PL81

The PL81 is a power valve intended mainly for the horizontal deflection output stage in television sets. Philips and Brimar sheets call it a line-output pentode; Tesla documentation instead uses the term beam tetrode. In either case, the documented role is the same: to generate the current needed for scanning while withstanding the brief high-voltage pulses that occur during flyback. Brimar describes its use with a booster diode in AC/DC receivers and with wide-angle cathode-ray tubes. Philips and Tesla sheets also document a different application, as a class-B push-pull audio output pair.

The PL81 has an indirectly heated cathode and a nine-pin noval base; the anode is connected to the cap on top of the envelope. According to Philips and Brimar sheets, its heater is designed for 300 mA and a nominal voltage of 21.5 V. It is therefore intended for a series heater chain: before any substitution, check both the pin connections and the chain current, rather than the type name alone. As typical characteristics, Philips gives about 45 mA anode current and 6.2 mA/V transconductance with 170 V on the anode and screen grid; at 200 V it gives about 40 mA and 6.0 mA/V. These are measurement points, not one operating condition for every television.

The distinguishing feature of a line-output stage is the difference between ordinary voltages and pulse voltages. Philips sheets specify up to 7 kV as the anode peak, but restrict it to very short pulses; the editions consulted give a maximum duration of 18 µs. The permissible continuous anode voltage is far lower, around 250 V in the stated limits. Brimar also specifies 180 mA as the maximum cathode current. Thus 7 kV is not a possible continuous supply, nor does it alone describe a circuit's ability to withstand stress: pulse shape, anode and screen-grid dissipation, and scanning conditions all matter.

A Philips warning concerns design over time: the published curves describe nominal new valves, whereas tolerances and ageing can reduce the available current. The manufacturer therefore advises allowing a margin in the line-output stage. A later sheet also stresses that the anode voltage must not fall too low during scanning, to avoid Barkhausen interference and loss of stabilisation. These practical points explain why a static valve test alone cannot reproduce its behaviour in the television set.

Audio use is documented with two PL81 valves in class-B push-pull. A Philips sheet gives examples at 170 and 200 V, with a 2.5 kΩ anode-to-anode load and output powers of about 13.5 and 20 W respectively under the specified test conditions. Tesla also describes this application. These figures concern a pair, its circuit and its stated measurement method; they cannot automatically be applied to a single PL81 or a deflection stage. Tesla lists 21A6 and N152 among similar types, while Brimar uses the designation PL81/21A6. These references help locate documentation, but any substitution still requires comparison of the connections and applicable limits.