The first stage of the Ten-Tec Corsair 560 "FINAL AMP 80565" is also a class B design.
It is a bit different though because the feedback is via direct connections between the transistor Base and Collector terminals. Unlike the second stage, the feedback does not pass through the stage's input transformer (in this case T2).
One of the symptoms the rig has, is that there was a 50% drop, or more, in output power.
Then there seemed to be no power output at all.
Another symptom is that there does not appear to be any obvious bias level problem in the final stage of the amplifier. Nor do the MRF458 transistors show any sign of damage.
Also the output transistor of the Low Level Driver 81037 circuit which drives the first stage of the FINAL AMP 80565, uses the same transistor type (MRF476). This transistor is known to have a partial failure mode (loss of gain).
Taken together these indicate that the problem is in the first stage of the FINAL AMP 80565 circuit.
Image thanks to ARRL