Single ended triodes of low to no degeneration are not the same current profile as a JFET. The Child Langmuir law has a 3/2's exponent, so you get 2nd and 3rd

Single ended triodes of low to no degeneration are not the same current profile as a JFET. The Child Langmuir law has a 3/2's exponent, so you get 2nd and 3rd for the single ended case. Further, virtually all guitar amps have a push pull amplifier output (of questionable symmetry) operating into the speaker, so the even order terms are cancelling to some degree here. The general character of many tube amps is to make a smooth transition from predominantly 2nd order distortion to a mixture of odd order harmonics. This ends up as primarily the 3rd order as a guitar amp speaker makes a heck of an underdamped lowpass filter. Between the cathode bias bypass caps, the dc blocking caps, and the RC behavior of grid stopper resistors combined with the Miller capacitance, guitar amplifiers have a lot of internal bandwidth filtering that reduces IMD amplitudes. --- JFET current behavior in the saturation range is as you say, but not necessarily so in the ohmic region. Generally people using JFETs in these circuits use at least some of their ohmic range behavior. Since you have skill and experience in this area, and also have a tube amp kit, I suggest googling for "Tubes 201." This is an excellent summary article for the behavior of vacuum tubes. Also, Aiken amplification website, and Merlin Blencowe's book I mentioned yesterday you would find to be good reads.