Rex
Dignified but Approachable
Okay, I see what you're saying, @Joe Bfstplk , but consider:
Notice that the signal never quite gets all the way to the power rail. That's because the supply voltage sags to meet the signal. That sag is part of the power amp's reaction to the signal, and it affects the sweet spot. If you allow the Headroom meter to rise above 0 dB, you lose access to that information.
It does, though. If the spikes peak out below the rail, you're running the power amp clean. If they run to the rail with a little dwell, you're in light overdrive. If the troughs rise and they dwell a bit longer, you're in moderate overdrive. If you're slammed to the rail, your tone farts like cattle eating silage. You can use the same concepts to visualize the behavior of held notes as they decay.Many of the dirty amps like Recto and Mark-series Boogies put out a rather compressed, peak-free signal from the preamp. The method you suggest for watching how hard/long it hugs the 0dB mark on the meter doesn't really help understand how far they are pushing beyond the available headroom.
There is no formula for how much overdrive constitutes any sweet spot. That depends on the amp, the EQ, the signal... It must be determined by ear, whether there's a headroom meter or not. No level meter can tell you how the mids are behaving relative to other frequencies. You have to hear it.With a compressed preamp signal, the current metering fails to help find that other sweet spot just above the threshold of power amp distortion. It can be dialed in by ear if you have lots of time to tweak and listen, but it is easily measurable, and the propsed metering improvement would show that info....
Notice that the signal never quite gets all the way to the power rail. That's because the supply voltage sags to meet the signal. That sag is part of the power amp's reaction to the signal, and it affects the sweet spot. If you allow the Headroom meter to rise above 0 dB, you lose access to that information.