PX25 — Power Triode, UX4

Also commonly written as PX-25 or P.X.25.

Automatic bias resistance values differ between AC and DC filament heating: 530 Ω (AC) vs 1000 Ω (DC) at Va=400 V due to the rectification effect of the directly heated filament.

The Osram datasheet specifies Pa_max as 25 W. Some later sources and reissue manufacturers (e.g. Emission Labs) rate the tube at 30 W. The original 25 W rating is used here.

DO24, LK4200, P16, PP5/400, RD25A, and RD27AS are listed by radiomuseum.org as similar/replaceable types but may have differing characteristics or ratings — verify before substituting.

PX25A is a variant with unspecified differences. PX25_UX is a base variant using UX4 base (same as PX25).

Care should be taken to switch off the power supply when inserting or removing the valve from its socket, or when any adjustments are made to the circuit.

When two valves are employed in push-pull or parallel, similar auto bias circuits and oscillation stoppers should be applied to each individual valve.

Sufficient air circulation should be provided around the valve to prevent overheating.