For older gear, I use a Variac device together with a 6-recepticle toroidal isolation transformer and adjust line voltage that produces 6.3 VAC and/or 12.6 VAC measured at the tube socket heater pins. That results in 115 VAC when using my Drake B Line. Voltage is monitored continuously with an RCA WV-120A line voltage monitor but confirmed with a Fluke True RMS DMM.
My older pre WWII gear almost always operates at 110 VAC to achieve the correct heater voltage at the tube sockets.
Before adding the Variac under the operating table, I was modifying gear with CL-90 Thermistors. That works fine too but thermal dissipation from the CL-90 adds to inter-cabinet heat.
Paul, W9AC
Paul Christensen