The 0D3 is a gas filled discharge valve that exhibits a near constant voltage across the valve for currents within the range 5mA to 40mA. In fact the regulation is 150 plus or minus 5.5 Volts. The construction is of a central cold cathode within a cylindrical anode. The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 85 mm tall. Reference: Datasheet. Type 0D3 was first introduced in 1939.