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The 39/44 variable μ pentode follows the pattern for RF valves of the 1930s. The control grid g1 is taken to the top cap to minimise the interaction between the grid and anode circuits. The anode ac resistance of 1M Ohms clearly shows the high impedance circuits that it would be used in such as AGC controlled RF and IF amplifiers in superhet radio receivers.

The 1950 Brimar manual lists the 39/44 as obsolete and suggests that it could be replaced by a type 6K7G in many circuits with a change of base to IO.

The classic envelope is 37 mm in diameter and, excluding the UX5 base pins, is 100 mm tall.

Reference: Data-sheet. Type 39/44 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.