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

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 126, 551, and 555, and as a pass tube in the power supply of the 513, 581 and 585.