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![]() The Layman’s Science of Tube Processing
By Randall Caba
But don’t let that old high school migraine that’s now spreading across your cranium keep you from reading on. We’re going to keep this tale simple for both our sakes.
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So, what happens after we weld a tube onto the manifold, hook up the bombarding wires then open the vacuum stopcock? Well, vacuum develops within the tube. That’s obvious. But…
What is vacuum? The answer: To lessen electrical resistance. Otherwise, electrical bombarding would require far greater voltage to move electric current through the tube. We would need more powerful bombarding equipment and, no doubt, additional safety measures. We would also need a stronger glass tube that could withstand higher stresses. At around 2 mm or 3 mm of pressure (vacuum), it becomes easy to light a tube; force electricity through it. But there is another reason to begin bombarding the tube at this pressure. At this pressure there are enough particles remaining inside the tube to impart sufficient momentum upon others. This causes heat. But why create heat?
Tube fever Electrical bombarding at this pressure generates heat and lots of it. During this ever more turbulent state, pressure inside the tube rises. It rises due to increased temperature and because vaporized impurities add to the total number of particles. Remember why, as the first step in tube processing, we reduce pressure in a tube? We do this to lower electrical resistance. As the pressure raises so does electrical resistance. So, this is when the tube usually begins to flicker. And from experience we already know if we don’t reduce pressure inside the tube by opening the vacuum stopcock, the tube will go out. The electrical resistance grows too high to overcome. Yet, even before the tube goes out, electricity is seeking an alternate, easier path to flow. Hopefully, it finds this route via the Jacob’s Ladder or other safe pathway we’ve installed on our equipment. But let’s say we kept the tube lit and its exterior has reached proper processing temperature, about 450 degrees Fahrenheit. The interior of the tube is far hotter, enough to vaporize most all organic materials. Now one might think this is the best time to stop bombarding and evacuate these high energy, impure particles. But it isn’t. Why? Because there is some chemistry to finish.
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Electrode chemistry Electrode shells are a blend of metals designed to survive the hostile environment created during tube processing. Most electrode shells are coated with a chemical blend; an emissive coating that emits electrons during bombardment, a type of shield. These free electrons protect the electrode shell by neutralizing heavy particles called CATIONS (cat~ions). A cation is an atom missing one or more electrons. During electrical bombarding, they number probably in the billions. They are drawn to the shell by strong electromagnetic forces, sufficient force to knock away the metal atoms that make up the shell.
The destruction of the shell by cation pulverization is called sputtering. See Figure 1 for an over-simplified graphical description of how sputtering during tube processing is kept in check by the emission of electrons.
![]() This emissive coating also absorbs any impurities remaining in a tube after processing. However, to perform this function properly, the coating must be chemically converted after its job of protecting the shell is complete. Conversion is accomplished by briefly raising the electrode shell temperature to about 1600 degrees Fahrenheit. Increasing both electric current and vacuum raises the shell temperature to this level. Once the electrodes are converted and heated their entire length a cherry red color for several seconds, the current is stopped while the vacuum pump goes about its job removing hot, vaporous impurities. It is important that the tube ultimately be evacuated to a pressure (vacuum) of about 1 to 5 microns. This assures the tube is clean. A clean environment contributes to a long-life tube, one that requires less voltage to light, and discourages bad behaviors like color changes or mercury stains too.
Fill it up please Once the tube is properly processed and gassed then comes the tip-off. We heat a small section of the tubulation tube connecting the tube to the manifold. Once molten, the tubulation tube shrinks until it closes and seals. But why does it shrink then seal? Because the molten tube is under vacuum pulling it closed. Actually, a more realistic view is the molten glass collapses because atmospheric pressure, the weight of the air surrounding the tube, presses it closed. Once pressed together, the tube remains sealed because glass molecules have an affinity for one another, a molecular attraction that binds them together. During the final phase of tube processing, burn-in, the tube electro-chemically stabilizes. Any residual impurities are absorbed by the emissive coating’s chemical “getter” action. It is this final cleaning that assures the tube’s proper color and stable operation. I’ve always been entertained by the science behind processes and found the knowledge useful. I hope this basic introduction into the science of tube processing proved entertaining and useful to you.
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