The 1A3 is a signal diode and features a 1.4 Volt indirectly heated cathode. Normally dry battery valves are directly heated. Such a valve would find applications where the cathode is not taken to ground or where the filaments of other valves are run in series from a higher voltage supply. Two such valves could be used in a discriminator in an FM radio in the 1950s.
The KB/D identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the D identifies the maker as Mullard Radio Valve Co., Mitcham.
The diode is the small structure near the base. The rest of the envelope is empty. The two vertical rods hold the getter holder ring.
The heater wire is a single insulated thread running vertically through the anode cavity. Note the connection to the cathode.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
References: Data-sheet, DA90 & 3002 Type 1A3 was first introduced in 1943. See also1943 adverts.
