What is "ultra-linear"

Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube (also called "electron-valve") to a load (e.g. to a loudspeaker).

'Ultra-linear' is a special case of 'distributed loading'; a circuit technique patented by Alan Blumlein in 1937 (Patent No. 496,883), although the name 'distributed loading' is probably due to Mullard. In 1938 he applied for the US patent 2218902. The particular advantages of ultra-linear operation, and the name itself, were published by David Hafler and Herbert Keroes in the early 1950s through articles in the magazine "Audio Engineering" from the USA. The special case of 'ultra linear' operation is sometimes confused with the more general principle of distributed loading.