Just got brand new bullet mustang.After I tuned the string with fender tuner app,all strings are great except the 6th string(low E,thickest string).Though the app says it's tuned it’s loose and you can hear fret buzz and bridge buzz.But if you tighten the string all the looseness,the buzzing goes away but the string is not tuned to E coz it’s tuned up.how can resolve this issue?
If it's playing the E that it says it is, and the A on the fifth fret is the same A as the open A, it's in tune.
If it's too floppy for you, and you can hear string buzz, then you can either adjust the truss rod to get more relief in the neck, raise the saddle a bit, or go for a thicker low E, which will mean you get higher tension for the same note.
A Mustang is a short scale instrument.
Thicker strings would always be good, as you need less tension on a short scale to achieve the same pitch, compared to a long scale.
Personally, I use Ernie Ball 'Skinny Top, Heavy Bottom', 10 - 52, which keeps the top end reasonably light & the bottom 'firmer' than regular 10s.
You sure you got all strings on and functional, cause ringing, buzzing, and dead frets equate to truss rod adjustment. I'm not telling you to start cranking on your truss rod, just sight down the neck by holding the back of the guitar up to eye level. What you looking for is any mis-shape to the gradual curve of the neck, a weird warp, and you can actually see the frets to, with no strings on it. Sounds to me that you need a professional setup done to the guitar, or you maybe screwed with a warped neck and looking to replace the guitar. Take your guitar into a local music store, ask the tech, "How much for a professional setup?" If it's anywhere between 30-50 dollars it's worth having it done cause 10 of those dollars is a new set of strings. If he looks at it and finds a critical issue like warped neck, he would be the one to tell you so, before you spend any money. Good luck!!