The first generation vertical devices like the above D series were much lower gm than modern parts and far better behaved in the linear region with SOAR. There is no modern equivalent to my knowledge
This capacitor helps to increase the open loop gain of the amplifier and to reduce the distortion of the closed loop gain.
From the JLH article, concerning C13 and R4 with J1:
"they form a DC feedback, used to regulate the input current supplied to the input long-tailed pair so that the output voltage at the collector of Q5 is stabilized".
R21 and R25 adjust the positive feedback ratio to increase the open loop gain of the amplifier.
It's an unusual circuit for sure. I think J1, C13, R4 are used to adjust the right end of R21 to be at near gnd; like a dc servo. R25 is a mystery...it doesn't provide much PFB at all. Curious.
The circuit as a whole looks marginally stable.
By testing, on differents loads (resistive, capacitive, etc.) and also on different real loudspeakers etc,
I think one has to define what is stable. Is it like a chair with four feet and one is a little shorter. One may argue it is not stable, but the chair does not fall over. In an amplifier, what is stability? Is it thermally running away, does it oscillate with some weird load, does it distort severely, does the devices break down or latch up, does it clip symmetrically, does it produce a clean square wave into any load, does the output vary up and down, is the output power constant, does it stop working after a while, does it perform identically under any temperature conditions? Stable is a very nebulous concept.
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