Some of this stuff is silly when you consider the objective of NPC is to increase the amount of volume without increasing the amount of distortion. The last thing you want to do is "volt the final". These radios modulate the final and the driver simultaneously for a reason. You cannot fully modulate any transmitter in the hi level mode (the type these radios use) unless you apply modulated DC to more than one RF amplifier stage. As soon as you disconnect the final from the modulated 6 volt line and tie it to the main 12 volt line, there is no modulation applied to the final power amp. You just went backwards to make a watt meter look impressive.
Effective NPC is much more than just a diode. Broadcast stations use very complex multi band compressors in conjunction with negative peak clipping that is always followed by many poles of filtering to move the harsh transitions any useable NPC is going to create. Most of these CB mods have been developed by people who think the goal is to stop the negative peak from reaching RF cutoff and if you look at the results, they sometimes do that. Problem is that has nothing to do with the unwanted distortion and actually removes headroom required to avoid it. Again, going backwards.
Most of us know when we see flat topping on the positive peak, we have distortion. The same is true of the negative peak in that we have to pay more attention to the shape of the waveform than the aspect of RF cutoff. An AM carrier perfectly modulated to 100% with a 1Khz. tone will hit RF cutoff 1000 times per second but not cause any objectionable IMD.
This is because at 100% modulation there are no flat portions in the waveform at the top or bottom. If nothing is flat, there is no DC component. If there is no DC component, there are no sharp transitions in the positive or negative peak. That all adds up to a clean signal with a carrier that is constantly going in and out of RF cutoff but the shape of the original audio is never cutoff on the top or bottom.
Now if you do some mod to your radio that clips off the negative peak at say 95% modulation, you just took out 5% of your headroom before the negative peak could become flat. Since that diode is not followed by a filter network anytime its driven over 95%, its output will be clipped, flat and have sharp transitions. Exactly what you want to eliminate. Most people would get more out of a good compressor before even thinking about how to compress the negative peak independently.
