Help on tube question

Is it possible to replace a bad kt88 tube with a 6550c tube and do no damage? The VAC 160i uses 4 KT88's and with one of the tube bad, I have lost one side of sound. If it were ok to use the 6550c's, I could replace the 4 KT88's or just one. I know Monday I will get help from VAC. But, I just want to hear that sound again if I could and do no damage.
Thanks for any information or Opinions. Tra