WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Yes and no. The amount of NFB varies with the setting of the control so the improvement in linearity varies the same. More boost means less NFB and less linearity improvement and vice versa.
Exactly what Ian said. Here's an example from my book, using a 12AT7. At the flat setting the distortion is a factor of ten lower than the open-loop performance.
[IMGDEAD]http://s29.postimg.org/4a39emc8n/baxandall.jpg[/IMGDEAD]
Using a higher mu triode allowing for more feedback margin under boost conditions should help overall distortion, but the relationship of distortion to boost Ian and Merlin mention still applies. Using a more linear device to start with helps too.