The XFY10 is is a very small wire ended pentode and very probably was designed for hearing aid use. The 22.5 Volt anode specification fits with the small dry cells available.
The valve is the lowest power audio output valve we have seen at three mW. This power would be suitable for sensitive ear phone use only.
The thin glass tube envelope is oval in section 9 mm by 5 mm. The length excluding B5A wires is 38 mm.
Reference: 1040. Type XFY10 was first introduced in 1949.
Absolute Maximum Operating Conditions¶
Updated March 10, 2013. shape:thin.glass.tube construction:all.glass type:pentode age:1940.1950 base:b5a pins:5 pin:1.a pin:2.g2 pin:3.f+ pin:4.g1 pin:5.f-.g3