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 (@+19dbU)

60mA, -36V BIAS, 161Va

Other operating points (@+19dbU)

20mA, -36V BIAS, 136Va
40mA, -36V BIAS, 152Va
80mA, -36V BIAS, 168Va
100mA, -36V BIAS, 174Va
120mA, -36V BIAS, 180Va

Traced plate curves


Probably last TV line output pentode before transistors took over. Successor of PL5*** family. In triode mode linearity is good, 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 only 20ma to get into the linear region and stays there up to 110mA! Incredible achievement of tube engineering. But there is always a price to pay and in this case it’s extremely low μ of 4. Hence why all THD measurements are taken @+19dbU – I just ran out of input voltage. Quite microphonic but much better behaved (oscillation wise) then PL36. 

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