Another interesting article on low voltage space charge electron tubes

Low plate potential tubes - radio and television news - Jan 1957


http://www.rfcafe.com/references/radio-news/low-plate-potential-tubes-january-1957-radio-television-news.htm


It exaplains the operation of electron tubes such as 12K5.


The 12K5 is used on the Pleiades Electra 2 one elcteon tube and no other component headphone amplifier.