Filament Voltage

[Electronic M]

I once heard it explained that the lower heater voltage in the predicta tube was the result of 2 design limitations. They needed to make the cathodes shallower as part of making the gun shallower, and they had only one diameter of heater wire to work with. To get it shallower they couldn't use as much heater wire and thus the shorter wire had a lower voltage drop. The later 6.3V replacements they went to the engineering effort to get thinner heater wire that they could wrap to drop 6.3V and still fit in the cathode.

[old_coot88]

So, was Predicta the main driver behind the "flattest jug possible"?

[bandersen]

Yes. Philco developed the super short neck and were the first to use it in the Predicta line. They talk about it in their commercials.
https://youtu.be/PG0wpwq8FdQ?si=4hshsj0KEaQYfcCa&t=54

[MuzzcoVW]

Originally Posted by Electronic M

I once heard it explained that the lower heater voltage in the predicta tube was the result of 2 design limitations. They needed to make the cathodes shallower as part of making the gun shallower, and they had only one diameter of heater wire to work with. To get it shallower they couldn't use as much heater wire and thus the shorter wire had a lower voltage drop. The later 6.3V replacements they went to the engineering effort to get thinner heater wire that they could wrap to drop 6.3V and still fit in the cathode.

As an electrician that completely makes sense regarding the filament length vs the voltage needed and the resistance of the filament. From what I understand they were prone to cathode emissions failure. With the smaller filament, I wonder if any voltage variation to the CRT would help speed that up. I once worked one where there should have only been 2.35V at the CRT but it was running a volt high. I forget the cause... this was in 1990. Could this have sped up those CRTs demise, and also why some seem to last a long time?