The B21 is a double triode Class B battery valve from 1933.For more details see the article above.
The twin triode sections.
At the base can just be seen that each triode has a double grid. Both grids are joined at the base.
The pinch and lower components.
The identification is on the dome. Within can be seen the filament tension springs for the oxide coated filament, the two grid supports at each end and the fingers of the top mica gripping the inside of the envelope.
A clearer view of the tension springs and the white coating on the filament.The classic envelope is 42 mm in diameter and, excluding the B7 base pins, is 84 mm tall.References: Data-sheet & Data. Type B21 was first introduced in 1933. See also 1933 adverts. |
