UY21

The UY21 is a high-vacuum half-wave rectifier with indirect heating, designed for universal AC/DC receivers and 100 mA series heater chains. Its heater requires 50 V. Sources place it in the early 1940s, during the initial period of all-glass construction; the described Philips and Mullard examples have a Loctal base, identified as B8B in their profiles. The later UY41 instead uses the smaller B8A base, so it cannot automatically be treated as an identical replacement.

Philips explains the design compromise: providing sufficient current even for receivers with push-pull output stages while limiting heater requirements to accommodate more valves in the series chain. Published limits are 250 V RMS alternating anode voltage, 140 mA DC output, 550 V between heater and cathode and 60 µF filter-input capacitance. The 250 V figure is not guaranteed DC output voltage: curves show that output varies with load and circuit resistance.

The series protective resistance depends on conditions. For 170–250 V mains, the table requires at least 175 Ω with 60 µF, 125 Ω with 32 µF and 75 Ω with 16 µF; with 8 µF it lists zero. For 127–170 V the minima are respectively 100, 75 and 30 Ω for 60, 32 and 16 µF; up to 127 V the 60 µF row lists zero. These zeros belong to the specified table cases, not permission to remove protection in any supply. Two terminals connected to the same anode do not make the UY21 a full-wave rectifier. Documentation consulted: Philips, UY21, pages 54–55, Dutch and English editions, description, limits, resistances and curves.