25Z6G

The 25Z6G is a high-vacuum twin rectifier with separate cathodes, intended for universal radio-receiver supplies. Sylvania’s manual stresses use in transformerless sets, either for half-wave rectification or voltage doubling. The independent sections enable these different connections: supply behavior depends on the actual circuit, not simply on having two anodes in one envelope.

Indirect heating requires 25 V at 300 mA AC or DC. Sylvania describes the G version with an ST-12 glass envelope, octal base and any mounting position. In universal receivers its heater is in series with the other tubes’ heaters; the manual notes that its 25 V rating helps reduce power dissipated in the string’s series resistor. This explains the heater choice within the complete set.

For half-wave rectification, Sylvania describes paralleling both sections, anode to anode and cathode to cathode, with a 16 µF capacitor-input filter. Tung-Sol also allows separate use and explains that paralleling reduces tube drop. Its table gives 75 mA maximum DC per anode at 117, 150 and 235 V RMS, with minimum total effective source impedances per anode of 15, 40 and 100 Ω respectively. These are figures tied to the stated conditions, not three DC output voltages.

For voltage doubling, Tung-Sol distinguishes half-wave and full-wave circuits: both list 117 V RMS per anode and 75 mA maximum DC output, but different minimum source impedances, 30 and 15 Ω per anode respectively. Curves also show how DC voltage, load current and filter capacitance interact: doubling does not mean constant output voltage as load changes. Above 40 µF the sheet warns that extra supply impedance may be needed. Each configuration therefore requires its own table.

Limits common to the examined documents include 700 V peak inverse voltage, 450 mA peak anode current per section and 350 V DC heater-to-cathode potential. Listed drop is 22 V at 150 mA per anode: this measurement current is not the maximum DC output current. Sylvania relates the family to 25Z5; Tung-Sol’s December 20, 1940 sheet states that the dual-designated 25Z6GT/G replaces either 25Z6G or 25Z6GT. The documents thus preserve both electrical relationships and the evolution of commercial forms.