solid-state technology overwhelmingly dominates today's world of
electronics, vacuum tubes are holding out in two small but vibrant
areas. They do so for entirely different reasons. Microwave & RF
transmission technology relies on tubes for their power-handling
capability at high frequencies.
The other area - the creation and
reproduction of music - is a more complicated and controversial story.
complications and controversy stem from the fact that music is played to
be heard by human beings, whose nonlinear 'ear to brain' hearing systems are
far from fully understood. Since no one knows exactly how to model the
human auditory system, no one knows exactly what engineering
measurements are appropriate to evaluating the performance of audio
equipment. A smidgen of some kinds of distortion may sound worse to the
ear than larger amounts of other kinds. So ultimately, the only way to
judge audio equipment is by listening to it. Hence the controversy:
subjective human perception - especially when flanked by questions of
artistic merit - is made to order for arguments and disputation.
a commercially viable number of people find that they prefer the sound
produced by tube equipment in three areas: musical instrument (MI)
amplifiers (mainly guitar amps), some processing devices used in
recording studios, and a small but growing percentage of high-fidelity
equipment at the high end of the audiophile market. These areas employ
vacuum tubes of the type once known as receiving tubes, but now called
simply tubes. Not only has the use of vacuum tubes in these fields
defied the semiconductor tide elsewhere, but such use and demand has
even surged through the course of the past 1 1/2 decades.
vacuum thermionic devices hold sway over the US $100 million worldwide
guitar amp business. One rough estimate shows a 10-percent-per-year
growth in demand for tubes used in MI amplifiers and high-end audio
since the late 1980s, with no apparent slackening - even during the U.S.
recession of 1991-92.
much of the demand for audio tubes derives not from the United States,
but from Asia. In Japan, Taiwan, and mainland China, tube high - end
equipment enjoys a powerful cult status, and vintage U.S. and European
electric guitars and guitar amps are valued collectors' items.
Why Tubes - Subjective Reasons
The three areas
of tube audio tend to be mutually exclusive and appear not to influence
each other, even though all three directly involve the production or
reproduction of music. It is common to see the same tube types, such as
the popular EL34 power pentode, in electric guitar amplifiers and in
high-end stereo amplifiers. Often, too, these disparate products employ
similar circuit topologies.
amplifiers, it is estimated, consume as many as three out of four of the
world's production of audio tubes. This is hardly surprising, since the
tube guitar amp seems unshakably enthroned at the top of the rock 'n'
roll world. In this case, the use of tube amplifiers in the early rock
of the 1950s and '60s caused their distinctive distortions to become the
standard tonal effect for the electric guitarist. A cultural bias formed
during those years among U.S. and British musicians in favor of the
particular nonlinearities of those amps, which typically were quite
simple and had little or no negative feedback to improve their
As documented in
many books on electric-guitar technique; in magazines such as Guitar
Player, Guitar World,
Vintage Guitar, and
others; and on Usenet news groups such as alt.guitar.amps, the clipping
distortion and other sonic artifacts of '50s-designed tube amplifiers
supply the sonic signature required for a successful guitar amp.
Discussion of an
amp's merit frequently hinges on the clipping effect, which is often
described as yielding a sound like a brass wind instrument. The
saturation distortion of the output transformer, which couples the power
tubes to the speaker, also plays a key role in determining an
amplifier's sound. Another amplifier parameter - its touch
sensitivity - is affected by circuit nonlinearities and loose regulation
of the plate power supply.
Tubes have also
been cited, albeit not without controversy, as facilitating a controlled
so called infinite sustain effect because of the way their signal
compression interacts with acoustic feedback from speaker to guitar
string. These effects are well known among musicians, yet seem difficult
to reproduce accurately with solid-state equipment. The many designers
who have tried to build tube simulators over the past 30 years have
achieved varying levels of musical and financial success. Such equipment
has its supporters, but most amateur and professional guitarists remain
faithful to tube amplifiers.
Fliegler, vice president of marketing at Fender Musical Instruments
Corp., Corona, Calif., said in a private letter last year, "This is not
even a topic for discussion as far as I'm concerned...there is no
substitute for tube electronics in the hearts and minds of pros."
This kind of
thinking has influenced the professional audio world of recording and
mixing equipment. Since 1985, some studio engineers have been attracted
by what they perceive as the "soft" and "euphonic" sound of vacuum tube
electronics probably because of their experiences with vintage tube
amps, since modern tube amps can be and are made without these
characteristics. Tube enthusiasts usually contrast the soft sound of
tubes with the harsh sound of modern digital recording and mixing, which
may have more to do with the use of electrolytic coupling capacitors and
inexpensive op-amp ICs than with solid-state devices in themselves.
Regardless of the validity of their reasoning, studio engineers began
experimenting with old tube equipped condenser microphones, preamps,
limiters, and equalizers from the 1946-70 era. The result has been
twofold: street prices for vintage tube equipment have skyrocketed, and
numerous small companies have sprung up to manufacture tube equipped
devices, following a variety of design practices.
Why Tubes - Objectively
If cries of
fraud and derisive comments about "magical sound" sometimes greet the
use of tubes in audio equipment, there are also highly competent
electrical engineers who see definite advantages in tube equipment. An
example is John Atwood, consulting engineer and owner of One Electron
Co., Santa Clara, Calif. The erstwhile designer of application-specific
ICs and other solid-state logic circuits has managed to transform his
hobby of tube audio design into a full-time consulting business.
opinion, "Some of the differences in the audio qualities between tubes
and transistors have to do with the inherent physical properties of the
devices and with the circuit topologies and components used with each
type of device. There is no way around it: linear [triode] vacuum tubes
have lower overall distortion than bipolar transistors or FETs, and the
distortion products are primarily lower-order...the clipping
characteristic of tubes is actually not much softer than transistors,
but feedback tends to 'square-up' the clipping. Thus, the heavy feedback
in most solid-state designs gives them worse overload performance.
"A low- or
no-feedback design can be driven harder without audible distortion,"
Atwood continued. "High feedback also can lead to transient
intermodulation distortion (TIM), caused by clipping or slew-rate
limiting within the feedback loop."
distortion is not the only issue. In semiconductors, the shift of
characteristics with temperature along with their relatively low maximum
operating temperature has led to extensive use of Class-B amplifiers to
keep power dissipation down. In many designs, the result has been
audible crossover distortion, which often does not show up in published
specifications. Those specs are typically based on measurements made at
full power, where crossover distortion is at a minimum. Consumer tube
amplifiers use Class-A or Class-AB designs, which have vanishingly low
distortion even at low signal levels.
expert - Bruce Rozenblit, the owner of Transcendent Sound Co., Kansas
City, Mo., and a well known designer of tube amplifiers - said he sees
the output transformer as a major factor behind tube sound: "The warmth
is created by a large component of second-order distortion, and the slow
rise time of the output transformer causes a coloration that I would
describe as a smoothing effect...the transformer is a nonlinear element
that causes alterations of the signal in the time and frequency domains,
thereby altering the sound."
Whitlock, the president of Jensen Transformers Inc., Van Nuys, Calif.,
has a similar view. "Much of the perceived advantage of tubes is
actually due to the transformers which often accompany them," he said.
"Harmonic distortions tend to be rather high - 1 percent or more is
common - but are almost entirely low order (second and third harmonics).
Transformer distortions are frequency dependent in a way that most
electronic distortions are not. Intermodulation (IM) distortions are
created when a low frequency modulates the gain of a high frequency
passing through the channel at the same time. The ear is much more
sensitive to these IM distortions than to total harmonic distortion
(THD). For electronic distortions, one normally expects IM to be three
to four times the THD. In transformers, however, IM is actually about a
third to a quarter of the THD."
solid-state designs, Whitlock asserted that they "...depend on huge
amounts of negative feedback to 'fix everything', including crossover
distortion. Op amps commonly have open-loop THD in the 20 percent to 70
percent range. Stabilization generally requires open-loop gain to fall
at 6 dB per octave. This means that, for ultrasonic input signals, the
op amp has little gain margin to fix its own distortion. The ultrasonic
signals, along with distortion products, are fed to the next stage for
further distortion and intermodulation. This intermodulation creates
audible, but non-harmonically related, artifacts which contaminate the
noise floor and mask many subtle features of the music."
analog-circuit designers today appear to feel that tubes are inherently
noisy, Atwood disagrees. "In the audio region, 1/f noise tends to
dominate over shot noise - it varies considerably from device to device,"
he observed. "You need to select devices for low 1/f noise. ... Microphonics, though, are a problem [with tubes]."
Whitlock said he
saw things somewhat differently: "Higher operating voltages can expand
one end of the dynamic range envelope, but for audio, tubes are
generally noisier (without the help of transformers) at the other end."
Atwood qualified this statement with the comment that it is true only
for low-impedance, low-level sources such as some (noncapacitive)
In addition to
the active devices themselves, the passive components customarily used
with them apparently also favor tube designs. According to Atwood, the
low impedances of bipolar transistors necessitate the use of large
(high-value) coupling capacitors, which for the most part means
electrolytics. High dielectric absorption, imperfect high-frequency
characteristics, and aging of electrolytics put them at a sonic
disadvantage compared with the good-quality film capacitors nearly
universally used for coupling the stages of tube amps. "While it is
possible to build solid-state amps with no electrolytics, it is rarely
done," he said.
Whitlock said he
felt that the main problem with electrolytics is actually at low
frequencies: "The major problem with electrolytics is dielectric
hysteresis, which produces high THD at low frequencies, where there are
ever-increasing voltage swings across the capacitor." He also stated
that direct coupling is the most desirable scheme, whether
semiconductors or tubes are used.
are held by guitar amp designers. As Rick Perrotta, a longtime recording
engineer and cofounder of guitar amp manufacturer Matchless Amplifiers,
Santa Fe Springs, Calif., explained, when a transformer saturates, it
has a compressing effect. "When someone says that a particular guitar
amplifier has a big fat sound, that is due, in part, to the output
transformer saturating along with the output tubes. Solid-state amps
can't perform that trick."
High-End Audio Equipment
oddest subgenre of tube audio is the high-end audio component market.
High-end equipment is aimed at the most obsessive audiophiles, famed for
worrying about small details which most people ignore or cannot even
hear. Yet in spite of its obscurity and its notorious marketing driven
focus, high-end is the most vibrant and active area of all.
the high-end market belonged almost entirely to solid-state equipment.
The use of tube equipment in the field started as a semi-nostalgic
subculture, with the use by audiophiles of vintage hi-fi amplifiers from
before 1970. Unlike users in the music performance market, audiophiles
have often preferred tubes for their clean, smooth sound - in some cases,
far more detailed and life-like than most early solid-state equipment.
new tube equipment like Audio Research Corp., Minnetonka, Minn., and
Conrad-Johnson Design Inc., Fairfax, Va., appeared during the '70s. The
field was small and sedate until the late '80s, when interest (and the
number of manufacturers) began to skyrocket.
hi-fi market originated after World War II in the United States and
Europe, the latest aggressive rise in high-end sales has been
concentrated in the Asian countries, led by Japan. Much of the high-end
equipment made in the West is exported and sold in the Far East. Indeed,
Asian audiophiles see labels reading "Made in USA" and "Made in England"
as badges of status and quality.
The rise of
high-end sales was influenced by the statements of subjective audio
reviewers, whose nontechnical and rarely rigorous listening tests at
times encouraged near-hysteria among magazine readers. A positive review
in a powerful magazine such as Stereophile
can trigger hundreds or even thousands of unit sales, and turn an
unknown manufacturer into an instant success. A negative review can sink
a small firm just as easily (and has done so). This applies only to the
small North American market. In Asia, terms such as "audio nut" and
"audio mania" are often applied to the users and makers of high-end
equipment. In Japan alone, the market for tube equipment could easily be
five or 10 times as large as in the United States, the source of much of
the equipment. The high-end market in Asia seems much more chaotic and
ill-defined than elsewhere, with many audiophiles choosing to construct
their own equipment from scratch, using a wide variety of tubes and/or
The demand for
high-end tube gear in Asia has been fueled, to an even greater degree
than in the United States, by an obsession with obscure sonic
characteristics. Starting in 1973, Japanese "audio maniacs" were
exhorted to seek out the primitive Western Electric 300B audio triode,
originally used in late-'30s movie theater amplifiers. Similar edicts
were issued about other antique Western Electric tubes, such as the 205D
and 212E, and about '30s Western Electric amplifiers and loudspeakers.
And hi-fi components made in the '50s and early '60s, by highly regarded
firms such as Marantz America Inc., Roselle, Ill., and McIntosh
Laboratory Inc., Binghamton, N.Y., enjoy cult status in Asia. Such
obsessive desire has driven the street prices of some vintage audio to
many times their original retail prices, and has caused Marantz and
McIntosh to reissue some of their past models of "obsolete" tube hi-fi
Much of high-end
is conducted in a gold-rush fashion, with companies advertising exotic
connecting cables and acoustical treatment devices while making wild
claims about the supernatural results achieved. The result: negative
comments from the professional engineering fraternity. Items have been
published in the Journal
of the Audio Engineering Society,
in electronic-industry journals such as EE Times, and elsewhere
that attack the methods and conclusions of the audiophiles. In spite of
the bad press, high-end audio was estimated in a Wall
Street Journal article in
December 1991 to sell as much as $1 billion per year worldwide, enjoying
a 20 percent annual growth rate. Japan, alone, consumes $200 million per
year in high-end components.
may be exerted on high-end sound by the pro-audio world. Most
audiophiles appear to be between the ages of 30 and 50, and thus have
been exposed to rock guitar since childhood. Perhaps more to the point,
the growth of sales of tube equipment was also facilitated by the
availability of low-cost Russian, East European, and Chinese tubes. Like
the guitar and pro-audio areas, high end has expanded most since 1985,
about when these tubes first started to appear on the market in large
quantities throughout the United States, Western Europe, and the Far
East. At the same time, receiving-tube manufacture in the West has
virtually ended because of the comparatively small market.
Like the guitar
world, high end is hardly a monolithic market. Demand for tube
amplifiers has split into three pieces, with audiophiles choosing their
preferred design topologies from a list. Designers in the field usually
subscribe to only one of these topologies, and battles between the camps
flare up regularly. Not so long ago, nearly all high-end tube amplifiers
were of conventional classic hi-fi design, using push-pull 6550s or
EL34s with negative feedback of 20 dB or less. Audio Research and
Conrad-Johnson are among the largest builders of these types.
Guitar Amp Origins
It was in the
'30s that the idea of attaching a transducer, amplifier, and loudspeaker
to a guitar first caught on. Early guitar amps were used primarily with
lap guitars, usually made of solid wood or metal with no resonant body
to increase volume. Later amps were adopted by big-band guitarists, and
in the '50s, amplifiers became pretty much mandatory for the amalgam of
blues, country music, and jazz known as rock 'n' roll. Since transistors
did not enter wide usage until about 1960, all of the originating styles
of rock guitar were developed on tube amplifiers. Later, musicians
discovered that using a gain device before the guitar amp forced the amp
to clip heavily; they liked the resulting sound, and it became the
foundation of hard rock, later called heavy metal.
Since all the
past styles of guitar playing are still regarded as musically valid, the
market for guitar equipment has fragmented. Various manufacturers offer
arrays of amplifiers, preamps, effects processors, and other means of
electronically processing the guitar signal. Both reproductions of early
equipment and innovations are available. Most remarkably, the basic
designs of tube guitar amps tend to be based on a few prototypes that
date from the '50s or early '60s. Extra channels or gain stages are
added, tone controls are modified, sound effects (like reverberation and
electronic tremolo) vary, and speaker cabinets become available in
various configurations. Yet the basic circuits keep returning to the
same set of paradigms.
of guitar amps has been as influential as Fender. Between 1946 and 1965,
founder Leo Fender and his design team created most of the rock guitar
sound in the form of the amplification used with their solid body
guitars, such as the famous and widely copied Stratocaster model.
primitive design for a Fender amp is the Champ model. Being intended as
a low-cost amplifier for students and beginners, a typical Champ uses a
single 6V6GT or 6L6GC power tube. Because of its single ended power
stage - and the large amount of second-harmonic distortion thereby
engendered - the Champ had a sound often described as "soft" and "lush."
The amp's small, cheap output transformer saturated easily and gave very
poor low-frequency response. Early Champs used 6V6GTs, were extremely
primitive, and had no feedback, while later models had more complex
circuits and loop feedback. In spite of their crudeness, early Champs
are now valuable collector's items, and have been much imitated in
recent years. The Champ sound is a standard, typical of many low-cost
amps used on early rock 'n' roll records.
The most popular
Fender models among serious professionals are the Bandmaster, Twin,
Showman, and Bassman with push-pull 6L6GC or 5881 output tubes. The tone
of these models has a ringing quality much sought after. The peculiar
distortions from these amps and their matching speakers, in addition to
the inherently light regulation afforded by tube rectifiers, gives
distinctive inherent compression effects. This combination is what makes
possible the infinite sustain effect mentioned earlier.
Even though the
Bassman was intended originally for bass guitar, it was widely used for
lead guitar and became possibly the most copied guitar amp in history,
especially the 1959 model equipped with four 25-cm speaker drivers. The
Twin Reverb model is often modified with extra gain stages for more
distortion, producing a fair degree of compression and allowing the
lengthy sustain of guitar notes. It served as the prototype for many
modern amps with complex preamp sections.
1962, a new sound appeared in Britain. Jim Marshall, a London music
dealer, found that imported Fender amps were popular but too expensive,
and so he developed his own. While his first amp was a copy of the
Bassman, he later changed the output tubes to push-pull EL34s. These
European tubes were true pentodes, different in electrical behavior from
the beam tetrodes used in Fender amps. With the new tubes, Marshall's
amps took on a tone described as very distorted and "crunchy," which is
now considered the classic British blues-rock sound. Interestingly, the
EL34 had reliability problems when operated in deep clipping for long
periods, so in the '70s the U.S. distributor for Marshall amps switched
the output tubes to 6550 beam tetrodes. As the sound of these amps was
much more like very powerful Fenders, some preference arose among U.S.
musicians for a "harder" sound than Marshalls give with EL34s. New
distribution in the 1980s had EL34-equipped Marshalls entering the
United States, as Jim Marshall preferred.
The third common
guitar-amp design is that of the models AC15 and AC30 made by Vox
Amplification Ltd., London. These were often used in Britain and
throughout Europe, most notably by the Beatles at the peak of their
popularity. The AC15 uses two push-pull EL84 output tubes, the AC30 four
EL84s. Both models use self-bias of the output tubes, in Class A
operation and with no negative feedback, unlike many other push-pull
guitar amps. The result is a unique tone that varies greatly with
string-plucking force. AC30s were made available with a Top Boost
option, adding gain stages for further versatility. The Top Boost AC30
design is widely imitated by modern amp designers.
Bass Guitar Amplified Otherwise
A bass guitar
has different needs from a lead guitar. Bass is used to reinforce the
song rhythmically, working with the melody at a pitch several octaves
below. Nearly all bass guitars are solid-body types--simply larger
versions of regular electric guitars with very thick strings. Since the
bass sound is not always assisted by distortion, solid-state designs
have come to hold sway over this market. Any amp with a high damping
factor and capable of generating high powers at low frequencies can
serve as a bass amp. Yet a ground swell of interest in tube bass
amplifiers has surfaced since 1990.
Early bass amps,
such as Fender models, were essentially little different from guitar
amps. Then the Ampeg SVT was introduced in 1969. It dwarfed previous
bass amplifiers, producing 300 W from six 6146 or 6550 tubes. The SVT
became a standard much imitated, especially in the last 10 years. Modern
tube bass amps are usually very large, producing 200 W at least from a
set of 6550 beam tetrodes - as many as 10 in some models.
Professional Audio Gear
For all that
guitar and bass amps are targeted at professional musicians, they are
often considered somewhat apart from the professional audio market. The
usual definition of "pro audio" specifies sound-reinforcement and
sound-recording equipment. Each kind is used both in a live musical
setting and in the recording studio, yet they are somewhat
differentiated by application. For stage use, ruggedness and reliability
matter the most. In the studio, though, there is more concern about the
specifics of the device's transfer function, which may be exploited as a
The most general
use of vacuum tubes in the recording studio is inside condenser
microphones, used to record vocals. Each microphone model has its own
transducer assembly or "capsule" design, each having distinctive
variations in frequency response, phase shift, and signal distortion.
Even the finest and most carefully designed capsules are not exactly
perfect, so recording engineers have learned to exploit the
The advantage of
tubes in condenser microphones is their very high input impedance, which
does not load down the capsule significantly. Many recording
professionals agree that the soft clipping exhibited by the tube--along
with its high-voltage operation--can also be important, since some
singers have very powerful voices, capable of producing peaks far beyond
the dynamic range of typical solid-state electronics.
Almost as much
the rage as tube microphones are special preamps and so called direct
input boxes, which in studio signal chains serve to provide voltage
gain, impedance conversion, and (in the case of direct input boxes)
conversion from unbalanced to balanced connections. In spite of their
lesser usage in studios, compared with tube condenser microphones,
these preamp devices are also riding high on a resurgence of popularity.
Among the best-liked designs of this category are units that are
solid-state but for a single 12AX7 tube operated at a very low plate
voltage--as little as 12 V. This so-called starved-plate operation
delivers high distortion, which some equipment designers consider the
only useful characteristic of vacuum tubes in audio.
much favored is a category of tube preamps that are a lot like classic
circuits from the '50s and '60s. This would include products from
Tube-Tech (TC Electronic A/S, Risskov, Denmark), Anthony DeMaria
Laboratories (ADL), New Paltz, N.Y., and Manley Laboratories, Chino,
Calif. The standard design involves push-pull circuits, frequently using
12BH7 or 12FQ7 dual triodes along with 12AX7s and 12AU7s. Input and
output coupling transformers, to match impedances of 600-(omega) lines
to the tube circuits, are found in these devices, in spite of the
difficulties of making high-quality audio transformers.
A key use of
tubes in the recording chain is in signal compressors, also called
limiters or leveling amplifiers. In general terms, a high-quality audio
compressor consists of a preamp, a so-called sidechain with audio-signal
rectifier and peak detector, and a voltage-controlled attenuator or
amplifier driven by the peak-detected voltage. The control is arranged
so that signal gain in the preamp rises as the signal level on the input
falls, giving a narrowed dynamic range. Compressors of this type are
considered mandatory in recording and production studios, for a variety
of sonic effects. Rock music is often heavily compressed to give the
illusion of greater loudness.
tend to follow the examples of vintage equipment. Among the most
influential was the Teletronix LA-2A, in which a controlled attenuator,
consisting of an electroluminescent panel driven directly by the tube
peak detector, shone onto a light-sensitive cadmium-sulfide resistor.
Such devices have compression ranges of at most 10:1--not large compared
with the enormous compression ratios available with modern solid-state
of the tube compressors have apparently not affected their value.
LA-2As known to sell for $4000 on the street today were worth perhaps
$100 in 1980. Whereas all the makers of vintage compressors are
currently out of business or no longer producing such equipment, most of
the modern firms produce compressors, ranging from starved-plate types (Aphex
Tubessence) to vintage-style designs.
compressor of modern design is the Summit Audio DCL-200, a dual
compressor of hybrid design. Its input and output circuits, as well as
the peak-detecting sidechain, are solid-state, while 12AX7s are used for
situation exists with outboard audio equalizers (EQs). Essentially
complex tone controls, dedicated EQs are viewed as adjuncts to the EQ
circuitry built into the channel modules of modern mixing boards. Many
of the above companies also have tube-equipped EQs in their lines. Often
they are based on classic designs by long-defunct firms, such as Pultec
and Lang. The tube EQs often use passive RLC filters, buffered on the
input and output with tubes, whereas active filters are the usual basis
for modern solid-state EQs. The Summit Audio EQP-100, manufactured since
1985, uses tube gain sections and passive RLC filters combined with
input-output drivers based on op amps using discrete transistors.
specialized areas in which tubes find application include mixers and
rotating-speaker tremolo devices. The latter, pioneered in the '40s by
Leslie, have remained in favor for their distinctive phasing sound.
Leslie speakers are considered essential for use with the Hammond B-3
electric organ, a standard instrument for rock and jazz music for 40
years. In point of fact, Leslie speakers with tube amplifiers are still
being manufactured by Hammond Suzuki Corp., Addison, Ill., while Motion
Sound Inc., Salt Lake City, Utah, has introduced rotating-speaker
devices that incorporate tube based electronic circuitry.
In recent years,
however, the field has seen the rise of the single-ended amplifier. Most
of this started in Japan, with the preferred design using one or two
primitive power triodes and no feedback. Distortion is substantial,
though mostly second-harmonic in nature. Power output tends to be low,
necessitating the use of efficient speakers. A parallel phenomenon has
been the return to favor of highly efficient horn speakers, which were
scorned as too "colored" or inaccurate by audiophiles--until
single-ended amps became trendy. An example of a single-ended triode
amplifier is the Moth Audio 304TL.
The third and
least-used design is called output transformer less (OTL). Driving a
low-impedance speaker directly is difficult with tubes, calling for many
low impedance triodes or tetrodes in push-pull sets. Nonetheless, some
audiophiles consider the elimination of the output transformer, with its
electrical nonlinearities, to be worth the effort.
A modern OTL is
the Transcendent Sound amplifier, which uses eight EL509 power tetrodes
to produce 80 W into an 8-(omega) speaker load and is one of the few
modern tube audio devices to be covered by a U.S. patent. Transcendent's
Rozenblit feels that the output transformer is fine for guitar amps but
not for high-fidelity: "The warmth is gone, the detail and resolution
are much greater, and the rise time is much faster...[yet] I cannot
duplicate the...sound with solid-state devices that is generated by my
OTL. I wish I could, because I could sell 100 times as many units."
Choosing a Tube
It is obvious
that tube-audio designers like to use tubes from the same small group of
types. In guitar amps, preamp and output tube choice are usually
dictated by old Fender/Marshall/Vox designs. Preamp stages are made of
12AX7s; phase splitters and drivers are 12AX7s or else 12AT7s; and
output tubes are from one of five families: 6L6GC, 6V6GT, EL34, EL84,
when Matchless amps arrived in the late '80s, their use of EF86
low-noise pentodes in their input stages befuddled musicians; the amps
eventually proved themselves and came into vogue, in spite of the
unexpected tube lineup. In professional audio, the standard choices are
not fully settled. Vintage-style equipment uses 12AX7s, 12AU7s, 12AT7s,
and 12BH7s, sometimes 12AY7s and a few others. Low-cost equipment opts
for the 12AX7 because of its ready availability, since it is the most
popular preamp tube in guitar amps.
designers in the past relied on power tubes endemic in the late '50s and
'60s, primarily 6550 and EL34 types. The arrival of single-ended triode
amplifiers has encouraged wide use of the 300B and other primitive
triodes using directly-heated cathodes.
recent past years, tubes have developed a poor reputation for reliability. A side-effect has been the veneration of new old
stock [see Defining Terms] U.S. and European tubes, of types that are
still being manufactured but not by the original makers. Since each
manufacturer has its own proprietary formula for cathode coatings and
other materials, tubes of the same type but from different manufacturers
often sound slightly different. Since the standards have already been
settled as being new old stock types like the RCA black-plate 6L6GC and Telefunken smooth-plate 12AX7 [see Defining Terms, p. 28], those types
of tubes have increased in value along with the vintage equipment they
were used in originally, even if the tubes end up in new equipment. Some
boutique guitar-amp makers and high-end firms deliberately equip their
new production with scarce NOS tubes, charging the customer accordingly,
while professing their aural superiority and limiting the sales of their
product (and, perhaps, also ensuring the product's obsolescence when
tube stocks run out).
tendency has been for tube amplifiers (of both guitar and high-end
types) to be designed and built by technically unqualified people. As
Transcendent's Rozenblit said, "Properly designed tube equipment is
extremely reliable. I have repaired many pieces of tube gear from the
late '50s or early '60s where all of tubes still worked. Usually, a
filter capacitor dried out and failed. The poor reputation comes from
basically two factors. One is that from about 1970 to about 1990, the
quality of vacuum tubes went through the floor. With the resurgence of
tube audio, very high-quality tubes are now being manufactured. The
second reason is that many contemporary tube designers don't know what
they are doing. They are not engineers, they are salesmen. Tubes are
forgiving and easy to work with, which allows the less sophisticated to
easily enter the marketplace."
designers have long attempted to produce tube simulators, using
solid-state analog circuits like diode clippers and compressors to
produce transfer functions and distortion akin to those of tube guitar
amps. Although these products have attracted only a limited following to
date, the recent appearance of solid-state amplifiers based on digital
signal processing (DSP) and using physical modeling algorithms, has
stirred some interest among serious musicians.
To quote Art
Thompson, technical editor of Guitar Player
magazine, San Mateo, Calif.: "Transistor amps have definitely come a
long way towards their ultimate goal of sounding exactly like their
tube-powered counterparts, but I think most pro players and industry
people have realized that this is a futile objective...." As to the
success of DSP guitar amps like the Line 6 products from Fast Forward
Designs Inc., Culver City, Calif., Thompson said, "We've tested some
impressive tube-simulation devices, and better ones are undoubtedly on
professional audio market, the final frontier for vacuum tubes looks to
be in the mixing console. This device is the heart of a working studio,
serving as primary signal router, EQ and effects controller, and the
tool to mix down multiple recorded channels to a stereo master music
program. Mixers used in most studio settings are large and complex and
often equipped with computer-controlled automated mix-down capability.
Although expensive, large mixing desks are frequently based on low-cost
op-amp ICs such as the 5534, and use electrolytic capacitors for
coupling the audio signal from stage to stage.
engineers have found this scheme wanting, hence recent introductions of
tubed mixers. Companies producing such mixers are Summit Audio, the UK's
Manley Laboratories and TL Audio Ltd., the latter being the sole
producer of full-size mixing consoles with tubes at this time. The
market may expand in the future, as interest in audio-tube electronics
interesting new development is the introduction since 1990 of new tube
types specifically designed for audio applications. Russian, Slovakian,
Serbian, and Chinese factories currently opt to imitate the popular
audio tubes of past types, such as 6L6GC and 12AX7. Yet the market has
been open to some new tube types not directly based on any U.S. or
European tube or modifications thereof. These types have aroused some
interest from original-equipment makers, even though they seldom fit
In the guitar
and pro-audio markets, new versions of classic power tetrodes and
pentodes include, from the JJ/Teslovak Co., Cadca, Slovakia, the KT88S
and E34LS, which are modified and uprated in power dissipation rating
from their KT88 and EL34 prototypes. New high-power triodes, such as the
SV811/572 series and 3CX300A1, have been introduced by Svetlana. Even
more than other fields, the high-end market has seen an explosion of
power triodes: the VV30B and VV52B from KR Enterprise Co., Prague, Czech
Republic; Svetlana's SV811/SV572 series; the AV30B SL and AV62B SL from
AVVT Technologies, Prague, Czech Republic; and many versions of the 300B
from two different factories in China, two in Russia, and one in
Slovakia--not to mention the revival of Western Electric and its resumed
manufacture of the original 300B.
The KT90 beam
tetrode, made by Ei Electronic, Nis, Serbia, has proved so popular for
audiophile amps that a Chinese factory has even introduced its own
version. Also winning a following in high-end audio are some Russian
tubes unknown in the West until recently, such as the 6C33C-B from
Electronpribor Ulyanov, Ulyanovsk, Russia. More new types are expected
to be introduced, and some out-of-production types will be reintroduced,
over the next several years.
enjoy an unshakable position in the design of guitar amplification.
Their use in the recording studio and in home audio often seems like a
fad, but it has very old roots and is therefore unlikely to disappear
overnight, as fads do. Technical reasons for the use of tube electronics
in some applications look solid and are well defended both by
professional designers and by the users, who define and drive the
marketplace. So long as musical tastes demand them, vacuum tube audio
equipment and replacement tubes will be produced well into the next
century, and probably thereafter.
This article was originally
written in 1998.