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The U404 is a half-wave rectifier for AC/DC radio sets without a transformer. The 100 mA heater is for heater chain use across the mains.

The U404 data-sheet gives the maximum reservoir capacitor as 16μF and has a graph for a reservoir capacitor of 32μF. The Wireless World book gives the maximum reservoir capacitor as being 50 μF and must have 180 Ω of series resistance to limit the current surge. In all cases the design has to limit the charging peak current to within the cathode rating of 700mA.

Thanks to Jonathan Marsh for sending the image.

The thin glass tube envelope is 19 mm in diameter, and excluding the B8A base pins is 60 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type U404 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.