6gn8 pentode model, try it , while i model triode section:

.SUBCKT 6GN8 P G2 G K ; LTSpice tetrode.asy pinout

* .SUBCKT 6GN8 P G K G2 ; Koren Pentode Pspice pinout

- PARAMS: MU=27.1 KG1=589.05 KP=105.29 KVB=12 VCT=0.2 EX=1.4 KG2=523.91 KNEE=27.71 KVC=1.811
- KLAM=2.225E-8 KLAMG=3.282E-4 KNEE2=17.44 KNEX=82.8 KNK=7.813E-4 KNG=0.04312 KNPL=1.281 KNSL=0.06875 KNPR=69.6 KNSR=32.48
- CCG=11P CGP=0.1P CCP=4.2P VGOFF=-0.6 IGA=0.001 IGB=0.3 IGC=8 IGEX=2
*----------------------------------------------------------------------------------

- Vp_MAX=400 Ip_MAX=80 Vg_step=1 Vg_start=0 Vg_count=20
- X_MIN=39 Y_MIN=106 X_SIZE=754 Y_SIZE=478 FSZ_X=1296 FSZ_Y=736 XYGrid=false
- Rp=1400 Vg_ac=20 P_max=5 Vg_qui=-9.5 Vp_qui=300
- showLoadLine=n showIp=y isDHP=n isPP=n isAsymPP=n isUL=n showDissipLimit=y
- showIg1=y isInputSnapped=y addLocalNFB=n
- XYProjections=n harmonicPlot=y dissipPlot=n
- UL=0.43 EG2=150 gridLevel2=y addKink=y isTanhKnee=y advSigmoid=n

RE1 7 0 1G ; DUMMY SO NODE 7 HAS 2 CONNECTIONS

E1 7 0 VALUE= ; E1 BREAKS UP LONG EQUATION FOR G1.

+{V(G2,K)/KP*LOG(1+EXP((1/MU+(VCT+V(G,K))/SQRT(KVB+V(G2,K)*V(G2,K)))*KP))}

RE2 6 0 1G ; DUMMY SO NODE 6 HAS 2 CONNECTIONS

E2 6 0 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))} ; Kg1 times KIT current

RE21 21 0 1

E21 21 0 VALUE={V(6)/KG1*ATAN((V(P,K)+KNEX)/KNEE)*TANH(V(P,K)/KNEE2)} ; Ip with knee but no slope and no kink

RE22 22 0 1 ; E22: kink curr deviation for plate

E22 22 0 VALUE={V(21)LIMIT(KNK-V(G,K)*KNG,0,0.3)(-ATAN((V(P,K)-KNPL)/KNSL)+ATAN((V(P,K)-KNPR)/KNSR))}

G1 P K VALUE={V(21)*(1+KLAMG*V(P,K))+KLAM*V(P,K) + V(22)}

* Alexander Gurskii screen current, see audioXpress 2/2011, with slope and kink added

RE43 43 K 1G ; Dummy

E43 43 G2 VALUE={0} ; Dummy

G2 43 K VALUE={V(6)/KG2*(KVC-ATAN((V(P,K)+KNEX)/KNEE)*TANH(V(P,K)/KNEE2))/(1+KLAMG*V(P,K))-V(22)}

RCP P K 1G ; FOR CONVERGENCE

C1 K G {CCG} ; CATHODE-GRID 1

C2 G P {CGP} ; GRID 1-PLATE

C3 K P {CCP} ; CATHODE-PLATE

RE23 G 0 1G

GG G K VALUE={(IGA+IGB/(IGC+V(P,K)))(MU/KG1)

+(PWR(V(G,K)-VGOFF,IGEX)+PWRS(V(G,K)-VGOFF,IGEX))}

.ENDS

*$