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The John Lawrence Collection
This BTH valve seems to be labelled B4 but the B4 has a 6 Volt filament.
The filament voltage is marked as 2.8 Volts, suggesting dry battery operation. The internal envelope silvering obscures the electrode assembly.
The balloon envelope diameter is 35 mm and, excluding the B4 base pins, is 85 mm tall.
 
Pin Connections