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The 6SK7 is a variable μ pentode for RF and IF amplifiers in superhet receivers. The gain of the device being controlled by a negative voltage derived from rectifying the final IF signal. In this way strong signals do not overload the receive chain because the various controlled stages have their gain reduced.

The American Type designations such as 6SK7 refer to a single ended version of another Valve in this case the 6K7. Tungsram are a European manufacturer but this was made in the USA and so is a re-branded import.

The 12SK7 had identical characteristics apart from the heater. The 12.6 Volt heater being used directly across a vehicle battery.

The plain paint on the reverse.

The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 51 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type 6SK7 was first introduced in 1938. See also 1938 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.