*http://www.diyaudio.com/forums/software-tools/101810-spice-simulation-49.html#post1420730
*K170 and J74 models are set to an Idss of about 12mA while K246 and J103 are set to an Idss of about 6mA.
*For other Idss values, adjust the model BETA parameter.
*In a first approximation, BETA=Idss*Vto^2 where Vto is the threshold voltage, already defined in each model. 

.model J2sk170gr NJF(Beta=59.86m Rs=4.151 Rd=4.151 Betatce=-.5 Lambda=1.923m
+ Vto=-.5024 Vtotc=-2.5m Cgd=20p M=.3805 Pb=.4746 Fc=.5
+ Cgs=25.48p Isr=84.77p Nr=2 Is=8.477p N=1 Xti=3 Alpha=10u Vk=100
+ Kf=111.3E-18 Af=1)