Amplifier 14: Pentode Push Pull Amplifier with Cathode Feedback

Amplifier 14 is a real pentode amplifier with lots of power. The screen grids are directly connected to the high voltage power supply. They seem to deliver no feedback (*). The cathodes again are connected to the secondary winding of the OPT. This creates strong local feedback. Click here to download the measurement results.

The sound character of amplifier14 is warm, but not too colored. The amp sounds very dynamic with lots of power. Not all details are noticeable, this amp is on its way to high end. I consider this an excellent amplifier for Jazz and acoustical guitars with lots of details and emphasis on open dynamic and detailed tonal structures. When in overdrive, the tonal structure stays easy and soft, excellent for a warm blues sound.
What makes this amplifier to sound much better than amplifier 13. Maybe it was the number 13? Jokes aside: in amplifier 13 something strange happened. First I applied positive feedback at the screen grids, enlarging the flaws of the tube! Then I used negative feedback through the cathodes, where the flaws were corrected again. This is like stimulating children to play in the dirt and to punish them afterwards for being dirty. Nothing good can come out of such. In amplifier 14 a better job is done. There might have been some distortion, but this is directly corrected by the effective cathode feedback. Also there is some feedback of the screen grids because the voltage between screen grids and cathodes is not constant. This effect is small, but present (*). The final result of this good approach is that the amp maintains full power, while its internal distortions are drastically reduced through the very effective cathode feedback.
The –3dB high frequency range is large. This is caused by a small effective Millar capacitance between the control grid and the anode. The screen grid indeed is able to function as a screen, because its voltage is constant at Vo. In the next amplifiers I can tell more about this important influence of the Miller capacitance.

Next time: amplifier 15