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

20mA, -36V BIAS, 180Va

Other operating points

40mA, -36V BIAS, 200Va
60mA, -36V BIAS, 210Va
80mA, -36V BIAS, 220Va,60mA
100mA, -25V BIAS, 180Va

Traced plate curves


TV line output pentode, successor of PL500 and PL36 (=EL500 with 27V heaters). In triode mode linearity is good (worse than PL500), but spectrum suffers from high level of uneven harmonics (5th particularly) that you can’t get rid of no matter what operating point you choose. This is common to all line output pentodes I have measured and could be culprit of “harsh” sound reported by a lot of users. Tube needs lower current then PL500 (<20ma) to ge into the linear region which is very large! THD doesn’t change up to 100mA. Not microphonic and much better behaved (oscillation wise) then PL36. 

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