[codefox1]

Agree that many rectifier failures are due to faulty filter caps or shorts, and output tubes do age less gracefully than small signal tubes. And weak mixer/oscillator tubes may work fine in some sets, and not at all in others. A decent tube tester will help you identify complete duds and help somewhat in selecting matching pairs for example. Nothing beats having a stash of "known good" examples for first level differential diagnosis. Taming higher line voltage is always a good idea.