Originally Posted by
nickb
First, until you get to some level of refinement, keep it simple. this means lose as many non-essential components as possible and simulate one section at a time. The schematic below will give you the idea.
I actually did this in the static analysis. At some point I wanted to see the complexity of the power supply and the effect of the NFB loop as well.
Lastly, you can estimate the power out and THD when stepping the input using '.four' and '.meas' commands. Note the TRANS statement parameters are crucial to get meaningful results.
I followed especially that suggestion and got results like the following (using a model of the JJ 6L6GC which AFAIK has been derived from µ-tracer-data):
Code:
.step in=0.9 (Remark: step 4)
N-Period=1
Fourier components of V(out)
DC component:0.0836239
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+3 1.099e+1 1.000e+0 -161.96° 0.00°
2 2.000e+3 5.030e-1 4.577e-2 6.81° 168.76°
3 3.000e+3 1.060e+0 9.644e-2 -118.59° 43.36°
4 4.000e+3 1.861e-1 1.693e-2 135.32° 297.28°
5 5.000e+3 1.267e-1 1.153e-2 -28.53° 133.43°
6 6.000e+3 9.270e-2 8.435e-3 -132.46° 29.50°
7 7.000e+3 4.893e-2 4.452e-3 101.51° 263.46°
8 8.000e+3 3.807e-2 3.464e-3 -35.18° 126.78°
9 9.000e+3 2.748e-2 2.500e-3 -157.58° 4.38°
Total Harmonic Distortion: 10.920017%(10.923670%)
...
Measurement: power_out
step vrms**2/8
1 4.09692
2 5.0647
3 6.19534
4 7.6529
5 9.67627
So with a 6L6 i can obtain about 7.5 W at 10%THD.
After these results i concentrated on the EL84 version. I both versions i had to struggle a bit with stability issues, especially with the EL84. I am not sure to which degree these are artefacts by the models and to which degree they are real.
Code:
...
.step in=0.55 (Remark: Step 4)
N-Period=1
Fourier components of V(out)
DC component:0.466336
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+3 9.276e+0 1.000e+0 -178.70° 0.00°
2 2.000e+3 2.531e-1 2.729e-2 21.10° 199.80°
3 3.000e+3 7.622e-1 8.216e-2 -165.69° 13.01°
4 4.000e+3 1.307e-1 1.409e-2 105.87° 284.57°
5 5.000e+3 1.182e-1 1.275e-2 -17.80° 160.90°
6 6.000e+3 6.183e-2 6.666e-3 175.17° 353.87°
7 7.000e+3 5.235e-2 5.644e-3 62.64° 241.34°
8 8.000e+3 3.023e-2 3.259e-3 -64.73° 113.97°
9 9.000e+3 2.397e-2 2.584e-3 153.30° 332.00°
Total Harmonic Distortion: 8.916135%(8.923619%)
...
Measurement: power_out
step vrms**2/8
1 2.86495
2 3.71463
3 4.65839
4 5.58837
5 6.25489
6 6.67498
7 6.96952
5.6 W at 9% THD (Step 4) ... that's pretty close to the predictions of the data sheet for an anode voltage of 250 V (Valvo, Telefunken). Even the values for k2 and k3 do match reasonably well.
Overdrive:
when the EL84 is overdriven, the output power can reach 8 W. To my understanding that means a heavy thermal overload of the anode - and indeed, my amp does not like to be overdriven for a long time.
With the 6L6GC the fully clipped output corresponds to about 12W which should be on the safe side
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