#1
So i recently purchased a bassman 100 head and had a broken 12ax7 tube in it that the amp needed to work. The guy told me a 12ax7 sound better than 12at7 in the amp so i bought a new one and put it in. After about a couple weeks the amp stopped working. All the tubes light up except for the one i just replaced. Whats going on.
#2
I can't tell you what is going on exactly.

Can we assume that your other identical thread is just a mistake because you didn't realize the first thread went through? Happens.

What I would do is try swapping some of the preamp tubes around and document as you go so you can keep track of things. Basically report back any differences in tone/tube brightness you might find.

This is an old amp so there is a very good chance that there is something else wrong in which I recommend you take it to an amp tech (that knows what they are doing) and have the amp looked over entirely.

That is what I would do.