Hello, Dalibor and Vladimir.

I'm from Tel Aviv area, Israel.
Love vacuum tubes from both sides - electrical and aesthetic (i'm designer).

Thank you for the information, and of course for the hard scanning work!
You have beautiful blog with good pictures!
As Vladimir said Kovar (Rodar) alloys work the best with the borosilicate
glass.
You can try to make the kovar glass metal seal by yourself, but it's little
bit problematic to get the material.
If you want to experiment I have some amount of kovar wire and I can share
some for free.
Anyway, try to flatten the metal in the place of seal, it will release
glass tension.

Daniil.