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The Fotos Bi-Grille is a bi-grid valve from the 1920's. The French Fotos company produced some of the earliest hard vacuum valves.

Space charge valves like this were initially popular in the 1920s for Unidyne receivers using very low HT voltages. When it was found that ordinary triodes performed just as well or better the space-charge or bi-grid valve fell from use.

This example has a spherical bulb and blue glass our other Bi-Grille has a tubular envelope and barium gettering.

Thanks to Joe Gruber for sending the images.

The balloon envelope is  mm in diameter, and excluding the special base pins is  mm tall.

Reference: ³⁰⁰². Type Bi-Grille was first introduced in 1925. See also 1925 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶