Sensibly equivalent¶ to:
The NR57 is an audio output valve, in fact a type ML4 from M-OV made for military use in WWII. With a 4.0 Volt heater it was designed to be mains driven, so it is surprising to find the output power is so low. However, the datasheet says that it was for use as a driver for a larger output valve.
The classic envelope is 42 mm in diameter and, excluding the B5 base pins, is 98 mm tall.
References: Datasheet & 1043. Type NR57 was first introduced in 1929.
Absolute Maximum Operating Conditions¶
Updated December03, 2012.