All tube operating points below are by no means definitive good sound recipe for your particulate tube project. Lowest THD point is not necessary best sounding one and there is always some variance even in a NOS tube batch from the same manufacturer!  Below information is only useful as a general starting point and is ment to get you in a ballpark when no measurement equipment is available.

Lowest THD operating point spectrum

40mA, -31V BIAS, 240Va

Other operating points

30mA, -31V BIAS, 230Va
60mA, -26.7V BIAS, 230Va
20mA, -28V BIAS, 200Va
80mA, -16.8V BIAS, 180Va

Traced plate curves


TV frame buffer pentode. In triode mode linearity is good (same as all PL5*** family), but spectrum is different. It has same large amount of higher harmonics, but it’s not dominated by uneven ones (5th particularly). Instead it’s more monotonically falling spectrum which should  sound much better then other TV pentodes. Tube needs only 20ma to get into the linear region and stays there up to 60mA. Quite microphonic but much better behaved (oscillation wise) then PL36. Really good candidate for some 5W SE amp. 

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