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The CL4 audio output pentode was designed for single ended use in universal mains receivers. The 4 Watt output was quite respectable.

The CL33 is electrically identical but has the more successful IO base. Whereas the pen36c is engineered on an British B7 base.

The side of the anode.

The three grid supports and additional vertical supports.

The cathode tube is a rectangle with rounded sides.

The classic envelope is 44 mm in diameter and, excluding the Ct8 base contacts is 123 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type CL4 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.