pspice to ADS20xx netlist conversion

Can someone advise on how to convert the following pspice netlist (a vacuum tube, 6146) to something that will run on ads20xx. I setup a design in ads using SDD but i'm having trouble in convergence. the problem seems to be in the current generation in the grid networks.


*************************
.SUBCKT 6146 A S G K
*
* Calculate contribution to cathode current
*
Eat at 0 VALUE={0.636*ATAN(V(A,K)/10)}
Eme me 0 VALUE={LIMIT{V(A,K),0,10000}/130}
Emu mu 0 VALUE={PWRS(V(G,K),1-(LIMIT{-V(G,K),30,9999}-30)/2000)}
Egs gs 0 VALUE={LIMIT{V(A,K)/10+V(S,K)*4.5+V(mu)*19,0,1E6}}
Egs2 gs2 0 VALUE={PWRS(V(gs),1.5)*16E-6}
Ecath cc 0 VALUE={LIMIT{V(gs2)*V(at),0,V(me)}}
Esd sd 0 VALUE={LIMIT{V(gs2)*V(at)-V(me),0,99999}}
*
* Calculate anode current
*
Ga A K VALUE={V(cc)}
*
* Calculate screen current
*
Escrn sc 0 VALUE={2.5E-4*PWRS(V(S,K),1.5)*V(gs2)*(1.1-V(at))+V(sd)}
Gs S K VALUE={V(sc)}
*
* Grid current
*
Gg G K VALUE={PWR(LIMIT{V(G,K)+1,0,1E6},1.5)*(1.25-V(at))*870E-6}
*
* Capacitances
*
Cg1 G K 13.0p
Cak A K 8.5p
Cg1a G A 0.24p

.ENDS

**********************

Thanks
rfc