Tube tester for a Predicta 21EAP4

[charokeeroad]

Originally Posted by Eric H

I don't think 21SFA is a valid number, it probably just means a 21" Short Focus Aperture. 21EAP4 is the original 21" Predicta CRT number. I would go by whatever number is glued to the CRT, not the housing.

The last two Predictas I got had 6.3 volt tubes, I found it out through simple trial and error, if it won't light at 2.3 then go up slowly to 6.3 and see if it does, just don't start at 6.3. They also had stickers saying they had been converted to the 6 volt tube but I wasn't taking any chances.

I also question is 2.35 is the correct voltage? I also see some information that says they are 2.68 volts and my B&K has a 2.68 volt mark on the meter but not a 2.35?

The problem with testing that way is that the filaments on those tubes are fairly robust. One could run a 2.3v filament at 6.3V for several hours before it would burn out. One could walk way and think they had a 6.3v CRT because it held up for a few minutes at that voltage only to put it in a set and have it burn out or never work at all. The Current test is fast and definative. I haven't found one Predicta CRT that was marked with the correct voltage. I'm sure they're out there but I haven't found one, Most of the time I can't find any markings on the CRT at all. I usually get some glow on a 6.3v CRT at the 2.3V setting.