The mount drawing for the now-eBaygenic “single-plate” 2A3 audio triode, with its 20 filaments. This original version carried a reputation for being hard to amke in terms of equal tension on the filaments, and was soon superseded by new designs. The note about the tensioning springs testifies to this diffuculty. Image: RCA Standardizing Notice 3-1-2A3, Sept. 13, 1934, in the Dowd-RCA Archive, a holding of the AWA Museum.