12B4
The 12B4 is a triode with 5.5 W maximum plate dissipation, 1000 V peak plate voltage, and and 105 mA peak cathode current. It has a B9A (Noval) miniature 9-pin base.
The 12B4 is used in the 551 and 555, and as a pass tube in the power supply of the 513, 581 and 585.