parallel 12B4s (center left) as pass elements in 585
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 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.