EIA manufacturer code 522 identifies Tung-Sol, based in USA. D-shaped getters, ribbed plates, smooth top dome glass.
The EIA (Electronic Industries Alliance) manufacturer code system was used on vacuum tubes, transistors, and other components to identify the factory of origin. The two-letter code stamped on a tube's glass envelope or base identifies where it was made, which is a key factor in assessing its quality, rarity, and value to collectors.
Browse TubeDealer listings and filter by brand to find tubes made by Tung-Sol. Many collectors and audio engineers specifically seek tubes with the 522 code for their known factory quality.