Sebastien,
I did not see any additional negative feed-back in the Essen circuit, though it is possible that I was not looking hard enough.
Some radios with similar PA stages do have an additional 100 Ohm - 200 Ohm resistor, inside the radio, parallel with the speaker, like R182 in that snippet that I enclosed.
(That output capacitor to the speaker, just before R182, it is probably 1000uF correctly and not 100uF!)
Such a resistor does not interfere with the working of the PA-stage when a speaker is attached, it barely dissipates any power, it sole purpose is to "keep the circuit happy" when there is no speaker attached. The DC voltages within the PA-stage are nearly the same in this case as with the speaker.
It is possible that the Mannheim radio has such a resistor installed, though the schematics does not show it.
Chris,
I collected a few GE-transistor models during the years.
Here is a thread about the "fake" AC128 models, and a pointer to an Excel program that helps modeling transistors:
https://www.diyaudio.com/forums/soli...odel-rare.html
(Someone mentioned that the program is no longer at the original site, though it still could be found with Google.)
Someone made a careful model of the AD161/162 transistors that I saved, I think it was on that site too.
There are a few models of old Ge-transistors in the LTspice example circuits too.
I also understand that Pspice has a utility to generate bjt models, though it is not enabled in the demo version.
Regards, Peter