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.
Very overlooked sleeper. Probably because common-cathode-diode part – which can be omitted and doesn’t effect triode operation at all! This tube is 1/2 5751 (=12AX7,ECC83) triode but with a much cleaner spectrum! Maybe not great for guitar amp or distortion box, where it will have less coloration, but ideal for hi-fi audio as hi-gain block. Lowest THD operating point is well suited for red LED biasing (-1.7V). Not microphonic, quiet and sounds really nice. Available for sale.