RL12P35 German Transmitting Pentodes

It was pure lust and love at first sight. I found the RL12P35 german transmitting pentodes and couldn’t resist in buying them. A very nice set of NOS Telefunken and Valvo valves with also NOS sockets. These pentodes look like a de-rated LS-50/GU-50 and very interesting candidates for a nice PP amp:

These have been used by Lorenz to build the classic and lovely PP amp LVA/BA30 RL12P35 Amp with anode to grid feedback (i.e. a la Schade) like the below amplifier:

When I submitted to the mercy of the uTracer, I had to work around the 300V limitation of my uTracer v3 (I do have the 400V upgrade kit but I haven’t got around to modify it yet). So I looked into the data sheet to find an operating point to measure my valves against:

The valves measured really well, most of them above 90%. The test point used was Va=250V, Vs=200V, Vgk=-15V which aimed for Ia=110mA:

From my traces I could see that the screen current should be about 25mA, Ra closer to 24-25kΩ and transconductance around 4.5mA/V which is predicted from the data sheets.

The generated Spice model matches really well:

 

Here is the Spice model of the pentode: RL12P35-pentode-150V:

****************************************************
.SUBCKT RL12P35-pentode 1 2 3 4 ; A G2 G1 C;
* Extract V3.000
* Model created: 28-May-2016
*
* Traced and model created by Ale Moglia / valves@bartola.co.uk
* www.bartola.co.uk/valves
*
*
*
X1 1 2 3 4 BTetrodeDE MU= 5.01 EX=1.331 kG1= 676.1 KP= 14.9 kVB = 1008.8 kG2= 3519.4
+Sc=.12E+00 ap= .034 w= 16. nu= .24 lam= 11.8
+ Ookg1mOokG2=.119E-02 Aokg1=.46E-06 alkg1palskg2=.119E-02 be= .151 als= 3.10 RGI=2000
+ CCG1=16.5P CCG2 = 0.0p CPG1 = 0.05p CG1G2 = 5.7p CCP=10.4P ;
.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

Enjoy!

Ale