Looks like you’ve gotten a bad tube… Saying that, it isn’t likely it’s related to your scanning issue.
If it’s an upgrade I’d check it’s TEM resonance mode. I can see by M3 that it’s not right. The unmarked spot in the center should not exist.
On closer inspection of the spot at m1, it’s more doughnut shaped, it is lighter in the center, so there is less power there.
If you’re out of TEM00 mode, it may never align/work correctly.
Most tubes have a better beam quality at high power, you may below the bottom of your useful range. An RF tube is on or off, but they consistently has a better beam quality, some of which is from it being on at 100% power.
