AL3

The AL3 is a Philips Miniwatt power output pentode, indirectly heated at 4 V and 1.85 A, with a European P8A side-contact base. The preserved sheet and its curves document it in 1935. Its characteristic figure is high transconductance: Philips specifies 9 mA/V at an operating point of 36 mA anode current. For an output valve, this parameter describes how anode current responds to grid voltage; it is distinct from power delivered to the load.

Our sources describe a history connected with the arrival of high-transconductance output pentodes in continental Europe. The 1935 Philips AL3 is described as having a cylindrical cathode and transconductance of 9 mA/V, compared with about 2.5 mA/V for the earlier types cited in the account. In 1936 Telefunken introduced the closely related AL4 with an improved profiled cathode. The same account states that Philips, Tungsram and other makers subsequently adopted the AL4 designation for existing AL3 production without changing its construction. This is a particular feature of designation history: the name change does not prove that every AL4 from every manufacturer has the same internal structure.

At the Philips operating point, the anode and screen grid are at 250 V and the control grid at −6.5 V, with 36 mA anode current and 4 mA screen current. The stated internal resistance is 50 kΩ. The sheet associates this example with 4.5 W output, an anode load stated as a maximum of 7 kΩ and a drive voltage of 3.5 V RMS. That table does not specify a distortion percentage, so none should be reconstructed arbitrarily. Current curves and anode characteristic families complete the picture of operation as bias changes.

The main limits are 250 V operating anode voltage, 9 W anode dissipation and 55 mA cathode current. The screen is limited to 250 V and 1.5 W; maximum heater-to-cathode voltage is 50 V. The 9 W figure concerns heat dissipated at the anode, not available audio power. Likewise, the 550 V figures for anode and screen without current must not be confused with operating voltages. When restoring equipment, the 4 V heater, side-contact base and stage conditions must therefore be checked along with the valve designation.