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The EF55 pentode is physically the big brother of the EF50 with which it shares common base connections.

The valve is a single ended RF pentode with very high mutual conductance and a sharp cut-off. The mutual conductance was almost twice the value for the EF50. It was designed for video bandwidths where the high mutual conductance allows a flatter passband to be achieved whilst maintaining a reasonable stage gain.

The reverse of the can is marked CV173. This appears to be the original identification with the Zaerix branding added later.

The EF55 is an early all glass constructed valve with equi-spaced pins and a spigot for location into the valveholder.

The current rating places it as a power valve but it is designated for amplifier service. Running the valve at high current improves the small signal noise performance and mutual conductance figure.

The wide glass tube envelope is 34 mm in diameter and, excluding the B9G base pins, is 79 mm tall.

References: Data-sheet & ¹⁰⁴⁰ Type EF55 was first introduced in 1946. See also 1946 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.