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
12.5mA, -0.7V BIAS, 131Va
Recommended operating point spectrum
10mA, -1.6V BIAS, 150Va
Other operating points
8mA, -1.74V BIAS, 144Va
5mA, -0.87V BIAS, 90Va
Traced plate curves
Comment
Another linear soviet pentode with a nice spectrum. Definitely usable. Available for sale.