AL2

The AL2 is an indirectly heated audio output pentode offered, according to our sources, in 1935 alongside directly heated AL1. Historical notes associate this choice with reduced hum and easier grid-bias generation. Philips’s 1935 text describes 9 W anode dissipation and a 4 V, approximately 1 A heater. Its P base has eight side contacts; the control grid connects on top, while cathode and suppressor are internally linked.

For single-valve class A, later Philips sheets give 250 V anode and screen, −25 V bias and 625 Ω cathode resistance. Anode current 36 mA, screen current 5 mA, transconductance 2.6 mA/V, internal resistance 60 kΩ. A 7 kΩ load and 14 V RMS drive give 3.8 W at 10% distortion. Philips dated 19 March 1940 instead gives 4 mA screen current at the same tabulated point; this edition-specific difference is preserved.

The 1940 sheet also documents two AL2s in push-pull: 250 V anode and screen, common 350 Ω cathode resistance and 6.6 kΩ anode-to-anode load. Maximum pair output is 11.5 W at 3% distortion. Anode currents are 2 × 33 mA idle and 2 × 41 mA at maximum signal; screen currents rise from 2 × 3.5 to 2 × 7 mA. These are two-valve circuit figures, not one AL2’s output or current. Preserved curves also compare fixed- and self-bias arrangements.

Philips explains that anode and screen may use one supply, allowing for voltage drop in the anode feed path. If the loudspeaker is switched out, the anode must remain powered to avoid screen overload. Cathode-resistor bias requires at least 2 µF bypass capacitance, preferably a small 25 µF electrolytic. These notes belong to the manufacturer’s circuit and help explain its historical implementation.

Philips limits are 250 V anode and screen, 9 W anode dissipation, 1.5 W screen dissipation and 50 mA cathode current. Separate no-current ratings are 550 V anode and screen. Heater-to-cathode figures are 50 V and maximum 5 kΩ resistance; control-grid resistance is at most 0.7 MΩ with self-bias, 0.3 MΩ with fixed bias. The 1940 drawing gives maximum diameter 46 mm and height 115 mm, later sheets 47 and 117 mm. Curves at other voltages characterise their own circuits rather than establishing new general limits.

Tungsram TAL2 provides an interesting comparison: tabulated transconductance 2.5 mA/V versus Philips 2.6. Tungsram lists 3.8 W at 10% in its table; its introduction gives about 3.85 W at 10% and 4.5 W allowing 12% distortion, illustrating the link between output and permitted distortion. The larger figure is not presented as Philips AL2 performance under identical conditions. Relevant photos include a Philips Miniwatt AL2-marked specimen; similar shapes or names do not replace comparison of sheets.