DeForest Tubular Audion
1916

Prior to about 1915, DeForest enjoyed what was largely a monopoly position in vacuum tubes and the company was able to impose very restrictive sales policies. At the time the only way to obtain a DeForest Audion was to purchase a complete detector. Even replacement bulbs would only be provided when the defective bulb was exchanged.

This all changed in 1915 when E. T. Cunningham introduced the Audio Tron, a triode detector tube. They were good quality and Cunningham sold the tubes with no restrictions, so DeForests' business began to suffer. To meet the competition DeForest introduced the Tubular Audion in 1916. Note that the ends of the tube are sealed with tar.

Advertisements from the October, 1916 issue of QST magazine