I liked that Mike Pappas said that with envelope detectors distortion happens at 90% negative. So many AM hams and others do not seem to realize this. What seemed to finally kill tube rigs for broadcast was tube supply and cost and the electric utility bill. When you have tubes lit up day and night every day you have to re-tube a rig every 12 to 18 months. When NOS tubes from Richardson put the cost up around $2K it became intolerable for a 1 KW AM. We hams are lucky--we can run our triodes and tetrodes for the rest of our lives and not have any lose emission provided they get treated right and I think most of us know how to do that.