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

3mA, -9V BIAS

Other operating points

10mA, -2.46V BIAS, 265 Va

Traced plate curves


Another vari-μ pentode with somewhat OK spectrum. THD is close to other popular valves with simple resistor load. Certainly good enough as cathode follower. Available for sale.

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