How a Vacuum Tube Works-Penta Laboratories
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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.
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
Between 1907 and the 1960s, a staggering array of different vacuum tube families was
developed, most derived from de Forest's invention. With a very few exceptions,
most of the vacuum 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. Penta's PL300B version, plus many other brands, continue to
be very popular with audiophiles around the world. Various vacuum tubes were developed for radio, television, RF power, radar, computers, and specialized applications.
The vast majority of these vacuum tubes have been replaced by semiconductors, leaving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 vacuum
INSIDE A VACUUM 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 vacuum 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 vacuum tubes compromise at a level of vacuum that is adequate
for the vacuum tube's application.
First, let's talk about the parts of the vacuum tube.........
Today, nearly all vacuum tubes use one of two different kinds of cathode to generate
1) 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
2) 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 vacuum tube, however, so it is rarely used in really big power
3) 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.
- Vacuum 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 vacuum 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 vacuum 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
- Small-signal tubes almost always use oxide cathodes. Good-quality vacuum 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 PL300B, 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 vacuum 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.
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 vacuum 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
A vacuum 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 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 Penta PL300B,
PL811A/572 series, and 572B. Power triodes come in "low-mu" (low gain) and
"high-mu" (high gain) versions. Low-mu triodes like the PL300B 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.
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.
See section F, "audio beam tetrodes", below.)
Large ceramic tetrodes are often called "radial beam tetrodes" or simply
"beam tetrodes", because their electron emission forms a disc-shaped beam. The
wires on their control and screen grids are aligned, a special trick which
E. Other grids--the pentode
By adding a third grid to the tetrode, we get a PENTODE, a Penta original design for power grip tubes. The third grid is
called a suppressor grid and is inserted between the plate and the screen grid.
It has very few wire turns, since its only job is to collect the stray
secondary-emission electrons that bounce off the plate, and thereby eliminate
the "tetrode kink". It is usually operated at the same voltage as the cathode.
Tetrodes and pentodes tend to have higher distortion than triodes, unless
special circuit designs are used (see ULTRALINEAR, below).
EL34, EL84, and EF86 are true pentodes. The EL34 is widely used in
guitar and high-end amplifiers as the power output tube. The smaller EL84 is seen in
lower-cost guitar amps. The EF86 is used as a low-noise preamp in guitar amps and
professional audio equipment. One of the few large high-power pentodes is the
5CX1500B, often seen in radio transmitters.
There were vacuum tubes with more than three grids. The pentagrid converter tube,
which had five grids, was widely used as the front-end frequency converter in
radio receivers. Such vacuum tubes are no longer in production, having been fully
replaced by semiconductors.
F. Audio Beam Tetrode
This is a special kind of beam tetrode, with a pair of "beam plates" to
constrain the electron beam to a narrow ribbon on either side of the cathode.
Also, the control and screen grids have their wire turns aligned, much like the
large ceramic tetrodes (above). Unlike the ceramic tetrodes, the grids are at a
critical distance from the cathode, producing a "virtual cathode" effect. All
this adds up to greater efficiency and lower distortion than a regular tetrode
or pentode. The first popular beam tetrode was the RCA 6L6, introduced in 1936.
Beam tetrodes still made today include the 6L6GC and KT88SC; the former is
most popular in guitar amplifiers, while the latter is the most common power
tube in modern high-end audio amplifiers for the home. Today this design is seen
only in glass tubes used in audio amplifiers, not in ceramic power tubes.
G. The heater inside the cathode
An oxide-coated cathode can't heat itself, and it has to be hot to emit
electrons. So, a wire filament heater is inserted within the cathode. This
heater has to be coated with an electrical insulation that won't burn up at the
high temperatures, so it is coated with powdered aluminum oxide. This is an
occasional cause of failure in such tubes; the coating rubs off or cracks, so
the heater can touch the cathode. This can prevent normal operation of the tube.
And if the heater is running from AC power, it can put some of the AC signal
into the amplifier's output, making it unusable in some applications.
Good-quality vacuum tubes have very rugged and reliable heater
We want a good, hard vacuum inside a tube, or it will not work properly. And
we want that vacuum to last as long as possible. Sometimes, very small leaks can
appear in a tube envelope (often around the electrical connections in the
bottom). Or, the vacuum tube may not have been fully "degassed" on the vacuum pump at
the factory, so there may be some stray air inside. The "getter" is designed to
remove some stray gas.
The getter in most glass tubes is a small cup or holder, containing a bit of
a metal that reacts with oxygen strongly and absorbs it. (In most modern glass
tubes, the getter metal is barium, which oxidizes VERY easily when it is pure.)
When the vacuum tube is pumped out and sealed, the last step in processing is to "fire"
the getter, producing a "getter flash" inside the vacuum tube envelope. That is the
silvery patch you see on the inside of a glass tube. It is a guarantee that the
tube has good vacuum. If the seal on the tube fails, the getter flash will turn
white (because it turns into barium oxide).
There have been rumors that dark spots on getters indicate a vacuum tube which is
used. This is NOT TRUE. Sometimes, the getter flash is not perfectly uniform,
and a discolored or clear spot can occur. The vacuum tube is still good and will give
full lifetime. THE ONLY RELIABLE WAY TO DETERMINE THE HEALTH OF A VACUUM TUBE IS TO TEST IT ELECTRICALLY.
Glass power tubes often do not have flashed getters. Instead, they use a
metal getter device, usually coated with zirconium or titanium which has been
purified to allow oxidation. These getters work best when the vacuum tube is very hot, which is how such vacuum tubes are designed to be used. The 812A and 811 use
The most powerful glass tubes have graphite plates.
Graphite is heat-resistant (in fact, it can operate with a dull red glow for a
long time without failing). Graphite is not prone to secondary emission, as
noted above. And, the hot graphite plate will tend to react with, and absorb, any
free oxygen in the tube. The 572 series and 572B use graphite plates coated with
purified titanium, a combination which gives excellent gettering action. A
graphite plate is much more expensive to make than a metal plate of the same
size, so it is only used when maximum power capability is needed. Large ceramic
tubes use zirconium getters. Since you can't see a "flash" with such vacuun tubes, the
state of the tube's vacuum has to be determined by electrical means (sometimes
by metering the grid current).
I. Assembling the tube
A typical glass audio tube is made on an assembly line by people wielding
tweezers and small electric spot-welders. They assemble the plate, cathode,
grids and other parts inside a set of mica or ceramic spacers, then crimp the
whole assembly together. The electrical connections are then spot-welded to the
audio tube's base wiring. This work has to be done in fairly clean conditions,
although not as extreme as the "clean rooms" used to make semiconductors. Smocks
and caps are worn, and each workstation is equipped with a constant source of
filtered airflow to keep dust away from the tube parts.
Once the finished assembly is attached to the base, the
glass envelope can be slid over the assembly and flame-sealed to the base disc.
A small glass exhaust tube is still attached, and enters the envelope. The vacuum tube
assembly is attached to a processing machine (sometimes called a "sealex"
machine, an old American brandname for this kind of device). The exhaust tubing
goes to a multistage high-vacuum pump. The sealex has a rotating turntable with
several tubes, all undergoing a different step in the process.
comes vacuum pumping; while the pump runs, an RF induction coil is placed over
the tube assembly and all the metal parts are heated. This helps remove stray
gases trapped in the parts, and also activates the cathode coating.
- After 30 minutes or more (depending on the tube type
and the vacuum desired), the tube is automatically lifted up and a small flame
seals its exhaust tubing.
- The turntable rotates, and there may follow an electrical "break-in"
period where the tube is put through a series of operational stresses, such as
higher-than-rated heater voltages.
- Then the vacuum tube is rotated to the getter-flash station, where a combination
of RF induction heating and/or high-voltage discharge flashes the barium
- Finally the vacuum tube is removed, the base wiring is attached to the external
base (if it is an octal base type) with a special heat-resistant cement, and
the finished tube is ready for aging in a burn-in rack. If the vacuum tube meets a
set of operational specs in a special tester, it is marked and shipped.
J. Metal-ceramic power grid types:
If you want to control a LOT of power, a fragile glass tube is more difficult
to use. So, really big vacuum tubes today are made entirely of ceramic insulators and
metal electrodes. Otherwise, they are much the same inside as small glass
tubes--a hot cathode, a grid or grids, and a plate, with a vacuum in-between.
In these big vacuum tubes, the plate is also part of the vacuum tube's outer envelope. Since the plate carries the full vacuum tube current and has to dissipate a lot of heat, it is made with either a heat radiator through which lots of cooling air
is blown, or it has a jacket through which water or some other liquid is pumped
to cool it. The air-cooled vacuum tubes are often used in radio transmitters, while the liquid-cooled vacuum tubes are used to make radio energy for heating things in heavy industrial equipment. Such vacuum tubes are used as "RF induction heaters", to make all kinds of products--even other vacuum tubes.
Ceramic tubes are made with different equipment than glass tubes, although
the processes are similar. The exhaust tubing is soft metal rather than glass,
and it is usually swaged shut with a hydraulic press. All the equipment for
exhausting and conditioning the tube is much larger, since there is more volume
to exhaust, and the large metal parts require more aggressive induction heating.
The ceramic parts are usually ring-shaped and have metal seals brazed to their
edges; these are attached to their mating metal parts by welding or
WHY ARE TUBES STILL USED?
A. High-power RF applications
Many big radio stations continue to use big power tubes, especially for power
levels above 10,000 watts and for frequencies above 50 MHz. High-power UHF TV
stations and large FM broadcast stations are almost exclusively powered by
tubes. The reason is cost and efficiency--only at low frequencies are
transistors more efficient and less expensive than vacuum tubes.
Making a big solid-state transmitter requires wiring hundreds or thousands of
power transistors in parallel in groups of 4 or 5 at a time, then mixing their
power outputs together in a cascade of combiner transformers. Plus, they require
large heat-sinks to keep them cool. An equivalent vacuum tube transmitter can use only one vacuum tube, requires no combiner (which wastes some power), and can be cooled with
forced air or water, thus making it smaller than the solid-state transmitter.
This equation becomes even more pronounced at microwave frequencies. Nearly
all commercial communication satellites use a traveling-wave tube for their
"downlink" power amplifiers. The "uplink" ground stations also use TWTs. And for
high power outputs, the vacuum tube seems to reign unchallenged. Exotic transistors
still are used only for small-signal amplification and for power outputs of less
than 40 watts, even after considerable advances in the technology. The low cost
of RF power generated by tubes has kept them economically viable, in the face of
B. Guitar amps
In general, only very low-cost guitar amplifiers (and a few specialized
professional models) are predominantly solid-state. We have estimated that at
least 80% of the market for high-ticket guitar amps insists on all-tube or
hybrid models. Especially popular with serious professional musicians are modern
versions of classic Fender, Marshall and Vox models from the 1950s and 1960s.
This business is thought to represent at least $100 million worldwide as of
Why Vacuum Tube amplifiers? It's the tone that musicians want. The amplifier and
speaker become part of the musical instrument. The peculiar distortion and
speaker-damping characteristics of a beam-tetrode or pentode amp, with an output
transformer to match the speaker load, is unique and difficult to simulate with
solid-state devices, unless very complex topologies or a digital signal
processor are used. These methods apparently have not been successful;
professional guitarists keep returning to tube amplifiers.
Even the wildest rock musicians seem to be very conservative about the actual
equipment they use to make their music. And their preferences keep specifying
the proven technology of vacuum tubes.
C. Professional audio
The recording studio is somewhat influenced by the prevalence of tube guitar
amps in the hands of musicians. Also, classic condenser microphones, microphone
preamplifiers, limiters, equalizers and other devices have become valuable
collectibles, as various recording engineers discover the value of tube
equipment in obtaining special sound effects. The result has been huge growth in
the sales and advertising of vacuum tube- equipped audio processors for recording use. Although still a minor movement within the multi-billion-dollar recording
industry, tubed recording-studio equipment probably enjoys double-digit sales
D. High-end audio
At its low point in the early 1970s, the sales of tube hi-fi equipment were
barely detectable against the bulk of the consumer-electronics boom. Yet even in
spite of the closure of American and European tube factories thereafter, since
1985 the sales of "high-end" audio components have boomed. And right along with them have boomed the sales of vacuum-tube audio equipment for home use. The use
of vacuum tubes in this regime has been very controversial in engineering circles, yet the demand for tube hi-fi equipment continues to grow.
USING VACUUM TUBES
Bias is a negative voltage applied to a power tube's control grid, to set the
amount of idle current the tube draws. It is important to bias a vacuum tube to stay
within its rated dissipation. Otherwise, you DO NOT need to worry about small
deviances from the manufacturer's recommendations. Many times we have customers
asking us things like, "I replaced the audio tubes, the old audio tubes ran at 35 mA, the new ones run at 38 mA. I'm worried that I have to rebias the amp." This is NOT worth worrying about. Especially with guitar amps--they tend to run their tubes
at idle conditions which are conservative. Some high-end audio amps run their
power tubes quite hard--in that case, rebiasing is necessary. Many amps have no
bias adjustments at all, and are designed so that you do not need to concern
yourself with bias. This includes most Mesa-Boogie guitar amps, most amps using
EL84s, and many single-ended triode hi-fi amps. We suggest that users consult with the
equipment manufacturer, if possible.
B. When should I replace the vacuum tubes?
Practically speaking, you should only replace vacuum tubes in an audio amplifier
when you start to notice changes in the sound quality. Usually the tone will
become "dull", and transients will seem to be blunted. Also, the gain of the
amplifier will decrease noticeably. This is usually enough of a warning for vacuum tube replacement. If the user has very stringent requirements for observing vacuum tube
weakening, the best way to check vacuum tubes is with a proper mutual-
conductance-style tube tester. These are still available on the used market;
though new ones have not been manufactured in many years. One tester is being
manufactured today, the Maxi-Matcher. It is suitable for testing 6L6, EL34, 6550
and EL84 types. If you cannot get your own tube tester, speak to a service
technician for his recommendations. See our cathode section 2A above for some
idea of typical lifetimes for tubes.
Large ceramic power tubes are usually operated in equipment that has metering
of the plate current or power output. When the vacuum tube cannot reach the rated plate current or power output for the equipment, the vacuum tube is usually considered to be at the end of its normal life. The operating manual should give a more complete
procedure for estimating the health of the tube.
C. Blue Glow -- what causes it?
Glass tubes have visible glow inside them. Most audio types use oxide-coated
cathodes, which glow a cheery warm orange color. And thoriated-filament tubes,
such as the 811 and 572 triodes, show both a white-hot glow from their
filaments and (in some amplifiers) a slight orange glow from their plates. All of these are normal effects. Some newcomers to the tube-audio world
have also noticed that some of their tubes emit a bluish-colored glow. There are
TWO causes for this glow in audio power tubes; one of them is normal and
harmless, the other occurs only in a bad audio tube.
- Most glass power tubes show FLUORESCENCE GLOW. This is a
very deep blue color. It can appear wherever the electrons from the cathode
can strike a solid object. It is caused by minor impurities, such as cobalt,
in the object. The fast-moving electrons strike the impurity molecules, excite
them, and produce photons of light of a characteristic color. This is usually
observed on the interior of the plate, on the surface of the mica spacers, or
on the inside of the glass envelope. THIS GLOW IS HARMLESS. It is normal and
does not indicate a tube failure. Enjoy it. Many people feel it improves the
appearance of the tube while in operation.
- Occasionally a vacuum tube will develop a small leak.
When air gets into the vacuum tube, AND when the high plate voltage is applied, the
air molecules can ionize. The glow of ionized air is quite different from the
fluorescence glow above--ionized air is a strong purple color, almost pink.
This color usually appears INSIDE the plate of the vacuum tube (though not always).
It does not cling to surfaces, like fluorescence, but appears in the spaces
BETWEEN elements. A vacuum tube showing this glow should be replaced right away,
since the gas can cause the plate current to run away and (possibly) damage
PLEASE NOTE: some older hi-fi and guitar amplifiers, and a very few modern
amplifiers, use special audio tubes that DEPEND on ionized gas for their normal
- Some amps use mercury vapor rectifiers, such as types 83, 816, 866 or
872. These audio tubes glow a strong blue-purple color in normal use. They turn AC
power into DC to run the other tubes.
- And occasionally, vintage and
modern amplifiers use gas-discharge regulator tubes, such as types 0A2, 0B2,
0C2, 0A3, 0B3, 0C3 or 0D3.
These audio tubes rely on ionized gas to control a voltage tightly, and normally
glow either blue-purple or pink when in normal operation. If you are unsure if
these special tubes are used in your amplifier, consult with an experienced
technican before replacing them.
ALSO NOTE: these light sources cannot be seen in metal-ceramic tubes, because
their parts are opaque. As we said above, it is difficult to tell if a ceramic
tube has become gassy. Usually, in a large radio transmitter, a gassy tube will
arc over internally. (This does not damage the transmitter. It has protective
circuits.) The equipment operating manual should give more information on
D. What is Class A, B, AB, ultralinear, etc?
1. Class A means that the power tube conducts the same amount of current
all the time, whether idling or producing full power. Class A is very
inefficient with electricity but usually gives very low distortion.
- There are single-ended class-A, or SE, amplifiers.
They use one or more tubes in parallel, which are all in phase with each
other. This is commonly used in smaller guitar amps and in exotic high-end
amplifiers. Many audiophiles prefer the SE amplifier, even though it has
relatively high levels of even-order distortion. Most 300B high-end amplifiers
are SE. Negative feedback, which can be used to decrease the distortion of an
amplifier, is felt by some people to sound inferior. Most SE amps have no
- Push-pull class-A amplifiers also exist--they use two, four or more tubes
(always in pairs) which are driven in opposite phase to each other. This
cancels out the even-order distortion and gives very clean sound. An example
of a class-A push-pull amplifier is the Vox AC-30 guitar amp. Push-pull Class
A operation usually involves low plate voltages and high plate currents,
compared to Class AB operation below. The high currents might tend to wear out
the tube cathodes faster than in an AB amplifier.
- There are two kinds of class-A operation, which can apply to single-ended
--Class A1 means that the grid voltage is always more negative than
the cathode voltage. This gives the greatest possible linearity and is used with
triodes such as the 300B, and with audio beam tetrodes and
--Class A2 means that the grid is driven MORE POSITIVE than
the cathode for part or all of the waveform. This means the grid will draw
current from the cathode and heat up. A2 is not often used with beam tetrodes,
pentodes or triodes like the 300B, especially in audio. Usually a class-A2
amplifier will use tubes with special rugged grids, such as the 811 and 572
series of triodes.
Class A2 also requires a special driver circuit, that can
supply power to the grid.
2. Class AB applies only to push-pull amplifiers. It means that
when one tube's grid is driven until its plate current cuts off (stops)
completely, the other tube takes over and handles the power output. This gives
greater efficiency than Class A. It also results in increased distortion, unless
the amplifier is carefully designed and uses some negative feedback. There are
class-AB1 and class-AB2 amplifiers; the differences are the same as were
explained above--the tube's grids are not (AB1) or are (AB2) driven
3. Class B applies only
to push-pull amplifiers in audio; it SOMETIMES applies to RF power amplifiers
with one tube. It is like Class AB, except that the tubes idle at or near zero
current. This gives even greater efficiency than Class A or AB. It also results
in increased distortion, unless the amplifier is carefully designed and uses
some negative feedback. If careful design is not undertaken, the result may be
crossover distortion, which appears at the midpoint of the output waveform and
has very bad-sounding effects in audio. Most solid-state audio amplifiers use
class B, because the transistors undergo less heat stress when idling.
4. Ultralinear operation was invented by David
Hafler and Herbert Keroes in 1951. It uses only beam tetrodes or pentodes, and
special taps on the output transformer. The taps connect to the screen grids of
the tubes, causing the screens to be driven with part of the output signal. This
lowers distortion considerably. It is usually seen only in hi-fi amplifiers that
use power tubes such as the 6L6GC, 6550C, EL84 or EL34.
E. Why are different kinds of power supplies used in various tube amplifiers?
Why do some use tube amplifiers? Why do some use tube rectifiers, while others
use solid-state rectifiers, while still others have electronic
Tube rectifiers are still used in power supplies of some guitar amps, because
the current a tube rectifier can produce varies somewhat with the load. It is
quite different in response from a solid-state rectifier. Many audiophiles also
prefer this classic design for much the same reasons. Also, inexpensive
solid-state rectifiers can put "hash" into a power supply, because of their slow
transient capability while charging and recharging a filter capacitor 50/60
times a second. Special high-speed silicon rectifiers are available at high
cost. They are rarely used in products other than a few high-end amplifiers.
Tube rectifiers have faster transient response than most solid-state rectifiers,
also making them useful in some high-end designs.
Regulated DC plate power can be very helpful in a push-pull Class AB
amplifier. Because the amp draws greatly different current when at idle and when
delivering full power, a regulated supply "sags" less at full power, producing
better transient response in the amplifier. It is expensive to regulate the high
voltages in a tube amplifier, so it is done only in expensive top-line models.
Class A amplifiers have less need for regulation since they draw nearly the same
DC power at all times. It is dependent on the circuit design. The only way to
see if you need an amplifier with a regulated supply is to listen to it and
carefully compare it with similar amps with unregulated supplies. Regulation is
almost never used in guitar amps, since the DC power "sag" causes some signal
compression, which is considered part of the desired sound effect inherent to a
F. What are the advantages of an OTL amplifier over a conventional one with
an output transformer? Should I get an OTL? What about its reliability
OTL, or output-transformerless, amplifiers are special high-end products.
Because it is expensive and difficult to wind an output transformer for a tube
amplifier to achieve the best possible performance, some designers have chosen
to eliminate the transformer altogether. Unfortunately, tubes have relatively
high output impedances compared to transistors. So, tubes with large cathodes
and high peak emission capability are used---in many push-pull pairs. A
well-designed OTL is capable of the best audio performance available today. OTLs
usually require more maintenance and greater care in use than
transformer-coupled amps. In recent years, OTLs have gotten a bad reputation for
unreliability. This was only a problem with some low-cost manufacturers, who
have since gone out of business. A well-designed OTL can be just as reliable as
a transformer-coupled amp.
G. There's all this talk about "parallel feed", "shunt feed", SRPP, "mu
followers", and the like. Which should I use? What's the
Parallel feed and shunt feed are the same technique. Basically, a choke is
used to load the power tube (usually one, in SE mode), while the output
transformer is coupled to the plate of the tube through a capacitor. So, the
plate current of the tube does not flow through the output transformer. This can
be a very expensive technique to implement, since the choke must be as carefully
wound as the output transformer. It does offer a possible performance
improvement. You should try to audition a parallel-feed high-end amp before
buying it. This technique is considered too expensive for use in guitar amps.
SRPP circuits and mu-follower circuits are special designs which use a lower tube
(for gain), and an upper tube which serves as the plate
load for the lower tube. The upper tube also acts
as both a cathode follower and as a constant-current source for the lower tube. If
properly designed, either circuit can offer improved performance over an ordinary resistor-loaded tube stage. These
circuits are used only in preamp stages and in the driver stages of
power amps, usually SE types, in high-end audio.
If you want to learn more of the technical details behind vacuum-tube
electronic design, we recommend the following books .
- We recommend two recently-published books on circuit design, which the
novice can derive much information from:
THE BEGINNER'S GUIDE TO TUBE
AUDIO DESIGN, by Bruce Rozenblit (ISBN 1-882580-13-3);
and PRINCIPLES OF POWER, by Kevin O'Connor (ISBN 0-9698-6081-1).
- Classic textbooks on tube audio design which were recently reprinted are:
THE RADIO DESIGNER'S HANDBOOK, by Langford-Smith (ISBN
FUNDAMENTALS OF RADIO-VALVE TECHNIQUE, by J. Deketh
(ISBN 1-8825-8023-0); and PRINCIPLES OF ELECTRON TUBES by Herbert Reich
These books are more advanced and are not
recommended for the novice. They are available from Old Colony Sound Lab,
Antique Electronic Supply or other book dealers.
-All the books above are available from book dealers or from Antique
-If you want to learn more about tube materials and processes, the American
Institute of Physics currently publishes two classic books that are chock-full
of advanced information: HANDBOOK OF MATERIALS AND TECHNIQUES FOR VACUUM
DEVICES, by Walter Kohl (ISBN 1-56396-387-6); and HANDBOOK OF ELECTRON
TUBE AND VACUUM TECHNIQUES, by Fred Rosebury (ISBN 1-56396-121-0).
All of the books are available from large book dealers and from some of our
audio-tube Stocking Distributors.