*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 J2sj74 PJF(Beta=92.12m Rs=7.748 Rd=7.748 Betatce=-.5 Lambda=4.464m 
+ Vto=-.5428 Vtotc=-2.5m Cgd=85.67p M=.3246 Pb=.3905 Fc=.5 
+ Cgs=78.27p Isr=129.8p Nr=2 Is=12.98p N=1 Xti=3 Alpha=10u Vk=100 
+ Kf=26.64E-18 Af=1)