General Electric RB-1-4 Radiator Tube (ca. 1930s)

The above photo is a close-up of the tube's cathode. Since this is a dual focus tube, there are two filaments. The smaller of the two is used when image detail is important. The larger filament would be used when shorter exposures were required.   I believe that the first commercial double focus tube was produced in 1930.

The "1" in the tube's model number indicates that one of the filaments produces a fine focus while the "4" indicates that the other filament produces a broad focus. The tube is plugged into a rotary switch that is used to select which filament is to be used. In the top photo, the switch can be seen at the end of the cathode arm (right end of photo). The photo above right shows the tube's radiator fins for air cooling.

A 1935 General Electric advertisement (see at bottom of page) describes the tube as follows: 

"Why sacrifice radiographic detail? This double focus Coolidge tube offers you the right size focal spot for the work in hand.

Observation shows that many x-ray laboratories attempt too wide a range of radiographic work with one and the same tube -  expecting one size focal spot to serve them for both light and heavy exposures.

Regardless of whether this practice is an economy measure or to save the time of changing from one tube to another, the fact remains that far too often the resulting film represents a needless sacrifice in radiographic detail.

The RB-1-4 double focus Coolidge tube is the practical solution to this problem. It is the equivalent of two tubes, as it offers you the selective use of both a small and a large focal spot, by the simple turn of a switch. In other words, the right size of focal spot for the work immediately at hand.

It means economy, too, as RB-1-4 represents two Coolidge tubes in one, at the former price of one. Moreover, you will obtain the advantages of the Benson type (linear) focus, such as finer radiographic detail with greater energies; also tube construction which provides 25% greater heat storage capacity.  $125.00" 

Size: ca. 16 1/2" long (excluding rod for radiator fins), 3 3/4" bulb diameter

References

General Electric X-ray Corporation. Coolidge X-ray Tubes - Kenotrons. Bulletin No. 293. 1934.

General Electric advertisement. American Journal of Roentgenology and Radium Therapy. 1935.

Coolidge Type X-Ray Tubes               Museum Directory