Mind the gap! RCA 10BP4 failure

[Penthode]

On Saturday investigating the state of some old CRTs (kinescopes) I have. A 10BP4 RCA made in the 13th week of 1949 had absolutely zero emission. Upon investigating the tube gun, I made this discovery. Follow the cathode connection and you will see a 1/16" gap!

I suspect it started as a bad weld and the tube tester in trying to restore the connection blew it wide open.

This is the second RCA 10BP4 in 50 years I have seen the same problem: last one in 1972! But I have only dealt with less than a dozen 10BP4s in that time.

[Alex KL-1]

If I see correctly, seems to be fused. Is possible to occur with power tubes, but, for CRT, perhaps people with more field expertise can indicate if this can occur or not. Or if people catch what are the issue...

[Penthode]

Something blew the cathode connection wide open.

My guess is the CRT tester rejuvenator. When the cathode weld failed and the rejuvenation process was applied, the blast blew the cathode wide open.

[Alex KL-1]

Ah, I considered the normal usage and perhaps I read too fast the text... yes, make sense, I also bet this (the tester) or one trying to manually re-welding the cathode...

[Penthode]

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[Tim Tress]

In almost 50 years, I have encountered two 10BP4s with the same problem; the filament lights, but there is absolutely no emission. Both were from late 1940s sets, so maybe there were some issues during manufacturing.

[Penthode]

Check the cathode connection in the tube. My observation is the cathode opening due to a burst of high current which is enough to melt the fine metal link to the cathode pin of the kinescope.

[Chris K]

I guess a gap of that size means it's hopeless to try to reweld the cathode using that function in many rejuvenators, even with vigorous shocking of the CRT neck?

[Electronic M]

Originally Posted by Chris K

I guess a gap of that size means it's hopeless to try to reweld the cathode using that function in many rejuvenators, even with vigorous shocking of the CRT neck?

I wonder if the metal is ferrous enough that a VERY Strong magnet could be used to pull the more flexible side of the break close enough for a weld function to work.