AD1

The AD1 is a directly heated output triode for the audio stages of mains-powered receivers. Philips highlights a feature for its power class: it can operate with a 250 V plate supply, common in receivers, instead of needing a much higher voltage. Its filament draws about 0.95 A at 4 V and also serves as the emitting cathode; Philips sheets specify a type P base. This is an output tube, and the power figures in watts in the documents refer to audio power delivered to the load under the stated conditions, not to the power consumed by the filament.

For one triode in Class A, the sheets give 250 V on the plate, about −45 V grid bias and 60 mA plate current. With an optimum plate load of about 2.3 kΩ, output is 4.2 W at 5% distortion; internal resistance at that operating point is about 670 Ω. Full drive requires an input signal of roughly 30 V rms on the grid. A Philips technical article notes that this needs more drive than an output pentode and may require greater gain from preceding stages. In return, the triode's low internal resistance changes its interaction with the loudspeaker and output transformer impedance; load choice can therefore affect the sound as well as measured power.

The documentation also shows two AD1 tubes in a Class AB push-pull stage, producing about 9.2–9.3 W at 250 V on the plates depending on the chosen bias. This is the output of the pair and must not be confused with the single triode's 4.2 W. For self-biased circuits, Philips proposes a resistor between the centre tap of the filament winding and ground reference, bypassed by a capacitor. This detail matters because the cathode is the filament itself: there is no isolated heater as in indirectly heated tubes. The sheets also give different maximum grid-circuit resistances for self-bias and fixed bias, respectively 0.7 MΩ and 0.3 MΩ.

Philips' comparison between triode and pentode is also of interest in restoration. The AD1's low internal resistance tends to damp loudspeaker resonances, but the article warns that some loudspeaker and transformer combinations can reduce response at higher frequencies. These are observations about the complete system, not an unchanging property of the tube. To assess an AD1 in a real set, plate voltage, filament, bias, driver stage and output-transformer impedance should therefore be checked together.