You do need to watch that heater volage, and with that the bias changes,,,,so you need to "set it up" for that voltage your running ....you cant just plug up and go...Yea. I'm a little confused by claims that you can go down to 90v Ac input and get a better tone . The 6.3 v AC heater supply is not stabilized in these amps . Going from 120 to 90 may drop your heater voltage to less than 5V , and it's going to cut the emissions from the cathode of each tube substantially. Having faulted a number of amps with low heater voltage over the years, IME they usually sound like pud. Dropping the HT sounds like a better idea to me.