Thursday, May 23, 2013

Putting The Vacuum In Vacuum Tubes

Given that most of our hi-fi components these days have gone solid-state, does the quality level of the vacuum inside our vacuum tubes still matter?

By: Ringo Bones

Even though maintaining a near-perfect vacuum inside a typical thermionic vacuum tube – or valves as they are called in merry old England - that’s still in use in some “purist” hi-fi equipment these days is vital for the tube to do its intended function, there are probably more people more interested to know who’s in the running for this year’s Miss Teen Topanga than the level of quality of the vacuum in the vacuum tubes in current production. Given that most of our hi-fi components these days have already gone solid-state, does the vacuum quality inside a typical vacuum tube in current production still matter?

These days – as in well into the second decade of the 21st Century – thermionic vacuum tubes are horse and buggy technologically wise compared to other of our home entertainment gear that have since gone the solid-state route. It is primarily their musically and psycho-acoustically consonant to the human hearing sound of vacuum tubes that have still endeared them in purist high-end hi-fi and the electric guitar amplification world that hitherto most solid-state designs still can’t achieve its own version of “musicality” and purity of tone. But given that most vacuum tube manufacturing equipment in current use probably dates back before World War II, manufacturing thermionic vacuum tubes of both high quality and reliability given the near-perfect vacuum required is getting harder and harder as we headlong into the 21st Century.

Thermionic vacuum tubes need a high vacuum which has to be achieved during manufacture and maintained during its entire service life – typically 2,000 to 10,000 hours. Even if a satisfactory vacuum is achieved initially, through pumping and ignition of the getter, occluded gases in the metal electrodes and glass enter the vacuum over time, especially if the heat treatment to drive them out before the vacuum tube is sealed is perfunctory, or the metal-to-glass seal around the pins leak due to unmatched coefficients of expansion between the glass and pin materials. Vacuum tubes, after all, are still a triumph of 20th Century materials technology and carefully controlled production processes.

But thermionic vacuum tube design and manufacture can be more than just maintaining a near perfect vacuum inside its glass envelope. In a July 1943 issue of the Scientific American magazine, Dr. Harvey C. Rentschler told in a then recent meeting of the American Physical Society that gases can dissolve in the crystalline structure of metals. In his experiments during the previous eight years back then have led to the conclusion that atoms of gas – like oxygen, hydrogen, or nitrogen  - actually dissolve in the crystalline structure of some metals just as salt dissolves in water. These gas particles then “loosen” the electrons in this structure, causing them to be emitted from the metal more readily when heat is applied. This “explanation”, according to Dr. Rentschler, should result in longer-lasting vacuum tubes and accomplish important savings in the size and the number of electric batteries, generators, and other apparatus needed to supply the filament power. Thanks to Dr. Rentschler’s discovery, there are vacuum tubes designed and manufactured after World War II that can function with an anode voltage or power supply as low as the standard 48-volt phantom power in a typical mixing board or desk. For example, the 12AX7 preamp tube can function when supplied with an anode voltage as low as 45 volts DC and yet it is still perfectly happy in a circuit that runs on 250 volts DC power supply.

Even though a typical high quality vacuum tube has vacuum levels at 0.000001 Torr or millimeters of mercury (a typical atmospheric pressure on planet Earth at sea level is 760 Torr or 760 millimeters of mercury) there are places and conditions elsewhere in the cosmos that would put the levels of vacuum found in a typical high quality thermionic vacuum tube’s glass enclosure to shame. The Horsehead Nebula and related celestial mists are more rarefied than the highest or hardest laboratory vacuum – or manufactured vacuum tubes – scientists had ever created so far here on Earth, but in many interstellar regions of the Milky Way galaxy, these whispy mists are banked so deep, cloud on cloud, that they completely hide the stars and galaxies which lie behind them. And yet on average, they are 50,000 times more rarefied than the vacuum enclosed inside a typical vacuum tube.

Astronomical instruments that were considered state-of-the-art during the 1960s had also found out that the convection currents in the outermost atmosphere of the red supergiant star Betelgeuse are comprised of atoms that are more loosely packed than in the most perfect vacuum scientists has ever been able to create here on Earth. These astronomical instruments had even shown that the region surrounding the Horsehead Nebula and the outermost atmosphere of the red supergiant star Betelgeuse is more rarefied by a factor of 50,000 or more than the “vacuum” inside a typical high quality thermionic vacuum tube!

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