Review of Demountable vs. Sealed-off Power Tubes

The history of demountable and sealed-off types of power tubes is reviewed. Both made their appearance in 1923 as a result of the demand for higher outputs from individual tubes in radio transmitters. The invention of the glass-to-metal seal by Housekeeper made it possible to construct sealed-off water-cooled tubes with copper anodes. Another solution was found by Holweck, in France, who designed a demountable vacuum tube connected to a high-speed rotary molecular pump and continuously exhausted during operation. By designing his molecular pump Holweck could dispense with the mercury-condensation pumps and liquid-air traps which were highly objectionable as a part of standard equipment of a radio transmitter. The main advantage claimed for the demountable tubes by Holweck and other subsequent designers is "unlimited" life. Life of a sealed-off tube is usualy determined by the longevity of its filamentary cathode. In a demountable tube a burnedout filament strand can be replaced in a short time. The great majority of radio engineers did not favor the Holweck tubes because of the rotary pump having 4500 revolutions per minute and only 0.001 inch clearance between the rotor and stator. In addition, each time the tube is opened for the filament repair, the actual interruption in tube operation is much longer than the time reqluired to replace the burned-out filament; this is so, because the operating high voltages cannot be applied to the tube until the proper vacuum is re-established. This necessitates having complete duplicate equipment, if uninterrupted operation is of importance.