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The 3A/109B is a telephone repeater triode from STC. It features a B4 base cap and replaced the earlier 3A/109A with a UX4 base cap.

The CV1663 is the same valve and both Type designations appear on the glass.

The triode is a low power device and so the anode is left bright.

The oxide coated filament has three hairpin loops making a total of six emitting surfaces. The tension springs can be seen attached to the top mica.

The view through the dome. The grid wires are distorted by the glass.

The classic envelope is 44 mm in diameter, and excluding the B4 base pins is 101 mm tall.

Reference: Data-sheet. Type 3A/109B was first introduced in 1947. See also 1947 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.