The voltage between the two driver bases won't change much when you replace R42.But the current through the drivers will
Look upon C12 (and all the bias the parts attached to it) as an elevator.It can rise up and down, almost to the 60th floor and 60 floors down.But the height of the elevator always stays the same.
A fusable resistor is just a normal resistor that burns up in a controlled way when something is wrong.Up to you to use fusables or not.
R9 should be 1k. I assume you measured that disconnected, not in circuit.A wrong value here could upset the balance of the amp.
OK, But what voltage should I see across C12? How do i know when I have it set right?
Somewhere I think I was advised the voltages at the bases of the transistors listed in the drawing needed to be exact and so shouldn't the R29,30, and 34 voltages drops etc. Is that right? They are all high.
The voltage across C12 should be ~~~2volt.
Don't go by the bias voltage. Only the result (current) matters.
Bias is like the idle RPM of a car. Low, but ready to take off quickly: Class ABTurning the car off while waiting for a trafic light: Class B
Bias current is measured WITHOUT input signal.Re-adjust the bias after 20 minutes of warm-up time.
NEVER run the amp, or adjust bias, when the power transistors are not mounted on a heatsink !!!Thermal runaway kills the power stage very quickly.