This 6H8C was supplied as part of an amplifier kit. The OTK on the base indicates that it is Russian in origin and made to military specifications. The 6H8C is equivalent to the 6SN7GT.
The twin triodes secured to the pinch. The base carries the OTK stamp.
One triode end on.
The wide glass tube envelope is 28 mm in diameter and excluding the IO base pins, is 75 mm tall.
