52KU

The 52KU is a dual-diode full-wave power-supply rectifier with an octal base and 5 V heating. Radio Valve Data, 1958, lists it under both Cossor and Emitron, with a heater current of 2 A. Its two sections conduct alternately from the two halves of a centre-tapped high-voltage secondary. Thus 500–0–500 V RMS refers to each half winding relative to the centre tap, not to a DC voltage directly available at the output.

Both manual tables give the 52KU a maximum rectified current of 150 mA, maximum reservoir capacitance of 16 µF and minimum series resistance of 75 Ω. These values belong together: the filter input capacitor recharges in pulses, and circuit resistance limits current during conduction. Comparing DC current alone is insufficient when selecting a replacement. Capacitance, circuit resistance and applied voltages must also meet the documentation for the actual type fitted.

The CV valve register links the commercial Cossor 52KU to CV1863 in brackets, meaning the nearest CV type. This distinction matters: the CV1863 specification of 21 October 1946 describes an indirectly heated 5Z4G rectifier and specifies, for a capacitor-input filter at 50 Hz, maximum applied RMS voltage of 350 V, mean DC current of 125 mA, 16 µF and at least 30 Ω introduced externally per anode. These belong to that CV specification and differ from the 1958 manual’s 52KU row; they must not be combined into a new set of limits. The specification date does not establish the 52KU’s introduction year.

The retained photographs add a construction detail. One specimen carries Emitron branding; another, made in the USSR, also bears GZ32 and CV593 markings. The latter shows insulation-coated heaters, interconnected cathodes and a heater-to-cathode connection. This describes the documented specimen, rather than proving identical construction for every 52KU-marked valve. Multiple glass markings must therefore be read alongside electrical data, preserving the distinction between a relabelled specimen and the historical specifications of different types.