Sensibly equivalent¶ to:
The X21 is a heptode frequency changer. This self oscillating mixer would be called a pentagrid converter in the USA. This battery valve has a variable-μ characteristic for use in superhet circuits where the AGC voltage would reduce the gain for strong signals to prevent overloading and distortion. The data-sheet says that there is little interaction between oscillator and mixer and thus the valve can be used across most of the HF bands.
It is not possible to read the Type designation on the zinc paint but the size and bulb shape are exactly correct.
Faint but readable - the Marconi brand on this M-OV made valve.
The classic envelope is 45 mm in diameter, and excluding the B7 base pins is 119 mm tall.
References: Data-sheet & 1043. Type X21 was first introduced in 1934. See also 1934 adverts.
Absolute Maximum Operating Conditions¶
Updated February 22, 2018.