That's true. It is important to remember, whenever you have an issue, the first thing to try is to change the tube. It is the easiest thing to do.
If you can afford it, keep a set of "gold standard tubes", ones that you know that work to use when testing. In the case of a spare pre-amp tube, it doesn't cost that much. Power tubes are a different story.
Official Ampeg Portaflex Club #89