The 25A6G is an audio output pentode that was designed for single ended use in universal sets. The heater is designed for series connection across the mains supply, and the anode ratings reflect the low HT available when no mains transformer is available.
The internal coating is reasonably thin and the electrode assembly shape can be glimpsed.
In this enhanced image the basic structure can be seen. The screen grid winding is visible between the lower mica and the base of the anode. The insulation on the heater wire looks as if the heater loop is made from a coil of fine wire. Such heater wire was made by winding the thin tungsten wire on a molybdenum mandrel and subsequently dissolving the mandrel away.
The top view: copper control grid supports with heat radiating fins either side of the round cathode tube. The top wire of both outer grids can be seen as well as part of the anode.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 102 mm tall.
References: Data-sheet & 1040. Type 25A6G was first introduced in 1936. See also1936 adverts.
