All these tiny, precision-machined metal parts are supported by thin mica discs, an arrangement often referred to as the “cage”; there’s a great deal more structure within a tube than in a lump of steel—complex, highly-organised, delicate structure. What effect does cryo-treatment have on that delicate structure of mica and metal wires suspended within its fragile glass envelope?
It’s difficult to imagine how cooling a tube down to ultracold temperatures can be of benefit to the mechanical integrity of the cage, the glass envelope or the glass-to-metal seals at the base of the tube. It could be argued, rather than improving reliability cryo-treatment does the exact opposite: the severe cooling process, if anything, degrades performance. Here’s why. Subjecting a tube to a frosty and unforgiving −190 °C will result in differential rates of thermal contraction of the various materials within the tube; no ifs, ands, or buts, its physics. As the temperature plummets, the metal pins on the base of the tube radially contract; they shrink and pull away from the surrounding glass putting stress on the “Dumet” glass-to-metal seals (US Patent 1,498,908). Technically this known as tensile stress; the forces that can arise from differential rates of thermal contraction are staggering—measured in 100,000s Kg/cm² [see the chapter on “Glass-To-Metal Sealing” in Materials and Techniques for Electron Tubes (1960) by Walter H. Kohl]. Ultimately this can lead to a breach, allowing air to enter the glass envelope and compromise the vacuum; this typically results in excessive noise, shortens tube life and can even kill the tube outright.
And the same physical principles apply to the cage. Differing rates of thermal contraction of the mica spacers and metal electrodes within the cage assembly can cause movement, potentially loosening the entire assembly resulting in adverse microphony. Given this, it should come as no surprise that:
Tube manufacturers never designed or intended tubes to be stored at cryogenic temperatures.
It’s not recommended practice to mishandle electronic components in this way. It’s worth noting that electronics component manufacturers publish datasheets that specify temperature ranges for storing and operating their devices—none recommend storing their components at temperatures colder than a midwinter’s night on Mars. In fact, thinking about it, NASA install heaters on their Mars rovers, and deep space hardware, to prevent batteries and onboard electronics being damaged by the extreme cold—NASA’s engineers are well aware of the risks involved when subjecting electronic components to extremely low temperatures.
