Sensibly equivalent¶ to:
The W21 is a variable-mu amplifier pentode according to the data-sheet and has a 2 Volt filament. As it is from M-OV and not Philips(Mullard) it is probably a beam tetrode internally.
The W21 was made in two versions, this with a B4 base cap and a B7 based W21 version, both were released in 1937.
The filament is for accumulator use and the anode is designed for dry battery operation with a maximum anode voltage of 150 Volts. The operating conditions shown are for the near equivalent.
The Osram name on the zinc screening paint. The connection to the base can be seen within circle above the base.
The balloon envelope is 42 mm in diameter, and excluding the B4 base pins is 101 mm tall.
References: Data-sheet, 1040 & 1043. Type W21 was first introduced in 1937. See also 1937 adverts.
Absolute Maximum Operating Conditions¶
Updated June 21, 2020.