Cutoff and Auto Restore on Sencore CR70
 

I have a 12LP4 CRT that seems pretty dead, but it never hurts to ask.

The tube had a crusty old brightener on it when acquired, so I didn't have high hopes in the first place. It initially tested at zero emission and I couldn't move the needle at all for the Cutoff test.

The first thing I tried was cooking it on the tester overnight at slightly elevated filament voltage, but the emission remained at zero.

I tried the Auto Restore function. During the three cycles, the needle moved only slightly above the bottom of the Current scale.

It still flunks the Cutoff test -- even turning the Cutoff control all the way clockwise doesn't move the needle at all. Following the Auto Restore, the emission moves well into the Good zone if I turn the Cutoff control all the way CW.

If I'm reading the manual correctly, the low (absent) cutoff means it may light up but have awful contrast.

Is it worth trying anything else, or should I write it off?

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

It's d-e-d dead. Cutoff refers to the ability of G1 (or is it G2?) to control the electron flow in the tube, so if it's wide open and still reading poorly that also means the same thing is likely to happen in a real set. You could look at the cutoff knob as roughly analagous to the screen control in a set, once the cathode is used up it won't make a decent picture no matter how hard to run it.

Okey-doke. I have a weak spare that should work well enough to show a pic during restoration. I'll set this aside and not waste more time on it.

Thanks,

Phil Nelson

Did you try it with a lower bias ? Sometimes the cutoff is dead at 52V but will be responsive at 36V or lower. Of course, that's still a bad sign but does mean you might be able to squeeze a little more life out of it.

Yes, at -36v bias I can move the needle into the little black cutoff zone and then get an emission reading in the low Good zone. I'll give this CRT a try when the day comes. I have had other tubes look pukey on a tester but play surprisingly well in a working TV.

Phil Nelson

Duh, I spend too much time in the color forum. B&W tubes can display a picture sometimes, not all that bright, but color tubes are completely unacceptable with dead guns. Never had one that had good color tracking if one of the guns was run down.

I never worry about having to use the "wrong" bias setting to get the cutoff set properly. Honestly I can't see any difference between tubes that work with the correct bias vs. those that need a little lower bias. Both produce good contrast and grayscale in working sets. My guess is that a tube that works with the proper bias setting is probably a tube with more life left in it, but even that I can't confirm. So don't take the bias setting too seriously on the CR70.