While touring in Texas, our band frequently played small rural venues where the AC power provided was at best questionable. A substantial low voltage drop will make a tube amp sound strange i.e., " what is wrong with my power amp ? it sounds weird".
The bass player had a rack mount power-conditioner that was supposed to condition the power, but after plugging in to it … Hmmm … not so much.
After further investigation, I finally purchased a Voltage-Regulator by Furman Sound, the AR-1215, that actually grabs hold of the voltage and adjusts the incoming AC line voltage if it varies more than a few percent from the standard 117 volts.
Problem solved