the use of an absorbent rubber pad in the mounting of the glass envelope on its base. The valves are not fitted with the usual four-pin base; instead, the base carries four contacts which press against tongues in the special-holder (Ct4). Some saving in space is thus obtained, and this is accentuated by the small dimensions of the glass bulb. The diameter is only 17.5 mm, and that of the base 25 mm, while the overall length, including the base, is 33 mm.
The smallness of these valves renders them invaluable for use in compact receivers, and they have been particularly developed for use in deaf-aids and police receivers where small size and weight are essential features. Electrically, these valves are less efficient than normal types, and they are rated for a maximum anode potential of 100 Volts only, but are intended for normal working at 45 Volts. The L11 has an AC resistance of 12,500 Ω and a mutual conductance of 0.4 mA/V, while the H11 has a resistance of 30,000Ω and a mutual conductance of 0.5 mA/V.
It is understood that supplies of these valves are now available, and that a special holder for them is being marketed by Messrs. Wright and Weaire, Ltd., of 740, High Road, Tottenham, London, N17. The price of the valves has been fixed at 15s.
See The extras menu for more details.
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