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.
Soviet TV line output pentode. Triode mode linearity is excellent, but spectrum suffers from high level of uneven harmonics. Fortunately it’s not dominated by 5th (like in PL50* family), but has more benning falling profile like PL508. Tube needs only 20ma to get into the linear region. Quite microphonic but much better behaved (oscillation wise) then PL36. Another (together with PL508) good candidate for some 5W SE amp.