>
The emitter of Q10 is "clenched" up at around .8 volt below that positive supply rail. I guess it cannot swing much then.
It shouldn't. Remember I asked which stage has LARGE current swings? A steady voltage implies a small current swing, low 2nd harmonic distortion.
But 0.8V is the wrong answer. R111 is twice R123 and passes nearly the same current, should have twice the voltage. R123 looks like it should have 0.6V from D59 voltage. So R111 shoudl be 1.2V. Why is it less?
Base current of Q20 is "about" 0.5mA, but the current in R121 toward the bias diodes is only 0.8mA. And Q18 sucks base current too. So counting on thumbs, D58 D59 pass less than 0.3mA. And this assumes Beta=100. Beta is VERY variable. If Q20's Beta is a little low, D58 D59 are starved.
In general you would like to have bias-set current at least 10 times the base current. If you can assume Beta is mostly over 100, then bias current can be 1/10th of collector current. Since Q20 is probably supposed to pass 50mA, R121 should pass about 5mA, not 0.8mA. For a quick-fix, make R121 10K (3.9mA).
Here are my guesses of what it should be doing with ample D58 D59 bias:
Note that R10 is doing nothing in simulation. V21 is an ideal zero-impedance voltage source. You often need R10 in real life, but with an ideal V21 then R10 is just a waste of simulation effort. So Q9's base voltage IS 0.000VDC.
But assuming R113 is low-ish, I don't see why it isn't swinging properly. Everything except the diodes seems to have enough bias and swing. Is it failing in simulation or on breadboard?