The 4101D is the STC version of the Western Electric 101D telephone repeater triode from 1925. Standard Telephones and Cables separated from the parent American Western Electric. The Australian subsidiary made the same valve but their prefix was SY.
The filament is oxide coated.
The original 101D was introduced in 1921 and was still in production, in an improved form, until the 1950s.
The first 5,000 4101Ds were made with a seamed nickel base shell. This exhibit has a seamless shell and is thus later.
The nickel base shell has the STC identification pressed into the metal.
The construction features a glass support attached to the pinch the holds the filament tensioning springs.
The anode is a pair of plain bright plates, the grid is a pair of ladders either side of the filament. The base shell features a locating pip.
The glass support rod holds all of the electrodes in place.
The ladder-form grid and ribbon filament within.
The base has four short pins and the valve type identification. Short pins an a locating pip indicate a bayonet valve holder.
This image shows in detail how the glass rod is vital to the construction of this open structure. The filament is a pair of inverted V's.
The balloon envelope is 58 mm in diameter, and excluding the UV base pins is 98 mm tall.
References: Data-sheet & 1047 Type 4101D was first introduced in 1925.