The 4033A is an interesting valve with the electrodes inclined at 45 degrees and it was used in telephone repeaters. It was the professional version of the PA1. The valve was introduced in the mid 1930s but was difficult to mass produce and was blacklisted from use in military or essential commercial equipment in WWII.
The red printed lettering on the envelope.
The inclined anode is supported on three stout wire struts.
The anode is blackened and has fins at the sides and the back of the central cylindrical cavity that contains the indirectly heated cathode and grid.
The top mica and anode. The single plate is folded to form the side wings and ribs have been pressed into the metal to add rigidity.
The balloon envelope is 56 mm in diameter and, excluding the B5 base pins, is 107 mm tall.
