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The 12SA7GT is a pentagrid converter for use as a mixer oscillator in superhet receivers. The design is quoted by RCA as optimised to increase stability.

This exhibit has the base wires passing into the plastic base without further support. Whilst this may have been the way the valve was originally made it is more likely that the original metal skirt that protects the delicate wires by holding the base to the envelope has been lost. This has been seen in other valves where the skirt has remained in place but has sprung open. In these cases the skirt has not been filled with cement.

Thanks to Stef Niewiadomski for the image.

The wide glass tube envelope is  mm in diameter, and excluding the IO base pins is  mm tall.

References: Data-sheet, ¹⁰⁴⁰, ³⁰⁰² & ¹⁰⁴³. Type 12SA7GT was first introduced in 1939. See also 1939 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.