This project was for the fun of using tubes that are a bit off the beaten path.
The input gain stage is a Russian 6z51p tube designed for high frequency IF gain stages. Not perhaps ideal for audio use and was a bit of a challenge to make a solid input gain stage out of it but that was the fun in using it.
Next we have the output tube a 6p13s that was designed for use in TV sets as a line scanning driver of the CRT. Again a bit of a odd duck for a SE amplifier but provides some useful characteristics that allow a excellent SE amplifier to be created with this tube. It has a high S (mA/V) of 9.5 and so needs little drive voltage allowing for a relaxed driver stage. The 6p13s with only 150V on the screen can sink a peak plate current with Va=100V of over 280mA so plate current is a not a issue with this tube. That is more current than a 7591 can sink with it's screen sitting at a scary 400V. The low screen voltage requirements keeps the total screen dissipation down improving ruggedness of this tube.
The plate dissipation is rated at 14 watts but in my experience this is a very conservative value and running a bit above this level is not a issue for this tube. The 6n13s can with the right screen regulator make a good poor mans 7591 having the same high S of about 10 making it very high gain and easy to drive. Do not assume I am saying a 6n13s can be put in a circuit designed for the 7591, it can not but with a amplifier designed to take advantage of the 6n13s quite good performance can be had from this tube. I have used a pair of 6n13s in push pull with a resulting 25W RMS output at low distortion. The plate cap I see as a bonus making the tube look retro and cool to me.
One issue with this tube is that it is designed for low screen voltages unlike a 6L6, 7591 or 6BQ5 and is sensitive to changes in screen voltage making it inconvenient to supply. Also makes it unsuitable for UL type operation. In this design I locked in the screen voltage with a shunt regulator at 140V. This allowed for lots of plate current while keeping the required cathode resistor to a low value reducing power and plate voltage loss while keeping drive voltage to a modest value. The regulated screen supply results in a stable operation point for consistent performance.
Below is a spice simulation of the SE amplifier that produces almost 6 watts at clipping into 8 ohms at low distortion (below .5%) almost all second harmonic and noise.
At lower power levels distortion falls to about 0.08% again almost all second harmonic. With the absence of higher order harmonics that I find tiring in many amplifier designs, tube or solid state this amplifier delivers a pleasant listening experience at moderate power levels .
Spice models used in Pspice format are show below. Unfortunately WIX seems to not allow a simple TXT file named .LIB as required for spice so I have pasted the text content below.
****************************************************
; A G2 G1 C;
* Extract V3.000
* Model created: 11-Jun-2021
*
*
* 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).
*
*
****************************************************
.SUBCKT 6Z51P-PEN-EX 1 2 3 4 PARAMS:
+ MU= 81.2 EX=1.223 kG1= 31.2 KP= 377.3 kVB = 2297.2 kG2= 122.7
+ Ookg1mOokG2=.24E-01 Aokg1=.48E-08 alkg1palskg2=.24E-01 be=.14 als=2.28 RGI=2000
+ CCG1=11.5P CCG2 = 0.0p CPG1 = 0.0016p CG1G2 = 0.0p CCP=3.3P ;
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/(1 + be*V(1,4))}
G1 1 4 VALUE = {0.5*(PWR(V(7),EX)+PWRS(V(7),EX))*V(8)}
G2 2 4 VALUE = {0.5*(PWR(V(7),EX)+PWRS(V(7),EX))/KG2 * (1+ als/(1+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 6Z51P-PEN-EX
****************************************************
* A G2 G1 C;
* Extract V3.000
* Model created: 22-Jul-2021
*
*
* 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).
*
.ENDS
*
****************************************************
.SUBCKT 6P13S-EX 1 2 3 4 PARAMS:
+ MU= 9.32 EX=1.355 kG1=180.3 KP= 37.0 kVB =562.7 kG2=4869.8
+Sc=.21E+00 ap=.014 w=124. nu=7.71 lam=2479.7
+ Ookg1mOokG2=.534E-02 Aokg1=.25E-05 alkg1palskg2=.534E-02 be=.318 als=24.24 RGI=2000
+ CCG1=17.5P CCG2=0.0p CPG1=0.9p CG1G2=0.0p CCP=6.0P;
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/(1 + 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/(1+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 6P13S-EX
Comentarios