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 and recommended operating point spectrum

 2.5mA, -1.18V BIAS, 242Va

Other operating points

2mA, -1.62V BIAS, 262Va
1.5mA, -1.8V BIAS, 255Va

Traced plate curves

THD statistics


Triode is ECC83 but with much lower H3, H4, H5 levels, closer to 7025. Available for sale.

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