Stuart Irwin: The suffix G for "glass" is not mentioned in some schematics or in "Wireless World" valve books, Babani's Equivalent books etc. The 6AG6 is the same tube in glass. There is no US tube with this designation. As far as known by us, the 6AG6 / 6AG6G was only made in Britain - for instance by Brimar and by Marconi's Wireless Telegraph Co. Ltd. in Chelmsford. The Brimar's parent, STC Australia, imported them despite that they then also had a valve factury in Australia - for communication types only.
Literatur
Taschenbuch zum Röhren-Codex 1948/49
6AG6: Schematic from my documents.
Pier Antonio Aluffi
