I guess the designer can explain the operation to us mortals, looks a bit fishy/
Definitely fishy. The definition of stable that I used when I started out was that it doesn't oscillate. Actually, that's a more sophisticated definition...I started with it's stable if it's not on fire. Later I raised my game to: not much ringing. It wasn't until too much time had passed that I realized the vast chasm in my knowledge: stability is a very elusive creature.
My first amp was a Millard amp using ECC83 and an EL84 with transformers stripped from of an old broken radio gram in the sixties, that was when I knew audio will become my life. The chassis was also made of cardboard cut from a shoebox. It was amazing because it worked first time! Then I duplicated it because stereo became the thing, but the second channel hummed, probably because I knew nothing of grounding.
That is when I also learned that HT capacitors remains at 350VDC long after the amp was turned off
That is when I also learned that HT capacitors remains at 350VDC long after the amp was turned off
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One firework I remember happened when I had my amp on the lounge floor with the lid off (the lid was always off because I was constantly altering the circuit!). Anyhow, on this occasion, "instability" caused a tantalum capacitor to overheat, separate from its legs and launch itself into the air like a miniature flare. Much like the recent Musk rocket. It had a bright flame and a trail of gasses and drew a perfect parabola in the air, up and then down onto the arm of my new sofa, burning a small hole. I was not pleased.