I am trying to copy the work of ltspice that is in the blog that you gave me, bigun, but it is with some changes. I am using 12ax7 instead of 6sl7, I want to keep it with the original schematic components, and the frequency response in the simulation gives me different, as well as the amplification of the square wave. Does it have to do with the 12ax7 and the ltspice model or something else?