Is anyone familiar with the process to bias the TRL Dude Preamp

I was saddened to learn that Paul Weitzel has passed away earlier this year. I was wondering as I was in the process of replacing tubes in my TRL Dude, it appears there is a potentiometer for what may be biasing
of one of the tubes. Is anyone familiar with this process and can share it here? I am not sure which tube
gets biased either, but it would appear to be one of the three power supply tubes. Thanks in advance.