20BP4

Type 20BP4 is an early CRT with a Pyrex glass envelope.  CRTs made of Pyrex are not rebuildable by the normal CRT rebuilding process, since Pyrex has different thermal properties and a different thermal expansion coefficient than typical soft glass used in more modern CRTs.

The determination that the glass on this tube is Pyrex was made by Clinton Displays (Loves Park, Illinois, USA - a company that manufactures and rebuilds CRTs).  Since the material was unknown, they agreed to attempt a rebuild, the first step of which is the cutting of the glass on the neck of the tube.  Using their normal hot-wire cutting method, this attempt to cut the glass failed.  In their experience, this means that the material is Pyrex.

Another indication that the material is Pyrex rather than soft glass is that the weight of this CRT is less than what is typically seen on a modern CRT of this size.

Although this CRT is used in the DuMont RA-101 Westminster (1947-48) TV, the design of this CRT is in some respects more like a pre-war CRT with its use of Pyrex glass.