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

6mA, -8.5V BIAS

Recommended operating point spectrum

5ma, -3.4V BIAS

Other operating points

5ma, -7V BIAS
10ma, -1.7V BIAS
8ma, -1.7V BIAS

Traced plate curves

THD statistics


Legendary tube. THD analysis shows, that you need at least -3V grid BIAS to get them into linear region (out of grid current). So single (-1.7V) red LED bias is not good enough, at least 2 in series is needed (-3.4V).  Crank up the current as much as total power dissipation allows. 

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