| TRANSFIXR :ALL ANALOG ALL THE TIME
WITH NO FUCKING PEDALS |
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Back in 1904, British scientist
John Ambrose Fleming first showed his device to convert an alternating
current signal into direct current. The "Fleming diode"
was based on an effect that Thomas Edison had first discovered
in 1880, and had not put to useful work at the time. This diode
essentially consisted of an incandescent light bulb with an
extra electrode inside. When the bulb's filament is heated white-hot,
electrons are boiled off its surface and into the vacuum inside
the bulb. If the extra electrode (also called an "plate"
or "anode") is made more positive than the hot filament,
a direct current flows through the vacuum. And since the extra
electrode is cold and the filament is hot, this current can
only flow from the filament to the electrode, not the other
way. So, AC signals can be converted into DC. Fleming's diode
was first used as a sensitive detector of the weak signals produced
by the new wireless telegraph. Later (and to this day), the
diode vacuum tube was used to convert AC into DC in power supplies
for electronic equipment. |
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| Many other inventors tried to improve the
Fleming diode, most without success. The only one who succeeded was
New York inventor Lee de Forest. In 1907 he patented a bulb with the
same contents as the Fleming diode, except for an added electrode.
This "grid" was a bent wire between the plate and filament.
de Forest discovered that if he applied the signal from the wireless-telegraph
antenna to the grid instead of the filament, he could obtain a much
more sensitive detector of the signal. In fact, the grid was changing
("modulating") the current flowing from the filament to
the plate. This device, the Audion, was the first successful electronic
amplifier. It was the genesis of today's huge electronics industry. |
| Between 1907 and the 1960s,
a staggering array of different tube families was developed,
most derived from de Forest's invention. With a very few exceptions,
most of the tube types in use today were developed in the 1950s
or 1960s. One obvious exception is the 300B triode, which was
first introduced by Western Electric in 1935. Svetlana's SV300B
version, plus many other brands, continue to be very popular
with audiophiles around the world. Various tubes were developed
for radio, television, RF power, radar, computers, and specialized
applications. The vast majority of these tubes have been replaced
by semiconductors, leaving only a few types in regular manufacture
and use. Before we discuss these remaining applications, let's
talk about the structure of modern tubes. |
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INSIDE A TUBE
All modern vacuum tubes are based on
the concept of the Audion--a heated "cathode" boils
off electrons into a vacuum; they pass through a grid (or
many grids), which control the electron current; the electrons
then strike the anode (plate) and are absorbed. By designing
the cathode, grid(s) and plate properly, the tube will make
a small AC signal voltage into a larger AC voltage, thus amplifying
it. (By comparison, today's transistor makes use of electric
fields in a crystal which has been specially processed--a
much less obvious kind of amplifier, though much more important
in today's world.)
Figure 3 (Inside a miniature tube)
shows a typical modern vacuum tube. It is a glass bulb with
wires passing through its bottom, and connecting to the various
electrodes inside. Before the bulb is sealed, a powerful vacuum
pump sucks all the air and gases out. This requires special
pumps which can make very "hard" vacuums. To make
a good tube, the pump must make a vacuum with no more than
a millionth of the air pressure at sea level (one microTorr,
in official technical jargon). The "harder" the
vacuum, the better the tube will work and the longer it will
last. Making an extremely hard vacuum in a tube is a lengthy
process, so most modern tubes compromise at a level of vacuum
that is adequate for the tube's application.
First, let's talk about the parts of
the tube.........
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A. Cathode
Today, nearly all tubes use one of two different
kinds of cathode to generate electrons.
- The thoriated filament: it is just a tungsten
filament, much like that in a light bulb, except that a tiny amount
of the rare metal THORIUM was added to the tungsten. When the
filament is heated white-hot (about 2400 degrees Celsius), the
thorium moves to the outer surface of it and emits electrons.
The filament with thorium is a much better maker of electrons
than the plain tungsten filament by itself. Nearly all big power
tubes used in radio transmitters use thoriated filaments, as do
some glass tubes used in hi-fi amps. The thoriated filament can
last a VERY long time, and is very resistant to high voltages.
- The other kind of cathode is the oxide-coated
cathode or filament. This can be either just a filament coated
with a mixture of barium and strontium oxides and other substances,
or it can be an "indirectly heated" cathode, which is
just a nickel tube with a coating of these same oxides on its
outer surface and a heating filament inside. The cathode (and
oxide coating) is heated orange-hot, not as hot as the thoriated
filament--about 1000 degrees Celsius. These oxides are even better
at making electrons than the thoriated filament. Because the oxide
cathode is so efficient, it is used in nearly all smaller glass
tubes. It can be damaged by very high voltages and bombardment
by stray oxygen ions in the tube, however, so it is rarely used
in really big power tubes.
- Lifetime of cathodes: The lifetime of
a tube is determined by the lifetime of its cathode emission.
And the life of the of a cathode is dependent on the cathode temperature,
the degree of vacuum in the tube, and purity of the materials
in the cathode.
- Tube life is sharply dependent on
temperature, which means that it is dependent on filament
or heater operating voltage. Operate the heater/filament too
hot, and the tube will give a shortened life. Operate it too
cool and life may be shortened (especially in thoriated filaments,
which depend on replenishment of thorium by diffusion from
within the filament wire). A few researchers have observed
that the lifetime of an oxide-cathode tube can be greatly
increased by operating its heater at 20% below the rated voltage.
This USUALLY has very little effect on the cathode's electron
emission, and might be worth experimenting with if the user
wishes to increase the lifetime of a small-signal tube. (Low
heater voltage is NOT recommended for power tubes, as the
tube may not give the rated power output.) Operating the heater
at a very low voltage has been observed to linearize some
tube types-- we have not been able to verify this, so it may
be another worthy experiment for an OEM or sophisticated experimenter.
The average end-user is advised to use the rated heater or
filament voltage--experimentation is not recommended unless
the user is an experienced technician.
- Oxide cathodes tend to give shorter
lifetimes than thoriated filaments. Purity of materials is
a big issue in making long-lived oxide cathodes--some impurities,
such as silicates in the nickel tube, will cause the cathode
to lose emission prematurely and "wear out". Low-cost
tubes of inferior quality often wear out faster than better-quality
tubes of the same type, due to impure cathodes.
- Small-signal tubes almost always use
oxide cathodes. Good-quality tubes of this type, if operated
well within their ratings and at the correct heater voltage,
can last 100,000 hours or more.
- The world record for lifetime of a
power tube is held by a large transmitting tetrode with a
thoriated filament. It was in service in a Los Angeles radio
station's transmitter for 10 years, for a total of more than
80,000 hours. When finally taken out of service, it was still
functioning adequately. (The station saved it as a spare.)
By comparison, a typical oxide-cathode glass power tube, such
as an EL34, will last about 1500-2000 hours; and a tube with
an oxide-coated filament, such as an SV300B, will last about
4000-10,000 hours. This is dependent on all the factors listed
above, so different customers will observe different lifetimes.
B. Plate (anode)
The plate, or anode, is the electrode that
the output signal appears on. Because the plate has to accept the
electron flow, it can get hot. Especially in power tubes. So it
is specially designed to cool itself off, either by radiating heat
through the glass envelope (if it's a glass tube), or by forced-air
or liquid cooling (in bigger metal-ceramic tubes). Some tubes use
a plate made of graphite, because it tolerates high temperatures
and because it emits very few secondary electrons, which can overheat
the tube's grid and cause failure. See "H--the getter"
below for more about the graphite plate.
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C. Control Grid
In nearly all glass audio tubes, the
control grid is a piece of plated wire, wound around two soft-metal
posts. In small tubes the plating is usually gold, and there
are two posts made of soft copper. Grids in big power tubes
have to tolerate a lot of heat, so they are often made of
tungsten or molybdenum wire welded into a basket form. Some
large power tubes use basket-shaped grids made of graphite
(see D below).
Inside any modern amplifying tube,
one of the things to avoid is called secondary emission. This
is caused by electrons striking a smooth metal surface. If
many secondary electrons come out of the grid, it will lose
control of the electron stream, so that the current "runs
away", and the tube destroys itself. So, the grid is
often plated with a metal that is less prone to secondary
emission, such as gold. Special surface finishing is also
used to help prevent secondary emission.
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A tube with only one grid is a TRIODE. The
most widely used small triode, the 12AX7, is a dual triode which
has become the standard small-signal amplifier in guitar amps. Other
small glass triodes used in audio equipment include the 6N1P, 6DJ8/6922,
12AT7, 12AU7, 6CG7, 12BH7, 6SN7 and 6SL7.
Many glass power triodes are currently on
the market, most of them aimed at amateur radio or high-end audio
use. Typical examples are the Svetlana SV300B, SV811/572 series,
and 572B. Power triodes come in "low-mu" (low gain) and
"high-mu" (high gain) versions. Low-mu triodes like the
SV300B have very low distortion and are used in high-end audio amplifiers,
while high-mu triodes are used mostly in radio transmitters and
big high-power audio amplifiers.
Large ceramic-metal power triodes are often
used in radio transmitters and to generate radio energy for industrial
heating applications. Specialized triodes of many kinds are made
for exotic applications, such as pulsed radars and high-energy physics
work.
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D.Screen grid--the tetrode
Adding another grid to a triode, between
the control grid and the plate, makes it into a TETRODE. This
"screen" grid helps screen, or isolate, the control
grid from the plate. This is important is reducing the so-called
Miller effect, which makes the capacitance between the grid
and plate look much bigger than it really is. The screen also
causes an electron-accelerating effect, increasing the tube's
gain dramatically. The screen grid in a power tube carries
some current, which causes it to heat up. For this reason,
screen grids are usually coated with graphite, to reduce secondary
emission and help keep the control grid cool.
Many large radio and TV stations use
giant metal-ceramic power tetrodes, which are capable of high
efficiency when used as RF power amplifiers. Power tetrodes
are also sometimes used in amateur radio and industrial applications.
(Regular tetrodes are rarely used for audio applications because
of an effect called "tetrode kink", caused by that
secondary emission. Most of it is due to electrons bouncing
off the plate, some from the screen.) This greatly increases
distortion and can cause instability if not carefully dealt
with in the design.
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