You have missed my point. If the circuit was designed to "work" with a broad range of tubes, then that means it will not get the best out of any of those tubes.
To get the best out of a particular tube, the circuit has to be fine tuned for that particular tube. So a design that will "work" with more than one particular tube is not optimized for any particular tube and therefore not getting the best from any of the tubes.
The designer's posted load lines below prove my point.
See what I mean? He's running a 12au7 at less than 2ma idle current and 67 volts plate. There is nothing optimized about that operating point.
The 12ax7 is being run at about 1ma and 155 volts plate (wrongly marked as 127 vdc). That's not bad but the 100k load impedance is too low so the load line is more vertical than it should be cause the tube to produce excess harmonic distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
