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The 3A/144AY is a 3A/144A that has been tested to special limits to replace the 4104D in telephone repeater amplifiers. See also 4104E

The box anode and oxide coated filament ribbons.

Looking into the top mica. The filament ribbon and three tension springs can be seen as can the grid wire.

Another view through the top mica. The tension springs consist of five components.

The classic envelope is 44 mm in diameter, and excluding the UV base pins is 104 mm tall.

Reference: Data-sheet. Type 3A/144AY was first introduced in 1940. See also 1940 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.