FTX founder Sam Bankman-Fried has been sent back to prison less than two months before his fraud trial, after a judge alleged he had attempted to tamper with witnesses. The disgraced cryptocurrency tycoon has pleaded not guilty to charges of wire fraud and conspiracy to commit money laundering, as well as election finance violations, in connection with the spectacular collapse of his firm last year. Along with its sister hedge fund Alameda Research, FTX went bankrupt in November. At one point, the virtual trading business had been valued by the market at $US32 billion ($A49.2 billion). Justifying the revocation of…