The 33A/100A is a low power double triode. Peter has supplied the operating data for this valve but the attached data-sheet is for a very similar looking valve from the same company. The 33A/138A was not a direct replacement for this exhibit but is very similar.
The side view of the twin triode sections.
The base showing the paralleling rods for the filaments.
Looking at both sections gives the illusion of two grids. The anode construction is consistent with a power triode suitable for a power oscillator or frequency doubler or driver.
The tops of the grid supports are topped with cooling fins, again suggesting a power valve rather than a simple voltage amplifier.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 115 mm tall.