Hi Gary, you should keep your throat open all the time. The “ku” is much more likely to trigger your throat closing than “tu”; it’s just one of those things! As with most difficulties, go back to basics and start slowly then build up speed. BCH breath will help open the throat – really try and “yawn” the breath in then keep that open feeling at the back of your mouth as you play. Then work slowly on tu-ku, staying on one note, but try very light tonguing and think more daa-gaa than tu-ku, to get used to the sensation of the tongue interrupting (but not stopping!) the airflow, whilst keeping everything relaxed. It helps me if I to try and make tu and ku sound the same. When I started it was more like tu–gggaa, so look to get the back of your tongue moving in and out of the airflow as quickly as the tip. As you get more familiar with the sensation, you’ll be able to tongue harder and speed up and change note/harmonic. Hope this is useful and I’m not telling you stuff you already know!