Sensibly equivalent¶ to:
256 A256 C-56 CV611 ER56 G-56 PH256 PH56 VT-56
The Arturus brand is the one most associated with the blue envelope glass. This exhibit is an indirectly heated medium-μ triode. The type 56 identification is etched into the envelope dome and only visible under oblique lighting. The Type 56 had the then standard US AC heater of 2.5 Volts, Type 76 is the same valve but with a 6.3 Volt heater.
The anode is notched and stitched. The control grid can be seen at the right of the image.
The balloon envelope is 38 mm in diameter, and excluding the UX5 base pins is 87 mm tall.
References: Datasheet & 3002. Type 56 was first introduced in 1932.
Absolute Maximum Operating Conditions¶
Updated February 15, 2013.