18E (18E)

Apparently the tubes 2A5E, 18E, 42E and most probably the 6F6EG are special versions of 2A5, 18, 42 and 6F6G.

They distinguish against their normal versions in that way, as their grid # 3 (suppressor grid) is connected to grid # 1 (control grid), instead of being connected with cathode, as usually.

In Europe the tube company Tungsram followed this fashion by releasing some tube types of the same peculiarity and called them "Tetraforte". Known type were at least the PP415 as direct heated type and the APP4120 as indirect heated tube.

Apparently, both the PP415 and the APP4120 were also available as normal penthodes, with g3 connected to cathode.

It would be interesting to find articles and other information about these tubes in contemporary literature of the early 1930s. By now, we know very little, with exception of some schematics and base diagrams, where the g3 to g1 connection is visible.

Best Regards , Jacob

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