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

18mA, -20V

Other operating points

10mA, -16V BIAS

Traced plate curves


Another well known output pentode. Microphonic, but with μ of only 7 that’s not very problematic. Quite linear and much cleaner spectrum than EL84. I personally prefer the sound of triode connected EL86 over EL84. Available for sale.

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