In the late 90's the Japanese Shin-ichi Kamijo created special circuitry for tube amplifiers to bring the rough sound of a Pentode power amplifier closer to the soft sound quality of a Triode stage.
This was archived by adding a kind of "non-linear negative local" feedback by supplying a small amount of the modulated Anode voltage from the power Pentode before the putput transformer to the Anode of the preamplifier's Triode.
The special feedback circuitry results in:
1st: The triode now "sees" the limited frequency response and distortions due to the modulated Anode voltage of the Pentode. The feedback circuitry adds these errors to the audio signal. Due to the 180° phase shift of the inverting power amplifier stage (negative feedback) these distortions will be eliminated and the frequency response is flattened. In a way, the Triode "supervises" the Pentode power amplifier stage.
2nd: Normally the feedback path is connected to the Triodes Grid but in this case it’s connected to the Anode so the Triode "sees" only the Pentode's distortions (and eliminates it) but not its own harmonic distortions.
But why not using such a circuitry for a semiconductor power amplifier ?
The result is absolute astonishing! The end result is the well-known Triode sound! You do not hear the semiconductor “sound” with its distortions; only the warm Triode tube sound is present and nothing more!
There is no comparison to today’s tube / semiconductor hybrid power amplifier. Current Hybrid amplifier designs employ a semiconductor power stage that works autonomous without providing feedback control to the tube stage. The audio signal is feed through the preamplifier Triode tube stage without the benefits of feedback from the semiconductor power amplifier stage. Furthermore, there are distortions added by the semiconductor power amplifier stage. This results in a significant audible distortion on the speaker output compared to the STC architecture.
This kit comes fully inclusive with all parts including an external switching power supply, tubes and enclosures to build a plug-and-play amplifier as shown on the photo. Most parts on the board are SMT types with expensive 1% MiniMelf resistors in the audio path and are pre-assembled. You only need to mount and solder the through-hole components. This can be done in an evening.
Please note that up to 180V is present on the board (for the Magic Eyes Anode voltage) therefore this kit is not recommended for electronic beginners.