Gramophone Dreams

Sidebar: That Purple-Blue Glow

Toward the end of this column, Herb writes, "My current new-production Western Electric 300Bs have strong blue clouds and have worked perfectly since 2021. These Ray Tubes Reserve 300Bs have blue clouds just above the smoked part of their glass; I take that as a good sign."

Is it a good sign, or isn't it? Yes, it's a good sign, but it's worth thinking about why.

When something glows inside a vacuum tube, it's because electrons are colliding with atoms, ions, or molecules. Energetic electrons bump into them, putting them in a higher energy state. Light is emitted as they return to their ground states. If this happens between the cathode and the anode, it means you don't have a good vacuum. That's not happening here.

Why do Herb's 300B tubes glow that beautiful blue? Western Electric's explanation is right on the important points but questionable on the specifics. "A blue glow may appear in the dome of the WE300-B just below the upper mica and the top edge of the plate." Which is precisely where it does appear. "The glow is caused by the electrons striking the plate, volatizing the surface, and dislodging atoms of nickel. High speed barium-strontium electrons from the filament collide with the atom, tearing off electrons, ionizing them and causing a deep blue color even in a well evacuated tube. The resulting glow isa function of the plate voltage and the number of gas molecules in the envelope. In a good vacuum there are still billions of molecules. The electrons travel safely back to the positively charged plate." They don't supply a scholarly citation.

There are two ways to dislodge nickel: heat and sputtering. Some tubes do boil off material from their plates: graphite, tantalum. But the boiling point of nickel is 2913°C—that's much higher than even the temperature of the cathode/filament, which in this relatively low-temperature tube (note the orange glow, not blue or white) is in the range of 800–900°C. To volatilize nickel, you'd need far more heat. The other way to do it is to knock those nickel atoms right off. Physicists call this "sputtering." Like a pool ball flying off the table, you knock it out physically with momentum. But electrons are hopeless for this: They are far too light. The only way to sputter nickel is with heavy ions traveling at high velocity. They'd need to be negative ions; otherwise the plate voltage would repel them.

There's one more problem: The strongest spectral lines in ionized nickel are ultraviolet—invisible—and green.

What, then, is that blue glow? I think this cathodeluminescence—that's the technical term—is coming from the glass itself, specifically from impurities in the glass. Bombarded by electrons, they glow, for the same reason electron-bombarded gases glow. The photos I've seen are consistent with this interpretation.

What does this say about the quality of the tube? Stray electrons—the ones that escape out the top of the long cathode-anode structure—don't collide with anything until they reach the glass at the top of the tube. That's evidence of an excellent vacuum.—Jim Austin