EML 20-AM DHT curves and model

There is one DHT which attracted me from Emission Labs which is the EML20. If you’re looking for a mid-mu DHT valve these days, this one is the way to go. In order to meet with the two key requirements of linearity and low microphonic noise, EML made a great effort in producing this valve. 

For me it’s a great candidate as a driver or for a Spud amp for headphones. I will likely use them in several places, but will start with a preamp stage,  as you would expect from me anyway.

First step: tracing the EML20AM

Using the eTracer, I managed to plot the nice set of curves up to 650V:

The next step was to produce a nice set of curves to make it easier the process of developing the Spice model:

The model adjusts very well. Unfortunately there’s no capacitance data on the EML data sheet. I have to say that this is an area where they fall short. These days you would expect a detailed data sheet. That is not the case so far, I hope they improve on this in the future.

**** EML20AM TRIODE Composite DHT *****************************************
* Created on 01/21/2018 12:10 using paint_kit.jar 3.0 
* www.dmitrynizh.com/tubeparams_image.htm
* Plate Curves image file: EML20AM triode.png
* Data source link: www.bartola.co.uk/valves
*
* Model and curve traced by Ale Moglia
* (c) Bartola Ltd. UK, London, UK
* valves@bartola.co.uk
*
* Curved traced with eTracer
*
* Notes: missing capacitance data, not available in datasheet
*
*----------------------------------------------------------------------------------
.SUBCKT DHT_EML20AM 1 2 3 4 ; P G K1 K2
+ PARAMS: CCG=1P CGP=1P CCP=1P RFIL=3.33
+ MU=20.7 KG1=1230 KP=300 KVB=60 VCT=-3.68 EX=1.4 RGI=2000
* Vp_MAX=600 Ip_MAX=60 Vg_step=2 Vg_start=0 Vg_count=14
* Rp=4000 Vg_ac=55 P_max=9 Vg_qui=-48 Vp_qui=300
* X_MIN=75 Y_MIN=51 X_SIZE=509 Y_SIZE=556 FSZ_X=1178 FSZ_Y=705 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 
*$

The Spice Model works really well. You can download it here: EML20AM spice model

Still need to undertake the usual THD measurements for various operating points. It looks like it’s best to run it around 300V. My current DHT preamp HT supply will not fit the optimal operating point, but playing in LTSpice, I can implement this one easily with filament bias.

Hope this helps

Ale

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) View all posts by Ale Moglia