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Atwood; Mercury-Vapor Rectifiers in Audio
Mercury-Vapor Rectifiers in Audio
By John Atwood "Lynns Mercury Vapor rectifiers One of the more eye-catching features in a lot of home-built €śextreme€ť tube audio amplifiers are mercury-vapor rectifiers. Their hazy blue glow that is modulated by the current draw of the amplifier adds to the organic life that attracts people to vacuum tube amps. But concerns about safety have polarized the audio community, with some fearing that their homes may become EPA hazard sites! And, do mercury-vapor rectifiers have a sonic benefit in tube amplification? This article will try to answer this question. A follow-up article by my friend, Wally Chan, is a well-researched look at the safety of mercury-vapor tubes in the home. (note: This picture taken by and supplied by Lynn Olson.) Physics & History First, some history and definitions. Part of the breakthrough in technology that allowed radios to be run off of home AC power, rather than storage and dry batteries, was the development of inexpensive rectifiers. In the high-power industrial field, conversion of AC to DC was traditionally done with motor-generator sets, but these are expensive, noisy, and unreliable. High-vacuum rectifiers became available in the 1920s, but the early ones (e.g. 207, 81) had high voltage drops, making them inefficient. However, once the physics of gas discharges was understood, the low voltage drops in a gas discharge could be used to make a more efficient rectifier. Mercury vapor gives a voltage drop of about 11 volts, essentially independent of current flow. The first mercury rectifiers were large €śpool€ť rectifiers that used a hot arc discharge from the surface of the mercury pool to generate the electrons and ions needed to conduct current through the rectifier. The smaller ones took the form of large glass bulbs with glass arms coming out of the sides for each anode. The larger ones, handling thousands of amps, were built into water-cooled metal tanks. On a large industrial scale, these were very efficient, and used right up until the time they were replaced by silicon rectifiers in the 1960s and 70s. For smaller scale operations, hot-cathode mercury-vapor rectifiers were developed. These used oxide-coated cathodes and were processed like high-vacuum rectifiers, but a small amount of mercury was added before the glass bulb was sealed. Once the tube is warmed-up, the mercury vapor allows conduction as soon as the voltage from plate to cathode reaches the ionization potential. If a metal grid is placed between the cathode and plate, a thyratron is formed, where conduction can be inhibited by a negative voltage between the grid and cathode. Once conduction starts, in either a rectifier or thyratron, it doesnt stop until the anode voltage falls below the ionization voltage. The thyratron is analogous to the silicon controlled rectifier (SCR). The higher efficiency of a mercury-vapor rectifier made it standard practice in industrial, radio station, and ham-radio transmitter uses by the late 1920s. There were a few instances of mercury vapor rectifiers in the home in the late 20s, with the American tube types 82 and 83 being used. However, mercury vapor rectifiers have their problems. They are very sensitive to ambient temperature, not conducting if they are too cold and arcing backwards if they are too hot. If anode voltage is applied before the cathode has heated-up, positive ions will strip the cathode, drastically reducing the life-time of the tube. And then there is the health issue of mercury. In an industrial environment with trained technicians, the operating and handling issues of mercury vapor tubes could be tolerated. But as soon as decent higher-current high-vacuum rectifiers became available (e.g. 5Z3, 5U4G, 5V4G, etc.) mercury-vapor tubes completely disappeared from the American home. Arc rectifiers can also be made with noble gases, usually argon or xenon, but their ionization potential (hence voltage-drop) is higher, although still less than equivalent high-vacuum rectifiers. They are also susceptible to €śclean-up€ť, where the gas molecules are slowly driven into the metal plates over use, dropping the internal gas pressure. Mercury-vapor tubes dont have this problem, since only a small fraction of the mercury in the tube is vaporized, the rest being a liquid reservoir. In the early days of AC-powered radios, Raytheon developed the BH gas rectifier, which used the heat from a gas discharge to heat up an electron-emitting spot on the cathode, eliminating the need for a cathode heater. This evolved into the 0Z4 rectifier used in car radios. These so-called cold-cathode rectifiers were not too reliable and were finicky, requiring both a minimum and maximum current rating, and generated RF noise. The temperature requirements for military equipment is often far beyond what a mercury vapor tube could handle, both for low (think high-altitude bombers) and for high (think deserts or jungles) temperatures. At first this problem was addressed by high-vacuum rectifiers, such as the 1616 and 836, but by the end of World War II, a new class of xenon gas rectifiers was developed that could directly substitute for mercury rectifiers, but run over a much broader temperature range. The most common are the 3B25 and 3B28. Xenon/argon thyratrons were also developed to replace many mercury-vapor thyratrons. The only place where mercury still had an edge was for very high current or high power applications. By the late 1950s, high-voltage, high-current silicon rectifiers and SCRs started to displace mercury and xenon arc rectifiers, and by the early 1970s mercury-vapor tubes were only being made for replacement use. The environmental movement took hold in the 1970s, and by the late 1970s, hazardous materials in electronics (such as mercury, cadmium plating, and PCBs in capacitors and transformers) were phased-out where feasible. This was not a problem for mercury-vapor tubes, since semiconductor replacements had already eclipsed these tubes. However, many thousands of mercury-vapor tubes, both used and new, filled the closets and store-rooms of service shops, ham shacks, and radio stations, which is where the ones seen in modern home-built equipment come from. Mercury-Vapor in Audio RCA 866AWhat are the advantages of using mercury-vapor rectifiers in audio, aside from the nice blue glow? About the only one that comes to mind is the relatively low and constant voltage drop which makes them more efficient than high-vacuum rectifiers and preserves better voltage regulation under changes in current draw, such as would occur in a class-B amplifier. In an AM radio station or ham transmitter, this is quite important. However, most of the uses Ive seen of mercury-vapor tubes in modern amp designs were for single-ended or otherwise class-A applications where the current draw is nearly constant. Not having personally A-B tested the sonic difference between mercury-vapor and high vacuum rectifiers, Im not going to take a stand on the sonic issue. And, I certainly wont take up here the issue of solid-state versus tube rectifiers in general, other than to say that I do like to use high-vacuum rectifiers in some of my designs. Now for the down-side of using mercury vapor rectifiers. The need for preheating the cathode before applying high voltage is not just modest lifetime enhancement as it is for high-vacuum rectifiers, it is a necessity. Light up these tubes with high voltage applied just a couple of times and the oxide-coating will be shot. Thus a reliable time delay circuit or well-managed manual high-voltage switch is needed. A major problem with all gas or vapor arc rectifiers is radio-frequency noise. This is not the subtle switching noise from a silicon rectifier that takes a sensitive spectrum analyzer to see, this is noise that can blank out an AM radio. When these rectifiers were used in communications equipment or car radios, so-called €śhash filters€ť (composed of inductors and capacitors) had to be installed on both the AC and DC connections to the rectifiers. This noise is caused by the abrupt onset of conduction when the ionization potential is reached. The chaotic condition of the gas or vapor at the cathode surface causes this point of conduction to be somewhat randomized, hence the use of the term €śhash€ť for the kind of noise it generates. It can be argued that in a purely audio amplifier, this noise is irrelevant, but RF can be detected or intermodulated down into the audio band in sensitive circuits. Filtering can only do so much. I would rather not have these major noise generators in my high-resolution amplifiers. The peak-current capability of gas or mercury rectifiers is quite limited. This essentially restricts their use to choke-input power supplies. While choke-input supplies have some advantages over capacitor-input supplies, they require higher AC input voltages, a large choke that throws off a large AC magnetic field, and a large bleeder resistor to insure a minimum current load. In other words, you would never see a choke-input supply in something as compact or cost sensitive as a Fisher 500C receiver! For a large set-up where cost is not an object, choke-input supplies can work well. This is the only place a mercury or gas rectifier would find a home. No mercury-vapor tubes have been made, at least in the west, since the 1970s. Anyone wanting to use them in a new design will have to draw on the finite supply of used and N.O.S. (New Old Stock) tubes. An individual can probably accumulate enough tubes to serve as a personal lifetime supply, but it isnt feasible (or probably legal) for a company to sell products using mercury-vapor tubes. Ebay prohibits sales of devices containing mercury, including mercury vapor tubes, but this isnt strictly policed. The nice 866A with its €śworld€ť carton (used by RCA from about 1939 to 1942) was recently purchased on ebay. And finally we come to the health issues of mercury. Mercury is a neurotoxin that accumulates in the body and has effects from subtle mental to major injury, depending on the dose and form of the mercury. Elemental mercury, the kind in mercury-vapor tubes, is the least harmful, and can be handled without much harm. Almost anyone with a technical upbringing my age or older can remember playing with liquid mercury, putting it on silver dimes, and marveling at its liquid weight. We are still alive and (mostly) still have our minds, but, in retrospect, playing with mercury wasnt doing our nervous system any good. As Wally Chan points out in the next article, it is the mercury vapor that does the damage, and at room temperatures, the vapor is not too bad. However, the chance of tube breakage putting the rather small amounts of mercury in a mercury-vapor tube into your living-room rug is probably worth avoiding. In summary, using mercury-vapor tubes is a risk you should be aware of, and use only understanding the risks. They have their place in historically-correct equipment. In my opinion though, even aside from the risk, the operating hassles of mercury-vapor tubes makes them not worth using, except possibly for high-power class-B amps. However, the blue glow is nice! If you really want a glow, try the 3B28 - it is safer and longer-lasting, too." http://www.clarisonus.com/blog/?p=230#more-230 -- Message posted using http://www.talkaboutaudio.com/group/rec.audio.tubes/ More information at http://www.talkaboutaudio.com/faq.html |
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