The Type 43, is an American design of audio output pentode for single end use in receivers with a series heater chain. It is electrically identical to the IO based 25A6.
The Brimar brand name of STC on the base. The coating within the envelope obscures the electrodes but the RCA Type 43 has a clear envelope.
The base of the anode can be made-out through the glass. The internal space is cylindrical with side flanges out to the support rods. The wire grids can be seen as can the multi-loop heater.
The classic envelope is 44 mm in diameter and, excluding the UX6 base pins, is 98 mm tall.
References: Data-sheet, 1040 & 1043. Type 43 was first introduced in 1932. See also1932 adverts.
