systems are used for drying everything
from textiles to lumber, for curing
adhesives and melting plastics, for
food cooking, pasteurization and
sterilization, and even welding.
PHOTO 2. The Peavey
Penta Guitar Amplifier.
Most of us are already familiar
with the “glass audio” crowd — audiophiles who prefer the sound of vacuum tube amplifiers — but there’s a flip
side to this coin. All modern manufacturers of guitar amplifiers still feature
vacuum tube amplifiers as a major
part of their product line, and many of
them are using designs that haven’t
changed substantially for decades.
Vacuum tube amplifiers exhibit a
“soft clipping” when driven into overload, and this gives a form of distortion
that’s deficient in the harsh sounding
odd order harmonics produced by
solid-state amplifiers. Guitar players
since the 1950s have intentionally over
driven their amplifiers just to get this
sound, and there are many classic
vacuum tube amplifiers that are prized
for the particular sound that they give.
Photo 2 shows the chassis of a
brand new Peavy Penta guitar amplifier — notice that it’s all tubes!
appliances and automobiles — are also
vacuum tubes. VFDs
have a heated filament, a plate (the
and, even in the case
of multiplexed displays, a control grid.
Gas Discharge Tubes
The Thyratron is a gas filled tube
with a plate, cathode, and control grid
that’s used in on-off switching applications in much the same way that you’d
use a silicon controlled rectifier.
However, unlike SCRs, there are
Thyratron tubes that are able to switch
short pulses in the hundred megawatt
(!!) range. Such tubes are used today
to control pulsed radar transmitters
and X-ray machines.
Another interesting feature of
Thyratrons is that they can have a rise
time — the time it takes the device to
switch from off to on — that’s on the
order of a hundred times faster than an
SCR. That fast switching time alone is
enough to justify the use of a Thyratron
in applications such as “crowbar”
circuits for high voltage power supplies
or firing circuits for pulsed lasers, even
where solid-state devices could otherwise handle the power levels involved.
Two specialized Thyratrons — the
Krytron and Sprytron — are able to
switch in subnanosecond intervals.
This sounds like an esoteric concern,
but, in fact, it makes them ideal for
triggering explosives, particularly in situations where multiple charges need
to be detonated with precise timing.
This is exactly the situation you’d find
yourself faced with if you were building an atom bomb, and Krytrons were
used in the detonator for the original
Fat Man bomb.
Obviously, I don’t
know whether they’re still used for this
purpose today, but the sale and export
of Krytrons and Sprytrons are tightly
regulated by the US Government.
And, although they aren’t really
vacuum tubes in the sense that we’re
talking about here, it’s worth pointing
out that the ubiquitous fluorescent
lamps, mercury vapor lamps, and low
pressure sodium street lamps are all
gas discharge tubes. Neon signs,
Xenon flash lamps, and even gas
lasers all also belong to this category.
One of the oldest forms of vacuum
tube — the X-ray tube — has been
around since the late 19th century.
Besides the obvious application of medical imaging, X-ray tubes are used today
for security scanners (we’ve all seen
those at the airport), food inspection
systems, thickness gauges, and more.
Vacuum tubes are also still useful
We’ve all seen those cute new
LCD TVs and computer monitors, but
did you know that CRTs still account
for about three fourths of all television
sales worldwide? The trusty old cathode ray tube is not only the cheapest
display technology available, but it also
has the brightest display, best contrast
ratio, fastest response, and the best
color rendition. New technologies like
LCDs, plasma displays, and organic
LEDs are making steady progress, but it
will be a while before CRTs disappear.
While we’re on the topic, vacuum
fluorescent displays, or VFDs — those
blue green numeric, alphanumeric,
and graphic displays often found in
PHOTO 3. S11 X-Ray
Tube. Photo courtesy
of Source 1 X-Ray.
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