The 21LU8 is a Compactron triode pentode designed for colour television receivers. The medium-mu triode was designated for use as a frame oscillator and the pentode as the vertical deflection amplifier. The 21LU8 was designed for series heater systems operating at 450 mA the otherwise electrically identical 16LU8A was intended for series heater chains running at 600 mA. For equipment using parallel heaters the version was the 6LU8.
Although referred to as a triode pentode the construction is as a triode beam tetrode.
Face view of the triode.
The top mica: triode on the left with the beam tetrode mounted at 90 degrees to the triode on the right. Both sections have the control grids wound on copper supports.
The 12 pin base.
The envelope is 38 mm in diameter, and excluding the B12C base pins is 73 mm tall.
Reference: Datasheet. Type 21LU8 was first introduced in 1970.