UU8

The Mazda UU8 is an indirectly heated full-wave vacuum rectifier for AC mains equipment. Ediswan's sheet of 1 May 1940 primarily specifies television receivers and other equipment requiring up to 250 mA DC. This is the combined average current of both plates, not 250 mA per plate. The heater needs 4 V and 2.8 A. Heater and cathode share one terminal, which must be considered in the tube supply.

Maximum AC voltage is 350 V RMS per plate. The per-plate reference matters in full-wave rectification: it is not automatically the DC load voltage. At 250 mA output, the sheet gives two examples with 300 V RMS per plate: about 320 V DC with 16 µF reservoir capacitance and 295 V with 8 µF. At 350 V per plate the figures become about 380 and 355 V respectively. Reservoir capacitance must not exceed 16 µF.

A manufacturer note prevents oversimplifying the table: those DC figures assume very low transformer resistance and reactance. With usual transformer sizes, changing 8 to 16 µF does not necessarily raise DC voltage, but reduces hum ripple. The sheet also gives average DC-voltage/load-current curves for different secondary voltages and another capacitance condition. They help assess loaded supply behavior, not establish no-load voltage as a constant performance. No minimum series resistance is specified, so no nonexistent numerical value should be inferred.

Mazda describes a metallized bulb and Mazda Octal base. Viewed from the free end of the base, pin 1 joins heater and cathode, 8 is the other heater end, 3 and 5 the plates, 6 the metallizing. Pins 2, 4 and 7 are omitted. The external coating connection is thus distinct from rectifier-current terminals. Document maxima are overall length 114 mm and diameter 54 mm. Our sources also describe clear specimens alongside metallized ones; those specimen observations do not change the Mazda sheet's stated pinout and dimensions.

Documentation consulted: Mazda UU8 A.C. Mains Rectifying Valve, Ediswan Radio, Sheet 4/6, 1 May 1940, ratings, typical operation, connections and curves; Precision 660, P660-Tubelist test table, two UU8 test sections.