This morning when I turned the amp on, the left channel immediately started to pop and scratch. The left front tube began red plating rather brightly. I turned off the amp and moved the red plating tube to another socket. I turned the amp back on and the same tube began to red plate again. Obviously a bad tube. This tube was running fine the day before. Seems like the last thermal cycle caused the grid to short to the cathode.
Fortunately I have a spare EL34 that is running fine now.
I know we all have tube failures, but I just wanted to note how we shouldn't just turn on the cold amp and walk off. If I hadn't been there to see the failure on startup, I may have damaged the output transformer or other components.