But aren't there still a lot of tube 50s in backup role? A couple of years
ago WTOP still had a tube rig as backup.

710 WOR in New York City had a Continental 317C as their main
transmitter until about 1997, when they replaced it with a Harris
DX-50. The Continental is now their backup transmitter. They use it
weekly during Joe Franklin's show -- the last music show left on WOR.

WOR also used the Continental during that big blackout a few weeks
ago. They chose it over the Harris because it's less sensitive to
power fluctuations. (Northeastern NJ had its power restored more
quickly than the City itself, but people reported voltage swings of
their household AC all the way from 50 to 148 volts, and I doubt the
three-phase power at WOR's transmitter site was any better controlled
at the time.)