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.