PX25

The PX25 is a power triode with a directly heated filament. Osram recommends it for the output stage of audio-frequency amplifiers provided the equipment has an adequate anode supply; Marconi describes it as a low-impedance triode capable of about 5 W in class A. The makers' documents mention domestic receivers, small amplifiers and use as a driver for larger valves. Marconi also notes its use as a medium-wave transmitter and as a modulator: these are separate applications, not one audio circuit.

The filament requires 4 V and about 2 A, and the base has four pins. Osram's table gives three anode operating points that must be read separately: at 400 V it lists −31 V on the grid, 62.5 mA anode current and 25 W dissipation; at 350 V, −26 V and 57 mA; at 300 V, −21 V and 44 mA. It gives an amplification factor of 9.5, an internal resistance of 1,265 Ω and, for the first point, an optimum load of 3,200 Ω. These are figures from the Osram table, not 25 W of audio output: that last figure is anode dissipation at the stated point.

Marconi's sheet offers another class-A example: about 5 W output with a 4,000 Ω load, a 475 Ω bias resistor and grid voltage of about −30 V. Its 4,000 Ω load differs from the 3,200 Ω optimum given by Osram for its operating point; the conditions come from different sheets and arrangements. Marconi also describes the PX25 as a high-slope valve, while Osram gives about 8 mA/V in its own table.

Osram recommends separate automatic bias for each valve. With an AC-heated filament, the bias resistor is connected to the electrical centre of the filament or of the low-voltage transformer secondary to reduce hum. If two PX25 valves are used in parallel or push-pull, the sheet calls for individual self-bias circuits and oscillation stoppers. It also recommends sufficient air circulation to prevent overheating. These mounting instructions matter as much as the electrical ratings when designing an output stage with this valve.