About 5 years ago I built the first version of the Aa DHT preamp. It looked like this:
This preamp sounded great (as you would expect with this DHT valve). As wrote before, with a gain of about 30 is ideal if you need the gain. More importantly, it’s timbre and detail is unique but is challenging to use. It has an internal anode resistance of about 30KΩ, so best topology is what I presented before as choke loaded is a real challenge.
The filament bias configuration with the Russian wire wound resistor of 5Ω is perfect drop-in. Later I tried the SiC bias configuration which was also great.
Recently I was asked for the Spice model. I thought I had posted it before, but I was wrong. So here it is:
I traced the curves of a Valvo Aa valve which tested 97% (Ia=2.92mA/1.09mS @ 220V/Vgk=-2V):
And here is the manual fit of the DHT model:
Below is the model, and you can also download it from here: Aa spice model:
**** Aa TRIODE Composite DHT *****************************************
* Created on 10/13/2017 18:31 using paint_kit.jar 2.9
* www.dmitrynizh.com/tubeparams_image.htm
*
* Traced and model by Ale Moglia valves@bartola.co.uk
* (c) 2017 Ale Moglia and Bartola Ltd. UK
* www.bartola.co.uk/valves
*----------------------------------------------------------------------------------
.SUBCKT DHT_Aa 1 2 3 4 ; P G K1 K2
+ PARAMS: CCG=1P CGP=3.5P CCP=1P RFIL=7.6
+ MU=30.3 KG1=4740 KP=216 KVB=720 VCT=-1.33 EX=1.41 RGI=2000
* Vp_MAX=350 Ip_MAX=8 Vg_step=0.5 Vg_start=0 Vg_count=11
* Rp=4000 Vg_ac=55 P_max=1.5 Vg_qui=-48 Vp_qui=300
* X_MIN=69 Y_MIN=51 X_SIZE=437 Y_SIZE=429 FSZ_X=1134 FSZ_Y=689 XYGrid=false
* showLoadLine=n showIp=y isDHT=y isPP=n isAsymPP=n showDissipLimit=y
* showIg1=n gridLevel2=n isInputSnapped=n
* XYProjections=n harmonicPlot=n harmonics=y
*----------------------------------------------------------------------------------
RFIL_LEFT 3 31 {RFIL/4}
RFIL_RIGHT 4 41 {RFIL/4}
RFIL_MIDDLE1 31 34 {RFIL/4}
RFIL_MIDDLE2 34 41 {RFIL/4}
E11 32 0 VALUE={V(1,31)/KP*LOG(1+EXP(KP*(1/MU+V(2,31)/SQRT(KVB+V(1,31)*V(1,31)))))}
E12 42 0 VALUE={V(1,41)/KP*LOG(1+EXP(KP*(1/MU+V(2,41)/SQRT(KVB+V(1,41)*V(1,41)))))}
RE11 32 0 1G
RE12 42 0 1G
G11 1 31 VALUE={(PWR(V(32),EX)+PWRS(V(32),EX))/(2*KG1)}
G12 1 41 VALUE={(PWR(V(42),EX)+PWRS(V(42),EX))/(2*KG1)}
RCP1 1 34 1G
C1 2 34 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 34 {CCP} ; CATHODE-PLATE
D3 5 3 DX ; FOR GRID CURRENT
D4 6 4 DX ; FOR GRID CURRENT
RG1 2 5 {2*RGI} ; FOR GRID CURRENT
RG2 2 6 {2*RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS
*$
