The most likely cause of distortion from only one side of this tube mic preamp is a failing tube. Luckily, if your other channel is working fine, you have a known-good set of tubes. Universal Audio says the more likely tube to fail is the 12AX7 tube.

First, try swapping this tube (12AX7) and see if the problem is resolved. If so, this tube is one of the most common tubes available today, used in tube guitar amps and other tube preamps, and should be available at most local music stores and you can purchase it and get the unit operational easily and quickly. If it is still under warranty, you can call Universal Audio for a replacement to be sent out to you.

If this does not solve the problem, try swapping the other tube.

If this still does not solve the problem, your unit will likely need service.