1.  It would be good to know if your tone problems are with the lower octave key notes D,E,F and G, or if the problem is really the high octave key notes, A,B,C, etc.  I ask this because there are two octave keys on your horn, and if your tone problem is only with the lower or upper notes, that would be good to know.
2.  Although you say you are not biting, it is very normal for relatively new players to bite down in the upper octave without realizing this.  To test this out, relax your embouchure and open you throat before you blow, and see if the octave notes improve in tone.
3.  What is your reed strength?  A reed that is too soft will yield good tone on low notes, but thin tones on high notes.  If your Yamaha tenor mouthpiece is a 4C, then the tip opening is about 0.067", and in general, the smaller the tip opening for a particular brand of mouthpiece, the harder the reed needs to be.  If you are playing a 2.5 strength reed, you might see what a 3.0 will do (but you may lose some of the resonance in your lower notes).
4.  One thing to do is to have one of your friends play with your mouthpiece (clean it first) and your reed strength, and see if they also experience the issue.  If so, there could be an issue with the horn.

There are a lot of experiments that you can go through to determine if the issue is you, your horn, your mouthpiece or your reed selection.  But please take your time and work through the possibilities methodically.  If you rush this you can come to the wrong conclusion, and that can make your problems even worse.  Don't worry too much, 5 months is not a very long time on any musical instrument.