The PL104 is a tetrode thyratron with a mercury vapour filling. It was designed as a controlled rectifier for use in motor speed control or in stabilised supplies. For AC use it could be used for lamp control or for resistance welding equipment of up to 27 kVA. Substantial in every way and not a mass produced item.
The pinch stem has a large diameter and is used to anchor the collar that supports the outer cylinder.
The outer cylinder is connected to a base pin and is thus the second grid. The control grid connects to the side cap.
The g2 cylinder with insulator and the braided wire for the control grid passing through it.
The anode is a circular plate whose edges are turned up towards the top cap.
The classic envelope is 95 mm in diameter, and excluding the base pins is 265 mm tall.
Reference: Datasheet. Type PL105 was first introduced in 1954.