Extensive experimental work was done at GE during 1924 and 1925 and in July 1926 samples were submitted to RCA, which assigned the type UX222 in November 1926.
Production was started at Nela Park and the UX222 was announced by RCA in October 1927 it features a thoriated tungsten filament.
Construction uses a glass rod to hold the tops of the supports aligned. The control grid is connected to the top cap. The UX4 base is of the bayonet type and the locating pip can be seen on the base cap.
The grid that can be seen is the screen grid. It is close wound and is placed inside and outside the anode cylinder. The filament is a single vertical strand.
The inner grid can just be seen and has a more open pitch than the screen grid.
From the top end and looking through the hole in the plate joining the inner and outer sections of the screen grid the control grid can be seen.
By changing the lighting the inner grid can be seen more easily.
The balloon envelope is 44 mm in diameter, and excluding the UX4 base pins is 117 mm tall.
References: 1047 & 2003. Type UX222 was first introduced in 1927. See also1927 adverts.