Predicta Tandem restoration

[bandersen]

Originally Posted by old_coot88

I'm gonna stick my neck out a little bit here.
Ideally, and for longevity's sake, i would run the CRT heater voltage as low as possible consistent with a nice bright picture (irrespective of what current is flowing thru the shunt resistor.
The logic behind this is that the 2V Predicta guns were notorious for going flat prematurely.
Other discussions have mentioned voltages ranging from about 2.25V to 2.5V as being appropriate.
I would experiment with adding a little more shunt resistance, say a hundred ohms at a time, across the 18 ohms you've already got there. And see how low the heater can be run and still give a bright pic.
The name of the game is preserving emission in a CRT that's probably irreplaceable.


2V Predicta CRTs generally had drastically short life, even when bought new.

Does anybody know for sure the logic behind the 2V guns?

I wouldn't exactly call them irreplaceable, since I just replaced one I'll be using a 21FAP4 in another Predicta that will also require a little modification to work properly.

Perhaps I should add a zener diode clamp across the filament like some guys do with 3KP4s for extra protection ?

As far as I know, only the only the original 21EAP4 CRTs had the short life. This is a rebuilt non-Philco 21EVP4. Other brands used the low voltage heaters not just Philco. I assume it has something to do with reducing the heat / power consumption.