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Default 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


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