Hope this link might help you.
Does a constantly accelerating charged particle emit EM radiation or not?
This relationship is not an absolute formula.It is just a precise relationship , which has been derived experimentally. Also it is not only frequency $\nu$ mentioned but its maximal value $\nu_{max}.$
This formula only tells you maximum frequency.which you will get corresponding to small energy losses.
In the experiment also when you are applying such a high voltage , and the kinetic energy is very much huge and its take negligible time to cross the tube in (very minimal time).so one can very much safely ignore that.
