SRS551 Pentode Curves and Model

It’s been a long and weird year with COVID-19. This evidently has kept me a bit distracted and changed priorities (as probably has done to you as well). Anyhow, here are the belated curves I was asked to publish.

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The SRS-551 is a lovely powerful transmitting pentode from RTF. Similar (or close enough) to RS1003 and F3a. Much more accessible in price is the SRS-551. I have a nice stash of NOS valves somewhere lost in my attic storage. Either way, they sounded nice and could definitely live in my setup. The valve was intended for audio modulation in FM transmitters so it’s very reliable and linear device. Distortion was extremely low when I measured it but most importantly it could work really well with local feedback to squeeze out 14W in Single-Ended at nearly 490V/100mA bias point with an LL1623 OPT. The distortion was below 0.7% predominantly H2.

Here are the curves traced with uTracer back in 2016 and model developed using Derk’s tool:

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LT Spice Models

Below is the pentode model, you can download it from here as well:SRS551-VS100V

****************************************************
.SUBCKT SRS551-VS100V 1 2 3 4 ; A G2 G1 C;
*      Extract V3.000
*
* Model created:  3-Apr-2016
*
* Curves traced and model developed by Ale Moglia valves@bartola.co.uk
* www.bartola.co.uk/valves
*
X1 1 2 3 4 BTetrodeDE  MU= 24.02 EX=1.281 kG1=  89.0 KP= 136.4 kVB = 2598.7 kG2=  739.5
+Sc=.31E+00 ap=  .039 w=    15. nu=   .00 lam=    11.6
+ Ookg1mOokG2=.988E-02 Aokg1=.12E-04 alkg1palskg2=.988E-02 be=  .138 als=  5.27 RGI=2000
+ CCG1=27.0P  CCG2 = 0.0p CPG1 = 0.24p  CG1G2 = 0.0p CCP=15.0P  ;
.ENDS
****************************************************
.SUBCKT BTetrodeDE 1 2 3 4; A G2 G1 C

*
* NOTE: LOG(x) is base e LOG or natural logarithm.
* For some Spice versions, e.g. MicroCap, this has to be changed to LN(x).
*
RE1  7 0  1MEG    ; DUMMY SO NODE 7 HAS 2 CONNECTIONS
E1 7 0 VALUE=
+{V(2,4)/KP*LOG(1+EXP(KP*(1/MU+V(3,4)/SQRT(KVB+V(2,4)*V(2,4)))))}
E2   8 0 VALUE = {Ookg1mOokG2 + Aokg1*V(1,4) - alkg1palskg2*Exp(-be*V(1,4)*SQRT(be*V(1,4)))}
E3   9 0 VALUE = {Sc/kG2*V(1,4)*(1+tanh(-ap*(V(1,4)-V(2,4)/lam+w+nu*V(3,4))))}
G1   1 4 VALUE = {0.5*(PWR(V(7),EX)+PWRS(V(7),EX))*(V(8)-V(9))}
G2   2 4 VALUE = {0.5*(PWR(V(7),EX)+PWRS(V(7),EX))/KG2 *(1+als*Exp(-be*V(1,4) * SQRT(be*V(1,4))))}
RCP  1 4  1G      ; FOR CONVERGENCE A  - C
C1   3 4  {CCG1}   ; CATHODE-GRID 1 C  - G1
C4   2 4  {CCG2}   ; CATHODE-GRID 2 C  - G2
C5   2 3  {CG1G2}   ; GRID 1 -GRID 2 G1  - G2
C2   1 3  {CPG1}  ; GRID 1-PLATE G1 - A
C3   1 4  {CCP}   ; CATHODE-PLATE A  - C
R1   3 5  {RGI}   ; FOR GRID CURRENT G1 - 5
D3   5 4  DX      ; FOR GRID CURRENT 5  - C
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS BTetrodeDE

And here is the triode model I developed manually using Dmitry’s tool. You can also triode-wire the pentode model from Derk if you want to. You can also download it from here: SRS551-triode-model

**** SRS551 TRIODE ******************************************
* Created on 04/03/2016 19:56 using paint_kit.jar 2.9 
* www.dmitrynizh.com/tubeparams_image.htm
* Plate Curves image file: SRS551 triode.png
* Data source link: www.bartola.co.uk/valves 
*
* Traced and model developed by Ale Moglia valves@bartola.co.uk
*
*----------------------------------------------------------------------------------
.SUBCKT SRS551-T 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=3P CGP=1.4P CCP=1.9P RGI=2000
+ MU=21.6 KG1=240 KP=140 KVB=300 VCT=0.132 EX=1.4 
* Vp_MAX=350 Ip_MAX=200 Vg_step=2 Vg_start=0 Vg_count=8
* Rp=4000 Vg_ac=55 P_max=60 Vg_qui=-48 Vp_qui=300
* X_MIN=81 Y_MIN=52 X_SIZE=481 Y_SIZE=465 FSZ_X=1117 FSZ_Y=651 XYGrid=false
* showLoadLine=n showIp=y isDHT=n isPP=n isAsymPP=n showDissipLimit=y 
* showIg1=n gridLevel2=n isInputSnapped=n 
* XYProjections=n harmonicPlot=n harmonics=y
*----------------------------------------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))} 
RE1 7 0 1G ; TO AVOID FLOATING NODES
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1} 
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID 
C2 2 1 {CGP} ; GRID=PLATE 
C3 1 3 {CCP} ; CATHODE-PLATE 
D3 5 3 DX ; POSITIVE GRID CURRENT 
R1 2 5 {RGI} ; POSITIVE GRID CURRENT 
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N) 
.ENDS 
*$

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