UL84

The UL84 is a power pentode for the audio output stage. Philips documents it as a single-tube class A output valve and in class AB with two tubes in push-pull; other manufacturers' sheets also give class B and triode connection, obtained by joining the screen grid to the plate. Because different circuits are available, output power must be read together with the voltages, load and distortion of the test rather than treated as one value that applies to every use.

The indirectly heated filament requires 45 V at 100 mA and is intended for series heater chains supplied by AC or DC. The UL84 has a miniature noval base. At the characteristic point specified by Philips, with 170 V on both plate and screen grid and −12.5 V on the control grid, plate current is 70 mA, screen current 3.5 mA and transconductance 11 mA/V. For a single class A stage, the sheet gives an example of 5.1 W output with a 170 V supply and 2 kΩ load, at 10% distortion under the stated conditions. With two UL84 tubes in class AB push-pull, Philips specifies 14.3 W with a 200 V supply and a 3 kΩ plate-to-plate load, at 3.8% distortion.

The operating limits give a maximum of 250 V on the plate, 200 V on the screen grid, 12 W plate dissipation and 1.75 W average screen dissipation. The documentation distinguishes the 6 W peak screen dissipation from its average dissipation; the two quantities must not be confused. Another practical note concerns certain pins marked as internal connections in the La Radiotechnique datasheet: they may be internally connected to an electrode and must be left unconnected in the wiring. This precaution matters when replacing the tube or rebuilding a socket without the original schematic.