The Philips 21- "Key Base" tubes were initially designed in 1940 with the 9-pin base B9G like the EF50. The later series production from 1942 on however was changed to the 8-pin Locktal-base B8G designed by Sylvania. There are also DK21's with International octal bases.
Bibliografia
- - Manufacturers Literature 1940-1941
DK21_9pin: original Philips documents
Jacob Roschy
