The Z21 is a straight HF pentode and equivalent to the KTZ21 as they are different designations for the same design. The makers data state that it is particularly useful as a sensitive detector.
The Z21 was issued with B4 or B7 base caps.
The Osram brand label on this M-OV valve.
The view from the top reveals little. There are two sets of mica insulators for both holding the electrodes and for gripping the glass envelope.
The classic envelope is 41 mm in diameter, and excluding the B4 base pins is 102 mm tall.
References: Data-sheet, 1040 & 1043. Type Z21 was first introduced in 1937. See also1937 adverts.
