Q: Predicta replacement CRT

[judge]

I'm researching replacement CRTs for 21" predictas and I've seen several folks say that the larger heater voltage they typically need is provided by the differential between modern voltages and old voltages (let's say that is 5V). However, as these chassis have series heaters (all 21" models other than the continental), I assume the additional voltage across one heater would be pretty small? I don't have a schematic in front of me at the moment, so I guess I am making some assumptions here.

The corollary would be that if the voltage across the heater is significantly greater with modern line voltages, is that an issue for original CRTs?