Some say you can use 6f12p instead of DA3. I just almost completed sim with 2 models, one painted myself and one from the net. The overall gain is 1700X or 17dbs, cathode fully bypassed. Here are the models, if you're interested to sim futher and share the result noise level etc, I can upload the source asc. I have clean up floor noise in FFT sim result as attached.

A) Pentode Section

**** 6F12PI_P ******************************************

* Created on 02/07/2016 22:40 using paint_kip.jar

*

Model Paint Tools: Trace Tube Parameters over Plate Curves, Interactively
* Plate Curves image file: 6f12pi-p.png

* Data source link: <plate curves URL>

*----------------------------------------------------------------------------------

.SUBCKT 6F12P-P 1 2 3 4 ; P G K G2

+ PARAMS: CCG=6.6P CGP=1.4P CCP=1.9P RGI=1030

+ MU=112 KG1=101.8 KP=361.1 KVB=1.92 KVC=1.6 VCT=0.00863 EX=0.664 KG2=12

* Vp_MAX=320 Ip_MAX=37.5 Vg_step=0.5 Vg_start=0 Vg_count=8

* Rp=1600 Vg_ac=23.5 P_max=5 Vg_qui=-23.4 Vp_qui=240 UL=0.43 EG2=120

* X_MIN=102 Y_MIN=74 X_SIZE=699 Y_SIZE=646 FSZ_X=1359 FSZ_Y=856 XYGrid=true

* showLoadLine=n showIp=y isDHP=n isPP=n isAsymPP=n isUL=n showDissipLimit=y

* showIg1=n gridLevel2=n isInputSnapped=n

* XYProjections=n harmonicPlot=y harmonics=y

*----------------------------------------------------------------------------------

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

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

+{V(4,3)/KP*LOG(1+EXP((1/MU+(VCT+V(2,3))/V(4,3))*KP))}

G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1*ATAN(V(1,3)/KVB)}

* Alexander Gurskii screen current, see audioXpress 2/2011

RE2 8 3 1G ; Dummy

G2 8 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG2*(KVC-ATAN(V(1,3)/KVB))}

E2 8 4 VALUE={0} ; Dummy

RCP 1 3 1G ; FOR CONVERGENCE

C1 2 3 {CCG} ; CATHODE-GRID 1

C2 1 2 {CGP} ; GRID 1-PLATE

C3 1 3 {CCP} ; CATHODE-PLATE

R1 2 5 {RGI} ; FOR GRID CURRENT

D3 5 3 DX ; FOR GRID CURRENT

.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)

.ENDS

*$

B) Triode section

.SUBCKT 6F12P-T 1 2 3 ; P G C (Triode)

+ PARAMS: MU=112.5 EX= 1.66 KG1= 94 KP=760

+ KVB=12100.0 VCT= 0.00 RGI=4500

+ CCG=4.6P CGP=1.6P CCP=0.26P

E1 7 0 VALUE=

+{V(1,3)/KP*LN(1+EXP(KP*(1/MU+(V(2,3)+VCT)/SQRT(KVB+V(1,3)*V(1,3)))))}

RE1 7 0 1G

G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}

RCP 1 3 1G ; TO AVOID FLOATING NODES IN MU-FOLLOWER

C1 2 3 {CCG} ; CATHODE-GRID;

C2 2 1 {CGP} ; GRID-PLATE;

C3 1 3 {CCP} ; CATHODE-PLATE;

D3 5 3 DX ; FOR GRID CURRENT

R1 2 5 {RGI} ; FOR GRID CURRENT

.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)

.ENDS