It is important to make the distinction between AB biased vs. class C.
In a class C amplifier, the only thing turning on the pills is the RF input. The base of the transistors (pill finals) have a DC path directly to ground. Starting from normal RF drive, reducing the drive power will eventually cause gain to drop, often this can be abrupt, and then gain will be zero where the pills are completely turned off, even though there is RF drive, its not enough to turn the pills on. In this area where the pills are shutting off, the impedance is changing drastically, and the possibility for spurs or oscillation occurs. However, its my opinion if an amplifier goes spurious here, its a fault design and needs negative feedback or something.
Class AB or A is a completely different situation. A bias current, or voltage for mosfets, keeps the pills turned on even with no RF drive. With such an amplifier, if you key it up with no RF drive, you'll have a current draw. This should maintain the gain even at low RF drive levels. This is needed for clean SSB. When you talk far from your microphone on your SSB transmitter, you are underdriving your amplifer and there is nothing harmful about that.
In a class C amplifier, the only thing turning on the pills is the RF input. The base of the transistors (pill finals) have a DC path directly to ground. Starting from normal RF drive, reducing the drive power will eventually cause gain to drop, often this can be abrupt, and then gain will be zero where the pills are completely turned off, even though there is RF drive, its not enough to turn the pills on. In this area where the pills are shutting off, the impedance is changing drastically, and the possibility for spurs or oscillation occurs. However, its my opinion if an amplifier goes spurious here, its a fault design and needs negative feedback or something.
Class AB or A is a completely different situation. A bias current, or voltage for mosfets, keeps the pills turned on even with no RF drive. With such an amplifier, if you key it up with no RF drive, you'll have a current draw. This should maintain the gain even at low RF drive levels. This is needed for clean SSB. When you talk far from your microphone on your SSB transmitter, you are underdriving your amplifer and there is nothing harmful about that.
good post dudmuck (got'a love that screen name) 
although it sounds like you might have something there Booty ,? but SSB tends to work OK to a degree on class C as long as theres a delay there . Only thing I was ever concerned about was not giving any given amplifier enough amperage , now I know that will blow a few transistors fairly easy. )-:
