I have a hard time pronouncing the U in "But", I usually pronounce it "Bat". I also have the same problem with "butter & batter", "bus & bass", "cat & cut". Can you please tell me where to locate my tongue as I pronounce the U in such words? thanks for taking your time to read this e-mail.
It is not the tongue, it is the tension of the apparatus and the form of the mouth. For the /æ/ of 'bat', smile widely and tense your cheeks where the smile ends. For the /ə/ of 'but', relax the jaw and mouth completely and let the mouth drop open only enough to let air pass through, then simply voice an exhalation.