This exhibit is an early type T Pliotron for transmitter use. The pinch has the designation VT-12 written on it.
VT-12 is a low power oscillating triode used for telephone transmitters and was made by General Electric as a substitute for the Western Electric VT2.
The type no VT-12 was designated by the US Signal Corps. It was only used briefly before being replaced with the VT-14 which has a different filament design.
Thanks to Joe Gruber for sending the image.
The balloon envelope is 35 mm in diameter and, excluding the UV base pins, is 101 mm tall.
References: J Gruber, 3002 & 1047. Type VT-12 was first introduced in 1918.
Absolute Maximum Operating Conditions¶
Updated February 08, 2021.