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

10mA, -11.2V BIAS, 260Va

Recommended operating point spectrum

2.55mA, -12.9V BIAS, 250Va

Other operating points

20mA, -9.8V BIAS, 260Va
30mA, -9V BIAS, 270Va

Traced plate curves


Very microphonic and has quite dirty spectrum.

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