I’ve got hold of a NOS quartet of UX-120 Radiotron valves made in 1925. These are 95 year old beauties. Unbelievable how well conserved they are and operate.
This valve developed by RCA was the first output valve intended to be used with dry-cells (3 No. 6 cells in series). See the data sheet extract below:
There is no much data available on this valve, if you have some please share and let me know. Here are the curves I traced from a sample which measured 114%: