12LP4 with FANTASTIC emission...but weak cutoff..OK?

[rca2000]

I came across a 12LP4 tube in a box--NOT nos--and on the CR-70...it tests like NEW on emission--holds into good somewhat even at FOUR volts and does NOT drop off !!

BUT...the cutoff is NOT good. CANNOT get it too the good range of cutoff unless it run the filament VERY high--and like I said emissions is fantastiic, at 6.3 volts it is about 12 on the scale and does NOT drop hardly at all in life test.

SO--should I consider this a good tube ? Dag coating has NOT flaked off from what I can tell.

[DaveWM]

are you using the correct bias setting?

[rca2000]

I am...but I have to use that damn "universal adapter" and i HATE using that thing !! I had one control setting off and it showed near MAXIMIUM cutoff and emissions with NO voltage on the filamant and also showed a cathode short. . I got it right--and now get what I talked about. no shorts, great emissions just poor cutoff...

[DaveWM]

prob be fine. Best CRT tester is a working set.

[Tom Albrecht]

I often see cutoff out of range, and simply change the bias scale to bring it into range. If it tests OK for emissions on any bias scale that brings the cutoff into range, it seems to work OK in a set. Would be interesting to get a clearer picture of what the cutoff test corresponds to in terms of picture quality. Obviously it is trying the measure that range of grid voltage from low cathode current ("cutoff") to a high current ("emission") is within "spec," but it seems to matter less in real life than the tester would suggest.

[kramden66]

i really don't worry about the cut off and worry about emissions , i've seen more then one tube with emissions that are passable , bad cut off and even bad life test and they work and look fine

[Polaraligned]

Do we not need to set cutoff in range before proceeding to emissions?
The emissions test results vary with the cutoff setting.

[Eric H]

Originally Posted by Polaraligned

Do we not need to set cutoff in range before proceeding to emissions?
The emissions test results vary with the cutoff setting.

That's how it works with the B&K testers, you can make a weak tube check good by cranking up the cutoff voltage.
I have no experience with the CR70 so I can't say on that.

I have a tube I rejuvenated that has great emissions but the cutoff won't respond all, still it makes a bright raster.

[Phil Nelson]

I have had similar experiences with my CR-70. The cutoff may look bad, but if you turn it up the emission looks good, and when you put the CRT in a TV, it looks fine. I don't know exactly what the CR-70's cutoff test is checking, but the results don't seem very meaningful. The best tester by far is the TV.

Phil Nelson

[Kevin Kuehn]

Just my armchair theory - I think the key is that the cutoff setting has some range of control over the tubes emission. With little control there would be poor contrast in the resulting picture. Most sets have a brightness and contrast control to get the tube operating at a usable background level.

[BigDavesTV]

I have a set that's got a weak 12LP4, that would be a good candidate for set-testing your CRT!

[kramden66]

the cut off setting on my b&k tester does not effect the emissions , the g2 setting effects it , i think its the 465

[Polaraligned]

What the heck good is a CR70 if you can just crank up the cutoff knob and change the emissions reading? Because of that, I assumed that the cutoff had to read in the "box" before you would get a correct emissions reading.

[rca2000]

It has been my experience that if you can't get the cutoff into the box--get it as far as it will go and measure form there. It is usually pretty accurate.

Keep in mind...this tube tests into the good range at 4 volts on the filamant. and does not drop much at ALL on life test at 6.3 volts.

By that same measure....I have seen a number of tubes that would go into the cutoff box FINE....but did NOT have good emission or dropped quickly on life teat.

[miniman82]

Cutoff has to do with the ability to control the stream of electrons leaving the gun, so less ability to get proper cutoff at the proper voltage indicates a tube which will have decreased contrast- in essence, you have less headroom between full bright and full dark. My experience has been that tubes which test like this often work fine in an actual set, and I suspect many thousands of CRT's left the factory this way or developed a cutoff issue shortly after being placed into service. It's a bigger problem with color tubes of course, because it affects color tracking.