TV line output pentode, successor of PL500 and PL36 (=EL500 with 27V heaters). In triode mode linearity is good (worse than PL500), but spectrum suffers from high level of uneven harmonics (5th particularly) that you can’t get rid of no matter what operating point you choose. This is common to all line output pentodes I have measured and could be culprit of “harsh” sound reported by a lot of users. Tube needs lower current then PL500 (<20ma) to ge into the linear region which is very large! THD doesn’t change up to 100mA. Not microphonic and much better behaved (oscillation wise) then PL36.