Two diodes in input stage of power amplifier
The collector of \$Q_{19}\$ cannot be more than a diode drop above the bottom rail, because its base is tied to the collector. \$D_5\$ then prevents the collector of \$Q_{12}\$ from being more than two diode drops above the bottom rail. Since the \$Q_6\$ (I was wrong to write \$Q_8\$ before, in comments) base is tied there too, this prevents the base of \$Q_6\$ from being hauled up more than two diode drops above its emitter. That shouldn't be a problem in a well designed circuit. But it might help limit things in unplanned abusive situations with the circuit.

I wasn't quite as sure of \$D_6\$ before. But \$Q_{19}\$ and \$Q_{12}\$ do form up a mirror. Normally, a design (and the DC portion of the feedback from the output) will arrange things so that a slight voltage imbalance (a minor DC "tilt") at the bases of \$Q_1\$ and \$Q_2\$ supply the needed base current for the \$Q_6\$ VAS (which has to sink all the remaining current left over from the \$Q_5\$ source.) The applied signal will normally swing the base of \$Q_1\$ around a bit and this will vary the current squirted into the base of \$Q_6\$. This difference current shouldn't be leaving from the collector side of \$Q_2\$. But in the oddball case where it is (such as at certain frequencies where the phase delay causes positive feedback and the whole thing isn't working as it should), \$D_6\$ might then make sense as a path to allow the mirror and the diff-pair to continue functioning in this crazy situation. Normally, you don't design things so that you squirt out the current from the leg that is tied to the diode-connected mirror BJT. But I can imagine extreme cases where you'd figure adding this diode path makes some sense.



Actually, I'd be happy to hear the input (and criticism) of my betters here. I'm just a hobbyist and in cases where I've done something like this myself, I didn't bother with these diodes and everything was just fine. But that doesn't mean they aren't a good idea for making a circuit more bullet-proof. I may not be fully aware of the possible thinking here. But that's my take.