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?

[Electronic M]

Look at the datasheets for the 2 tubes. If the current draw is the same they will sub just fine in a series string. Back in ~1958 line voltage was assumed to always be 117V, but in 2019 line voltage is always assumed to be 125V...An 8 volt increase.

In an original series string with original CRT that should be divided evenly across the whole chain and not cause harm. The higher voltage CRT heater should absorb a good portion of that change and soften the amount the rest of the string has to absorb.

[judge]

Ah yes, subconsciously I was assuming that the resistance of the filament would have been larger too. Now it makes sense. Like you say, I need to read the data sheets!

[old_coot88]

A quickie calcumalation aid is:

Series = constant current (individual elements in the string may have different resistances thus different voltages).

Parallel = constant voltage (individual elements may have different resistances, thus draw different currents).