You gave links to two models so use one of them.
Click Edit in LTspice and select SPICE Directive and paste this
.Subckt 6AK5 A S G K
+ Params: Ex=1.919796584 Mug=28.62765031 Mus=5.93E+02 Kg1=1241.263033 Kp=87.71075724
+ Kvb=0.001045715 Go=-0.174882604 Kd=5.00E-02 Exd=1.018581504 Exsd=1.106610165
+ C1= -20.45944113 A1=-0.001923837 B1=20.33820425 X1=0.011665792 Y1= 0.000268033 Sr=0.032709138
+ Cgk=4P Cgs=.5P Cga=0.03P Cak=2.8P
* calculate the total space current based on Koren's triode equations,
* but with an equivalent triode plate volatage equal to the pentode Vs + Vp/Mus
En1 n1 0 VALUE={(V(A,K)/Mus+V(S,K))/Kp*LN(1+EXP(Kp*(1/Mug+(V(G,K)-Go)/SQRT(Kvb+(V(A,K)/Mus+V(S,K))**2))))}
Esc sc 0 VALUE={ (PWR(V(n1),Ex)+PWRS(V(n1), Ex))/(Kg1) }
* calculate the plate current capture ratio
Ecr cr 0 Value={If(V(A,K)<=0,0,C1+A1*V(A,K)+B1*((limit(V(A, G),1,1e6)**X1)/(limit(V(S,G),1,1e6)**Y1)))}
* the tentative plate current is total space current multiplied by capture ratio
Ept pt 0 Value={V(sc)*V(cr)}
* calculate the diode line current
Ed d 0 value={Kd*V(A,K)**Exd*V(S,K)**Exsd}
* the actual Plate current is the lesser of pt and d
Ep p 0 value={Min(V(d),V(pt))}
Ga A K value={V(p)}
* calculate the screen current capture ratio
* if Vg <= 0 then the screen gets all the remaining current, otherwise the grid gets some
Escr scr 0 value={Limit(if(V(G,K)>0,1-Sr*V(G,K),1),0,1)}
* screen current is anything not captured by the plate, multiplied by screen capture ratio
Esg sg 0 value={(V(sc)-V(p))*V(scr)}
Gs S K value={V(sg)}
* grid current is anything not captured by the plate or the screen
Gg G K value={V(sc)-v(p)-V(sg)}
* interelectrode capacitances
C1 G K {Cgk}
C2 G S {Cgs}
C3 G A {Cga}
C4 A K {Cak}
.ENDS 6AK5
Then add tetrode from components and give it 6ak5 for value.
You can also save it to a file and then add spice directive .inc yourfile.txt
And remember that spice will lie to you.
