Swap the tubes and find out. I have all tube components. Whenever something odd like that happens it is a tube and swapping will quickly identify it as tubes are split along channels.
I also always have an extra set of tubes around as backup. They don’t fail or act strange often, but it is good to have if the do.
I also always have an extra set of tubes around as backup. They don’t fail or act strange often, but it is good to have if the do.