The following are the current main tube manufacturers, grouped by country:

SED, (formerly Svetlana Electron Devices), St. Petersburg

SED is a major Russian power-tube manufacturer. Formerly a part of the huge Svetlana collective factory, that is now independent. SED products are now marketed outside Russia by PM Components using the SED (Winged C logo). Audio and Guitar types in production: EL34, SV6L6GC, SV6550C, SV300B, EL509, EF86, 6BM8, 6AS7, 12AX7A. (website: http://www.SED-USA.com)

Reflector Corp, Saratov

Reflector makes most of the tubes marketed by New Sensor Co. of New York under their "Sovtek" and "Electro-Harmonix" brand names. Some are old Russian tubes, others are made especially for export. They recently acquired the Tung-Sol, Mullard and Genalex trademarks and are making replicas of famous audio tubes from these long out-of-business companies. Types in production:

12AX7EH, 12AX7LP, 12AX7LPS, 12AX7WXT+, 5751, 7027, EL34WXT, 6550WE, 6550WD, KT88, 6L6WXT+,

5881, 6922, 6EU7, 7199, EF86, EL84, 300B, 2A3, 6A3, 6B4G, 6072A, 6973, 7868, 7591A, 7591XYZ, KT66, KT88,

5AR4,  and many others. Also variants of above bearing "EH" suffix. "Sovtek" also markets 5U4G,

5Y3GT, 6SN7, 6SL7, 6N1P, two 12AX7s (WA and WB) and a few other types made at other Russian factories, primarily Voskhod Kaluga.

Note: there is no "Sovtek" factory. "Sovtek" is only an American brand name owned by New Sensor.

(New Sensor currently owns the Svetlana "S" trademark name and logo in the USA, and is attaching this logo to Reflector-made tubes.) (Note: There was a factory in Kaluga called Voskhod Kaluga. They once made 6V6s and 12AX7s for New Sensor. Their continued production cannot be confirmed at this time.)

(Website: www.newsensor.com)

Shuguang Electrical Factory No. 1

Their main factory is located in Changsha, Hunan; and is owned by Korean firm, Samsung. Shuguang makes

most of the popular audio and guitar types, which are frequently rebranded by importers and OEMs.

Types believed introduction: 6L6GC, EL34 (3 types), 6550, KT100, 5AR4, 2A3, 807, 211, 845.

(website:http://www.shuguangtubes.com)

Liuzhou

They make many power tube types, which are marketed abroad. PM Components, a British firm, has "Golden Dragon" type

made by Liuzhou.

O&J Enterprises/Valve Art/ESTI

Reportedly makes the following: 6L6GC, EL34, 6550A, KT88, KT100, 300B, 300B-C60 (graphite plate 60w),  5300B (graphite plate 80w 150mA), and 6300B (graphite plate 80w 180mA), plus possibly other types,  at a factory in Changsha, Hunan. It is suspected some types might be made by Shuguang and Liuzhou under an   exclusive agreement. Note: The "Sino" factory in Beijing recently stopped making 12AX7, 12AT7, 12AU7, 6GH8, 6AN8  and other small tubes, due to a poor market and tough competition from Sovtek/New Sensor. Sino is apparently no longer making any tubes. Rumor has it that Sino's old tooling was purchased by Magic Parts, who have put it back into operation with a Chinese partner. There are other tube factories in China. Most make only large power tubes and keep a very low profile, such as the Jingguang factory that makes most of the ceramic tubes for Penta Laboratories. Some specialize in transmitting tubes such as the 811A, 812A, 813, 3-500Z, 100TH, 833 and others. Such transmitting types are often branded with old American/European brands and sold without notification that they are Chinese-made. Buyers should beware of such rebranded Chinese tubes.

Elektronska Industrija (Ei), Nis

Former Philips contract manufacturer, making audio tubes along with many other electronic products. Edicron is currently

marketing these.

Types in recent production include: 6CA7, 6CG7, 12AT7, 12AX7, 12BH7, 12DW7(7247), EL34, EL519, PL519, KT90,

KT99, EL84, possibly others.

NOTE:  A new development, according to a phone call from Charles Whitener, Western Electric has purchased the Ei factory in November 2006. More news to come soon about this from Whitener.

(website: http://www.eiexpo.co.yu/)

Slovakia (JJ Tubes)

In 1994, Jan Jurco, a business entrepreneur in Slovakia, the former Czech Republic, obtained tube manufacturing equipment from the former Tesla Tube company and opened JJ Tube manufacturing. Originally, they only made the 12ax7, EL34 and KT88. The power tubes were available in clear, red or blue glass. Their first factory was only about 10,000 sq ft. Now, they have expanded to become one of the most popular and well-respected tube company in the world. Their production facility is about 40,000 sq ft and they manufacture over 20 different tube types, tube amps, preamp and capacitors. Their current lineup of tubes includes: ECC81, ECC82, ECC83S, ECC803S, E88CC, EL34, EL84, KT77, KT88, 6L6GC, 6V6S, GZ34, EZ82, 2A3, 300B, ECC99, ECC832/7247. They now have an EF86 as well.

VTV: The only NOS left in any quantities are the JAN Philips/Sylvania items

made in the mid-1980s, because they are recently released government surplus. These types include: 12AT7WA, 6SL7GT,

5814, 5Y3GT, 6V6GTA, 6L6WGB and several other radio and test equipment types that are not useable in guitar amps. Most

of these are now in low supply or almost gone, so I wouldn’t wait too long to buy your lifetime stash.

A few years ago, a bunch of military surplus Brimar, Mazda, Mullard and Philips tubes originally intended for now-obsolete NATO

AVIONICS and related equipment, were released to European surplus dealers. For a while there were lots of Mullard CV4004

(box plate 12AX7), CV4003 (12AU7), Brimar 6V6GTs, etc. available for reasonable prices. Now there are apparently only some

CV4024 (12AT7) types available from some US and British tube dealers at decent prices. Once they are gone, the term "NOS"

will slowly fade out. I know for a fact that many stealth hoarders are snapping up good NOS audio types as fast as they can,

with the remainder being snatched (and used up) by audiophiles with fat bank accounts. Believe me, prices for premium NOS can

only go up, and up, and up, while the remaining stock dwindles. VTV already ran an article about rising NOS prices in issue 18.

Any real M-OV Genalex "Gold Lion" tubes will be selling for record figures soon--especially

KT77, KT77 and KT88s. NOS RCA 6L6GC Blackplates, once a $20 tube, now sell for up to $175 each. Telefunken 12Ax7s,

once a $10 tube, now fetch up to $150 each. This is a trend that will continue, as there are only tiny quantities of quality NOS

left and millions of tube sockets in audio and guitar amps to fill.

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The following is a comprehensive guide and series of FAQs for modern tube hifi and guitar amps, tube types, amp types, tweaks and general advice for the beginner and intermediate tube audio enthusiast.

Where did the tube come from?

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 electronics industry.

Between 1907 and the 1960s, a staggering array of different tube families
was developed, most derived from de Forest's invention. The vast majority of
these tubes have been replaced by semiconductors, leaving only a few types
in regular manufacture and use--mostly in specialized audio applications, or
in high-power radio equipment.

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, the 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.)

A typical modern vacuum tube 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), and
preferably less. 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--pumping is done for less than one
half-hour. This is a major reason why NOS tubes are superior to modern
production, in the old days tubes were simply pumped longer, because
production costs were less.

What's in a tube?

A. Cathode

Today, nearly all tubes use one of two different kinds of cathode to
generate electrons.
1) 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.

2) The other kind of cathode is 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, suc as the 211 and 845 triodes. The
thoriated filament can last a VERY long time, and is very resistant to high
voltages. It does use a lot more heater power, however.

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.

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.

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. Power triodes come in "low-mu" (low
gain) and "high-mu" (high gain) versions. Low-mu triodes like the 300B 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.

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. The result is higher voltage gain and
efficiency. Unfortunately, higher distortion is also produced.
See section F, "audio beam tetrodes", below.

E. Other grids--the pentode

By adding a third grid to the tetrode, we get a PENTODE. 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).
The 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.

There were 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 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, KT88 and 6550; the former is most popular in guitar amplifiers, while
the latter are the most common power tubes in modern high-end audio
amplifiers for the home. Beam tetrodes usually benefit from moderate
negative feedback. Many beam-tetrode amps sound every bit as good as any
triode amp; there is no magical effect exclusive to triodes.

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 tubes have very rugged and reliable heater
coatings. This is another area where NOS tubes are often superior to current
production.

H. The getter

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 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 tube is pumped out and sealed, the last step in
processing is to "fire" the getter, producing a "getter flash" inside the
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 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 tube is still good
and will give full lifetime.

THE ONLY RELIABLE WAY TO DETERMINE THE HEALTH
OF A 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 tube is
very hot, which is how such tubes are designed to be used. 
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, thus acting as an extra getter. The SV572 and 845
use graphite plates. 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, as in high-power tubes for radio transmitters.


WHY ARE TUBES STILL USED IN AUDIO?

A. 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 $150 million
worldwide today, perhaps more (it's hard to determine, as most of the makers
of tubes and tube amplifiers today are private companies who are secretive
about their sales.)

Why 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.

B. 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 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 growth today.

C. 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 tubes in this regime has been very
controversial in engineering circles, yet the demand for tube hi-fi
equipment continues to grow.


COMMON QUESTIONS AND ANSWERS

A. What is the best amplifier for my use? What kind of tubes should I look for?

Frankly, the best design from the standpoint of lowest distortion, widest
bandwidth, and best efficiency would probably be a push-pull triode
amplifier using low-mu triodes. The quality of the output transformer can be
critical (though not as critical as the quality of the speaker you are
using!!) Finding the right am to match a given speaker can be VERY
difficult. And there is limited help we can give you in this area, due to
the many manufacturers of speakers as well as amps. But we can make some
generalizations.

1) If you insist on using inefficient panel-radiator speakers, such as
Magnepans orelectrostatics, you will need BIG power. Magnepans can require 150 watts per
channel or more. Don't even think about SE triode amps. This is where the big
names in tube amps, such as Audio Research and Conrad-Johnson, have the
advantage--they make high-power amps for high-ego consumers.

2) There are some excellent high-efficiency horn speakers around today, such as
Edgarhorns, Avantgarde, Moth Audio and others. Try them. They will allow you
to run low-powered SE amps.

3) Some of the two-way small bass-reflex box speakers are very good. Many are
not so good. Not many are efficient enough to allow use with SE amps. And low
bass is always an issue. Box speakers are not called "monkey coffins" for
nothing.

4) Go to a high end dealer and ask for demos. Don't go around asking
half-informed questions; you will get very little information back, and
simply alienate dealers and other people. Learn what kind of sound you
want, and getthe equipment that gives it to you. And STICK WITH IT.

5) If you really insist on trying a broad range of high-end equipment, the only
place you can really go is the Winter CES high-end exhibit at the Alexis
Park hotel in Las Vegas in January, and to the "T.H.E. High-End Show", which is held at a
neighboring hotel at the same time. This is probably the best opportunity
to see and hear the broadest range of audiophile products. It also allows
some chance to encounter the designers of these products--most of them are reasonable and
rational men and women who love good sound.

6) Do NOT dismiss a given tube or amp type simply because some egotistical
guru told you to. The world is a complex and chaotic place, and great sound can
be found in some of the humblest places and most unlikely scenarios. If you
use only modern gear, you might be shocked at how good "junky" vintage tube amps
can sound--if properly set up.

Guitar amplifiers are a different area. Personal preference and playing
style are important in the choice of a guitar amp. The cleaner sound of 6L6
or 6V6 type tubes is preferred by most jazz, blues and popular music
players, while the higher distortion and muddier sound of a typical EL34
amplifier seems to have more popularity with heavy-rock guitarists.

B. What is Bias?

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 tube to
stay within its rated dissipation, and to balance the currents on each side
of the transformer in a push-pull amp. 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 tubes, the old 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.

"Self-bias" is often seen in SE amplifiers, and sometimes in push-pull amps.
This is the simplest method of biasing a tube, only a single resistor in the
cathode circuit causes the tube to make its own bias voltage. Although
simple and reliable, and eliminating the bias adjustment, self-bias amps
usually have higher distortion and less efficiency than amps with negative
grid bias.

C. How often should I replace the tubes in my amp?

Practically speaking, you should only replace 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
tube replacement. If the user has very stringent requirements for observing
tube weakening, the best way to check 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. 

D. My Tubes Glow Blue Inside -- 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 211, 845 and SV572 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.

* 1) Most modern 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.

* 2) Occasionally a tube will develop a small leak. When air gets into
the 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 tube (though not always). It does
not cling to surfaces, like fluorescence, but appears in the spaces BETWEEN
elements. A tube showing this glow should be replaced right away, since the
gas can cause the plate current to run away and (possibly) damage the amplifier.
NOTE: some older hi-fi and guitar amplifiers, and a very few modern
amplifiers, use special tubes that DEPEND on ionized gas for their normal
operation, as follows:

a. Some amps use mercury vapor rectifiers, such as types 83, 816, 866 or
872. These tubes glow a strong blue-purple color in normal use. They turn AC
power into DC to run the other tubes.

b. And occasionally, vintage and modern amplifiers use gas-discharge
regulator tubes, such as types 0A2, 0B2, 0C2, 0A3, 0B3, 0C3 or 0D3. These
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.


E. What is Single-Ended, 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
feedback.

Unfortunately, the high saturation currents in SE-amp output transformers
tend to limit the bass-frequency response of such amps. Unless you use a
subwoofer with a separate driver amplifier, the SE amp will tend to give
inferior performance. We also feel that keeping distortion down is important
(though not as important as engineers tend to think).
SE amps made with low-mu triodes can have very low distortion, IF they are
properly designed. Beware of some SE amps that are designed entirely "by
ear", especially those made by small companies run by audio-guru egomaniacs.
The amps made by such firms might NOT do everything well, and might not be
the sound you would prefer.

* 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.
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 or push-pull.
--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 sometimes with audio beam tetrodes and
pentodes. (Such amps are rare in the modern trend-driven and ego-ridden
world of high end audio today--most gurus lean either toward 
Class AB beam-tube amplifiers, or toward SE triodes.)

--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 SV300B, especially in audio. Usually a class-A2 amplifier
will use tubes with special rugged grids, such as the 811A or SV572 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. Most guitar
amps are push-pull Class AB. The biggest high-end amplifier manufacturers
love to build big Class AB amplifiers using 6550s or KT88 type beam
tubes--primarily because their wealthy customers often have very inefficient
speakers, such as electrostatic panels or Magnepans. (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 positive. AB2 is
rare in today's high end market, primarily because the egomaniacs who often
design such amps do not how to design a reliable and good-sounding AB2
amplifier....)

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. Class B
amplifiers can sound very good, if well-designed. The classic Altec 1570 is
an example.

4. Ultralinear operation is usually considered to be 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, 6550/KT88, EL84 or
EL34. All classic Dynaco amplifiers used ultralinear connection. It is VERY
hard on the screen grid of the tube, especially the EL34 type. So
good-quality EL34s are critical in such amplifier circuits.


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 regulation? Why should I care?

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, although NOT necessary for good sound, 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. SE Class A
amplifiers have less need for electronic 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 guitar amp.


F. What are the advantages of an OTL amplifier over a conventional one withan output transformer? Should I get an OTL? What about its reliability issues?

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 some 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 difference?

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 small possible
performance improvement. You should try to audition a parallel-feed high-end
amp before buying it--the weight, complexity and cost will be much higher
than for a conventional SE amplifier.

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. Whether they
actually make a big difference in the sound quality of an amplifier is a big
question--we have found that a well-designed conventional RC-coupled
amplifier stage can give outstanding performance. The type of tube used can
be far more important than the circuit topology!


H. How often should I replace tubes?

That can be a tough question, especially for an inexperienced tube audiophile.
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. NOS tubes tend to be better in the area of vacuum and of
cathode purity.

* 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 well-designed amplifier should not be a problem in this regard.
Beware of some of the amps built by smaller "guru" firms; those guys
sometimes run the heater too hot, because it makes the tube "sound better",
in their crazed opinion.

*The ONLY reliable way to determine if a tube must be replaced, is to
test it with a "mutual conductance" tester or something similar. Old testers
are becoming scarce and costly, and new testers are few and far between. The
"Maxi-Matcher" is suitable for testing power tubes and recommended. Preamp
tubes can be tested with a vintage mutual-conductance tester, or with the
recently discontinued George Kaye tester (if you can find one).


I. What about exotic cables and other tweaks?

For the past few years, Mike Vans Evers has been giving a little
demonstration at the Winter CES that really shakes up a lot of assumptions.
He sets up a cheap Sony CD player and a pair of common headphones. On the CD
player's power cord, he has attached one of his little exotic-wood cubes,
which supposedly contains no magnetic materials or wires. While you listen,
Mike slides the cube a few inches. Virtually all the participants claim to
hear a slight (VERY slight) change in the tonal quality of the sound. We
have witnessed this demo, and can attest to its veracity. The CD player has
highly-regulated power supplies, and Mike's little wood block cannot
possibly have any effect on the sound coming from it.

We like this demo, because it shows how sensitive the human ear can be to
the oddest little changes in sound. And Mike's products are reasonably
priced. However, such demonstrations have created a worldwide industry for
the gullible audiophile to support.Many of the "tweaks" out there are simple bunk, while some others are (barely) valid. But we do not take the opposite extreme, that of the scientific skeptics(such as Don Lancaster or Bob Pease, both longtime electronics industry
insiders who regard all audiophiles as mental cases). They claim that
nobody can possibly hear differences between cables, and swear by the blind
AB test (which can be, and often is, misused). There is a middle road, where
we suspect the real truth lies buried (strangled, more likely, by arrogant
cable dealers).

You can spend a little money on a cable, or you can spend a lot. We feel
that if you spend more than about $300 on an interconnect or $500 on a
20-foot speaker cable, you were probably ripped off. Above a certain level,
the tonal differences can be either unimportant, or the cable can introduce
major problems by adding too much capacitance, or somesuch (there are indeed
high-ticket cables that can cause high-frequency rolloff). Once you go
beyond a certain price point, you are asking for trouble and putting your
wallet into the hands of vendors looking for suckers.

Exotic power cords are in a similar category. Unless a cord is made of
precious metal wires, there really is no reason for such cords to cost more
than about $200-300.