Hello Everyone,

We have been having a particularly difficult time with the newest batch of Voltarc Blue. The tubes are cracking at welds and double backs after annealing and sometimes days after bombarding. We have tried several different annealing techniques with no luck. We ended up remaking the job with some old FMS BL55. Anybody having the same issue with new Voltarc Blue. Any ideas on what the heck is going on?

Cheers,

Todd

Hi Todd,

 I know blue can be problematic, but not sure if it is specific to Voltarc. I had the same problem a few months ago with some blue. Not sure the reason this happens but I believe it is something to do with the coating in the glass...maybe it's more prone to hold moisture. What I did to solve the issue (based on the suggestions of several other tube benders) was to process the tube immediately after finishing the bending of the unit. I have no explanation for why this works, but it does. I think the bombarding process anneals it a little, but I have heard others disagree about that. In any event, I had success with pumping the units immediately after bending. Good luck!

From what I understand the newer glass or "lead free" is the reason its harder to work with. My older stock that isn't lead free is easier to bend. The phosphor being different makes sense also. 

I just had this exact same issue with FMS BL 55 blue. And even worse than cracking tubes at double-backs, when I bombarded the units, everywhere I had heated the tube (literally EVERY bend) the phosphor was SUPER BAD. Like either it looked stained, or in some cases like it completely evaporated and the tube was near transparent. It was horrible - I was on a tight deadline and my project was completely screwed. 

FMS said they have not had any other reports of issues with that batch, but I can't beleive nobody else has seen the problem... it was super obvious and really really ugly. I have never seen this problem before with any glass.

One of the problems that is a factor is that the phosphors used have been changed in response to chemical availability and unfortunately we are often the unwitting test subjects for some of it.  In at least one case I am aware of, Voltarc blue phosphor was reformulated and it had a slight incompatibility with the glass that appeared to involve a somewhat hygroscopic nature that would effectively exacerbate the temperature gradient in the glass in areas that were worked and this would induce additional strain that could lead to failure.  As has been mentioned, promptly processing these units seemed to help with the problem and while there is definitely room for debate about how much, if any, annealing can take place at bombarding temperatures I did find that a look at a tube weld in a polariscope before and after bombarding showed less stress after the bombardment.  Granted I will not call it scientific as I only ran the test once or twice (hardly a suitable sample size) but it did seem tantalizingly possible enough to not reject the prospect out of hand.