BULBS

The kind of glass used in the manufacture of bulbs for transmitting tubes must meet specific requirements. It must have good mechanical strength, be a good electrical insulator, stand high temperatures, and should be easily freed of adsorbed gases.

Where heat-dissipation requirements are moderate and where bulb size is nor especially important, so-called "soft" glass is a suitable material. If the bulb size must be kept small, "hard" glass is employed. The important physical distinction between soft glass and hard glass is that the latter has an appreciably higher softening point (about 750ºC compared to 625ºC). Hard glass is generally employed for the larger air-cooled tube types, where bulb size is an important factor.

BASES

Base materials are of two general types-ceramic and plastic. Ceramics include glass (usually Pyrex) and various silicates, of which porcelain is an example. The plastic material in common use is Bakelite. Some tube bases are composed of metal shells with an insulating bottom disc.

The better grades of ceramic insulators cause less radio-frequency losses at high frequencies than most plastics suitable for use in bases. However, the use of ceramic bases is generally limited to tubes where fairly high r-f voltages appear between some of the base pins. A basic principle guiding the manufacture of RCA transmitting tubes is the use of those materials which provide a well-balanced tube design. The true measure of radio-tube value is optimum performance with minimum cost. In choosing proper materials to accomplish this result, RCA has the benefit of long manufacturing experience supplemented by intensive research and comprehensive operating experience.