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

 14mA, -3.4V BIAS

Other operating points

4mA, -4V BIAS
 4mA, -7V BIAS

Traced plate curves


Vari-μ pentode, hopelessly nonlinear. Not suitable for hi-fi audio without some sort of a negative feedback mechanism. Should work great as compressor or other effects box though 🙂 Available for sale.

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