Notice the presence of cathode resistor induced current feedback on both stages in this configuration. The input of the amplifier is protected aga ins RF by a RCRCR filter tuned at ~100 KHz. I live near an AM radio station and my audio gear needs good RF interference protection. This filter does not impact audio frequencies, but cuts off RF. The output transformer represents a 3K load when feeding a 8 Ohm driver.
With PSPICE, we obtained the following data for 1 kHz, 5 Vampl Sin wave input: Vout = 5 VRMS, THD = .6:
HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 7.062E+00 1.000E+00 -8.032E-01 0.000E+00 2 2.000E+03 2.563E-02 3.630E-03 9.083E+01 9.243E+01 3 3.000E+03 3.134E-02 4.438E-03 -4.429E+00 -2.020E+00 4 4.000E+03 7.746E-03 1.097E-03 -1.008E+02 -9.755E+01 5 5.000E+03 2.355E-03 3.335E-04 1.718E+02 1.758E+02 TOTAL HARMONIC DISTORTION = 5.847176E-01 PERCENT1st tube is biased at 17 mA and sees ~ 210 V of cathode-grid voltage. 2nd tube is biased at 62 mA and sees ~ 310 V.
Here the output probe is in the FFT - Fast Fourie Transform - mode:
In the next variant we added a bypass capacitor for 2A3 tube:
We also made several less apparent changes. The driver stage receives only 75V on the plate. We also re-biased the input tube: with 11k resistor it run less hot. These changes give considerable THD cancellation effect. With 3.5 Vampl we get 35 mV of 2nd and 3rd harmonics.
With this input, the amp delivers over 4 W RMS:
This configuration is capable of delivering more power with lower THD than standard 6sn7+R, 2a3 topology.
Still, it does not have a very good overloading behavior. For Vampl= 4 we get 5.5 W RMS, and the spectrum is bad:
lots of odd harmonics, definitely audible.
Interestingly, looking at THD, we can only conclude that the result is very good because 5.5 W RMS is much more than 3.5 W RMS or so that the traditional topology can deliver with equal THD. However, distortion of the traditional topology is predominantly 2nd-order. This show that spectrum data is much more revealing than THD.
we could expect increas of THD at moerate-to-high levels of signal but in fact with 3.1 Vampl as input we get almost 4 W RMS with .8 THD:
Here is the output of the FFT PSPICE analyzer:
NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 8.159E+00 1.000E+00 -1.739E+00 0.000E+00 2 2.000E+03 4.534E-02 5.557E-03 1.222E+02 1.257E+02 3 3.000E+03 4.620E-02 5.662E-03 -1.625E+01 -1.103E+01 4 4.000E+03 1.641E-02 2.011E-03 -1.157E+02 -1.087E+02 5 5.000E+03 7.431E-03 9.108E-04 1.550E+02 1.637E+02 TOTAL HARMONIC DISTORTION = 8.234891E-01 PERCENTThis is encouraging! How about overloading? Here we simulate pre- clipping conditions - input is raised to 3.6 Vampl.
NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 9.345E+00 1.000E+00 -1.772E+00 0.000E+00 2 2.000E+03 8.499E-02 9.095E-03 1.036E+02 1.072E+02 3 3.000E+03 1.776E-01 1.900E-02 -1.055E+01 -5.238E+00 4 4.000E+03 2.708E-02 2.898E-03 -1.242E+02 -1.171E+02 5 5.000E+03 7.944E-02 8.502E-03 1.686E+02 1.775E+02 TOTAL HARMONIC DISTORTION = 2.290172E+00 PERCENT
We got the same spectrum we saw previously - with lots of odd harmonics. However, this clipping happens due to screen current of 2a3, which is evident if we disconnect the second stage.
B+ 250 V Rc 5k HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 7.539E+00 1.000E+00 -2.185E+00 0.000E+00 2 2.000E+03 1.895E-01 2.514E-02 8.447E+01 8.884E+01 3 3.000E+03 6.120E-02 8.118E-03 -1.166E+01 -5.108E+00 4 4.000E+03 1.179E-02 1.563E-03 -1.089E+02 -1.002E+02 5 5.000E+03 2.574E-03 3.415E-04 1.643E+02 1.752E+02 TOTAL HARMONIC DISTORTION = 2.646387E+00 PERCENT
B+ 75 V Rc 5k HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 8.130E+00 1.000E+00 -2.063E+00 0.000E+00 2 2.000E+03 5.096E-02 6.268E-03 1.001E+02 1.042E+02 3 3.000E+03 5.681E-02 6.987E-03 -1.544E+01 -9.246E+00 4 4.000E+03 2.131E-02 2.621E-03 -1.110E+02 -1.027E+02 5 5.000E+03 9.697E-03 1.193E-03 1.586E+02 1.689E+02 TOTAL HARMONIC DISTORTION = 9.818634E-01 PERCENT1st harmonic increased 10%, 2nd decreased more than 3-fold, 3rd dropped slightly, 4th is slightly higher and 5th is 3..4 times higher. A tradeoff? How about if we have comparable 1st? With Vin 2.8 Vampl we get
B+ 75 V Rc 5k HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 7.603E+00 1.000E+00 -2.062E+00 0.000E+00 2 2.000E+03 3.353E-02 4.411E-03 1.077E+02 1.118E+02 3 3.000E+03 3.950E-02 5.195E-03 -1.620E+01 -1.001E+01 4 4.000E+03 1.267E-02 1.666E-03 -1.135E+02 -1.053E+02 5 5.000E+03 4.928E-03 6.481E-04 1.551E+02 1.654E+02 TOTAL HARMONIC DISTORTION = 7.045707E-01 PERCENTWe traded a 2-fold increase in the 5th to much lower 2nd, somewhat lower 3rd and a notch smaller 4th. This tradeoff is easier. However, 5th is extremely unpleasant to the ear, and humans are very sensitive to it. IS it better to lower 2nd 6 times by rasing the 5th 2x? Something to think about - everyone should make personal conclusion as tastes may differ.
Another interesting observation is how harmonics interplay across the power range. Here is one more notch - .2 Vampl as input:
HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E+03 5.437E-01 1.000E+00 -2.027E+00 0.000E+00 2 2.000E+03 2.212E-04 4.068E-04 -1.795E+02 -1.754E+02 3 3.000E+03 1.955E-05 3.596E-05 -1.616E+02 -1.555E+02 4 4.000E+03 4.200E-05 7.725E-05 -1.721E+02 -1.640E+02 5 5.000E+03 1.816E-05 3.339E-05 -1.731E+02 -1.630E+02 TOTAL HARMONIC DISTORTION = 4.169924E-02 PERCENThere, with 15 mWRMS of power and .04% THD we still do not see a more "natural" spectrum - as given by a "classical" 3-stage SET, instead the 3rd dips under the 4th and is equal to the 5th.
The following is a plot across output voltage range of .1... 8.2 V ampl:
or the same data but in logarithmic scale:
A "dip at 2nd" pattern at higher power gradually transforms into a "dip at 3rd" pattern as power goes down. Does it introduce changes in timbre that affect listener's perception> If so, how? IS this a desirable variation or not? These questions are still open. It is my opinion that the topologies described here can be implemented in such manner that adjustments can be done in real time, with a knob (a THDC knob?) or 2 separate knobs - for bias and plate volatage - and the future will tell which positions will be most preferable.
Non-linear TV tubes such as 6an4, tubes such as 6aq8 and high-mu high-current tubes such as 6c45pi may be promising as potentially allowing THDC at higher currents. 6c45pi maybe be the most promising since 6an4 seems to pre-distort "too much" for 2a3.