|
I had a 12AP4 at RACS for rebuild that cracked. The original intent was to have Pyrex stems made so no graded glass would be required. They quickly found the Pyrex stems were experiencing micro-cracking around the leads. After a few failed attempts they gave up and went with a soft stem and a graded ring. This appeared to be working in the first couple of tubes, but then cracks started appearing around the HV lead. These tubes don't use a button like a modern crt but just a lead exiting the envelope, just like the stems that were cracking. They were supposed to send my crt to a lab in Paris for failure analysis but this was just as things were coming to and end at RACS, and that never happened.
They never did determine exactly what was causing the cracking. Was it when the envelope was first opened and the stress relieved? Was it during baking? Was it during pulldown? Hopefully we will have a chance to experiment ourselves at the ETF, but there are only so many duds to experiment with. Perhaps we can persuade John Y who has been doing fantastic work on the 15G's to do some research on the Pyrex tubes so we have a better understanding of what's going on there.
RACS also mentioned the problem with Pyrex is getting the exact formula correct, and not knowing exactly what the original formula used for the crt glass was. These tubes were rebuilt by RCA back in the 40'a and 50's as there are a few floating around, so it was possible at one time, but that knowledge has been lost.
Here's my 12AP4 after rebuild before it started going down to air. So close....
|