Good news.
There was found (bought long time ago) a 50VA toroidal power transformer made by Giatras in Athens. (VA means volt times amperes ie watts). Voltage times Current is Power. Similarly to Force times Velocity equals Power in mechanical systems.
This transformer has a secondary of 9V. And another secondary of 75V.
The 75V secondary was connected to the BD139 operating in class A (electrons flowing all the time).
The 9V winding to the 4Ω Pleiades Linoleum Auratone 5C type speaker.
It worked. Sound is much louder than connecting the 4Ω speaker directly to the transistor.
Sound is very promising.
The 9V winding has inductance of about 8mH which looks fine. Maybe 25mH would be better as in the already being used EI lamination transformer. It may be found by experiment what sounds best.
The 75V winding has an inductance of 660mH which looks too much. The inductance on the cureemt used output transformer (see previous posts) is about 80mH.
Once a new winding of lower inductance is wound on this toroid, sound and impedance match should be better or optimized.
If things turn out well it is good news.
A world class amplifier operating in class A and no negative feedback would be very easy. With just one transistor, a battery in series with a fuse for safety, a biasing resistor, a toroidal transformer with windings of few turns.
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