It's almost like a joke, a tiny BD139 sounding almost huge around many rooms...

while operating in class A (electrons flowing all the time).


Just a tiny BD139 with heat sink, a big output transformer made of power transformer laminations, a tiny biasing resistor, an efficient full range baffle speaker (Zeiss Ikon)


It almost looks like a cartoon


This is the schematic:


https://euroelectron.blogspot.com/2019/05/pleiades-one-transistor-emitter.html


An output transformer is connected in place of 4Ω shown on schematic. The transformer transforms 4Ω to 60Ω so BD139 feels 60Ω and there should be close to maximum power transfer.


Biasing resistor is 17KΩ.


A 5KΩ series potentiometer to base for volume control.


Volume.


So loud with the efficient Zeis Ikon typ:31-06


For setup, signal path see previous post.


How does BD139 do this?


Almost scaring deep bass. Extended treble. Great midrange. Natural sound.