One of the most sought after vintage Western Electric items is the low powered valves referred to as Tennis Ball tubes, for obvious reasons; the 102D and 205D.
The 205 is an old audio triode, also called 205A, B, C or D. Sometimes called "tennis ball tube". Out of production for may years is is now gained status as Audiophile cult item, especially in Japan. No easy substitute, although Svetlana SV811-3 or SV572-3 could be used by changing sockets and providing 6.3v at 4 amps for filament.
Taken From 1925 Australian Newspaper
