Reproduced from VTV and
By Eric Barbour
(Note: This article does not include the full text, photos and
the original one published in VTV Issue #4.)
If you ever fnd yourself being harassed by a techie who insists upon
criticizing you for your interest in vacuum tubes, there is an easy
He can be silenced with a simple question. Ask him if any early type
integrated circuits will still be manufactured and used in new
say, the year 2030. If he's honest, the answer will be "no". Then tell
that the frst-ever beam power tube is still selling in the millions
and shows no sign of becoming obsolete.....after 60 years. That should
rid of him.
As of March 1996, the mighty 6L6 is celebrating its 60th birthday.
still being manufactured in Russia and China. And its popularity in
amps is assured for the conceivable future. Various "experts" in the
mainstream electronics industry, who relentlessly kill old
curse people who use them, can do nothing about the 6L6 ?p; it
be a dominant voicemaker of rock'n'roll guitar.
Many "experts" have tried to simulate the 6L6 guitar amp with various
semiconductor-laden gizmos, from complex analog computers to DSP
With varying technical success, and with little or no fnancial
There are numerous companies making 6L6 amps today; companies such as
Fender, Mesa-Boogie, Ampeg, Peavey, Kendrick, Victoria, Soldano, THD,
Electric and many others have staked some of their product lines on
6L6. So don't accept the mutterings about "dead technology".
In 1931, the audio outputs of radio sets were dominated by triodes
the UX-171 and UX-245. But even though push-pull 245s could produce 5
easily, there was ongoing pressure from manufacturers for
ever-more-effcient output tubes. The pentode was the answer at frst.
originated in Europe, with the frst American power types being the
P-704 and Arcturus PZ. These were very early types and had some
problems. They were quickly superceded by RCA's UX-247, released June
Suddenly you could get 2.5 watts out of a single tube, with easy drive
requirements and at only 250 volts! Millions of radios used the '47,
its descendant, the 42 with its 6.3 volt heater, was even more
38, 48, 59, 2A5, and 6F6 followed, as did European types such as the
AC/Pen, Cossor MP/Pen, Osram MPT4, Mullard PenA4 and numerous others.
But RCA engineers were pursuing more lofty goals: low distortion with
effciency. They were developing special power tetrodes, such as the 46
(intended for Class B push-pull and giving 20 watts from a pair) and
smaller battery-set types 49 and 52. Late 1932 saw the 48, which
the 46) was intended to have its screen grid connected only as a
grid, not in parallel with the control grid as in the 46. A similar
development in Britain was the Hivac "Harries". But the 48 was the
father of the 6L6, and all that came after.
This is a good place to describe the technical basics. In a triode
electrons are boiled off the cathode or flament by heat. The electrons
attracted strongly to the positively-charged plate. But to get to it,
must pass through the control grid in their path. By varying the
the grid, the electron stream is varied. Simple enough.
Unfortunately, there are three problems here. First, The maximum
that can flow at low plate voltages is limited, and this limits the
power output. Second, in order to get a high maximum plate current,
"mu" (voltage gain) needs to be low, requiring a high grid drive
The third problem is the capacitance between the plate and grid is
by the gain from the grid to the plate. This is the "Miller Effect",
makes triodes harder to use at high frequencies. So triodes have
their frequency response and effciency, when used as power amplifers.
In the 1930s the problems were diffcult to get around; triodes with
capacitance were eventually designed. But at the time, effciency was
improved by adding another grid (Fig. 2). This was called a "screen"
because it acted as an electrostatic screen between the grid and
reducing the plate-grid capacitance. This opened up the short-wave
because the screen allowed greater frequency response. It also
gain, as the fxed voltage on the screen made the plate current less
dependent on the variations of plate voltage. The resulting "tetrode"
became a standard for RF amplifers in radios, and the RCA 48 was about
good as a tetrode could be made for audio.
But when used for amplifying audio, tetrodes have a problem. The
emission can be attracted to the screen grid, which lowers the plate
current for low plate voltages. This is the famous "tetrode kink"
It is a source of distortion in audio, and represents some wasted
well. Because of this, a third grid was added between the screen and
(Fig. 4). The "suppressor" grid is widely spaced and is at the same
as the cathode. Thus, secondary electrons which bounce off the plate
be repelled away from the screen and back to the plate. The kink
disappears, and we have a "pentode". Gain and effciency are very high,
frequency response is excellent, distortion is lowered.
Even so, the RCA engineers knew that the pentode has problems. One
one is that the screen and control grids are wound with different wire
spacing. So, some electrons will pass through the spaces in the
grid, only to strike (or be defected in a useless direction by) a
screen-grid wire directly in that space. That electron is wasted
and does not reach the load. The electrons that strike the screen just
it up. A similar interaction can happen with the screen and
mostly involving the secondary electrons. And some electrons can pass
through gaps at the top and bottom of the grid assembly, or strike the
siderods of the grids. So the main electron beam can have a circuitous
route. Most of the wasted energy heats the screen grid, which in an
case can make them emit electrons, causing the tube's plate current to
In England, studies on secondary emissions showed that by spacing the
a critical distance from the screen grid in a tetrode, a "virtual
suppressor" is formed. (see side-bar) 1. Schade and his fellow RCA
engineers took this concept and perfected its implementation in
ways. First, they wound the control grid and the screen grid with the
spacing. The wires were aligned, so very few electrons would strike
screen. Second, the suppressor grid was replaced with a pair of "beam
plates" on either side of the grid structure. This assured that the
electrons reaching the plate were in the area where the critical plate
distance was right, insuring the "virtual suppressor" was effective.
result was extremely high effciency, high linearity and lowered grid
heating. The frst production version of this was encased in a metal
envelope with a then-new octal base. (Fig. 5) Thus was born the 6L6.
It was an immediate hit. All the major radio manufacturers started
in their audio output stages, essentially eliminating the triodes,
the '45 and 2A3, and elbowing out old tetrodes like the 46 and 48, and
pentodes like the '47 and 6F6. And new applications appeared;
operators found that it could give usable power in a transmitter, even
shortwave frequencies, and at far lower cost than previous tubes or
official transmitting version of the 6L6, the 807. The cost of public
address amplifers was affected by the new tube, as it was now
get 25 watts without using four 2A3s or expensive larger triodes like
50 or 300B. Only two 6L6s were now needed, at a fraction of the cost.
The 6L6 gave birth to a vast array of beam tubes (Figure 6). The 6V6,
and others were immediate developments, which gave lower power for
radios at lower cost. The 807 was the beginning of a series of beam
intended for radio transmitters, some of which are usable beyond 500
The 807 was the direct ancestor of the famous 6146 transmitting tube.
major VHF push-pull tetrodes of World War II, the 815 and 829 were
the 6L6. The 6550 was a high-power audio tube based on classic beam
principles. The first American television horizontal amplifier or
tube, the 6BG6G, came out in 1946., and was a repackaged 6L6. It was
followed by dozens of derivatives ending up in the monster color TV
tubes of the 1970s, such as the 6LQ6 and 6KG6/EL509.
To this day, new tubes are being developed that are descended from the
The KT90, KT99 and KT100 are examples. These recent audio tubes are
from TV sweep tubes.
The original metal 6L6 was a typical design for RCA at the time.
Metal-shell tubes were a passing fad of the 1930s, marketed to people
were afraid to replace their own radio tubes because of the danger of
injuring their hands on broken glass. The steel envelope was more
to manufacture and had real problems dissipating heat, so the fad was
virtually over by 1940. The metal 6L6 and its premium version, 1614,
often used in early jukebox amps and in many Zenith radio chassis, not
mention PA amps.
A few maniacal radio hams found that a metal 6L6 could be operated in
bath of transformer oil, allowing it to dissipate 150 watts for short
periods. The glass 6L6G, appearing in 1937, proved more popular with
conservative audio industry. It was common in nearly all WWII
and became nearly universal in PA amps right through the 1940s.
the G version had the same ratings as the metal style, it took over
During World War II, improvements were made in the glass envelopes,
after the war, the 6L6GA was introduced. It had the smaller ST-14
"coke-bottle" envelope. In the early 1950s, the 6L6GB came out, having
straight-sided S-12 envelope. These all had the same maximum ratings
After the war, an escalation in power ratings began. This had been
prefgured in the 1938 introduction in Britain of MOV's KT66, a more
powerful version of the 6L6. OEMs wanted more and more power, without
resorting to transmitting tubes. In 1947, Mullard introduced the EL37.
and the KT66 were more expensive in America than the 6L6s, so the
RCA/GE/Sylvania business continued as more and more dissipation was
demanded from the tubes. The result was a group of "supertubes", which
became standard for high-power American guitar amps and some hi-f
1954, a combination of better materials and a different maximum rating
system allowed the 6L6GC to raise the plate dissipation from 19 to 30
watts. In 1955, the 6550 was introduced. In 1958, the 7027 came out.
early 1960s, the 8417 was developed.
The 5881, introduced by Tung-Sol, was intended as a smaller 6L6
use in military and industrial equipment. Millions of 5881s were
into servo amplifers in aircraft such as the B-52 bomber, so this must
rugged and reliable tube. It was standard equipment in some home hi-f
amplifers, such as the classic Heathkit W-3 and W-4 series, Fisher
Pilot AA-410 and many others. Fender's Bassman was equipped with
this guitar amp (like many later models) is very demanding of its
tubes. 6L6Gs simply can not be used in such amps!
The 5932 was Sylvania's rugged 6L6 type. It was never used in audio
equipment and is extremely scarce. See below for more information on
variations on this tube. General Electric tried to make a super-6L6 in
mid-50s, and the result was the 7581. You can easily recognize a real
7581 by its pinkish fesh-colored base, which is virtually unique. It
the standard tube in the classic Harman- Kardon Citation 5 amplifer,
was rarely used otherwise due to its high cost. Tube manuals sometimes
the 7581 as an exact replacement for the KT66, although it is
quite different. Still, it has become a valuable tube due to its
tolerate the high voltages in post-1958 6L6 guitar amps.
TABLE 1: ESCALATION OF 6L6 RATINGS OVER THE YEARS
DISSIPATION MAX PLATE V MAX SCREEN V
6L6/G/GA/GB 19 W 360 V 270 V
KT66 (1940s-on) 25 W 500 V 400 V
EL37 (1947) 25 W 800 V 800 V
5932 (1950) 21 W 400 V 300 V
5881 (1950) 23 W 360 V 270 V
6L6GC (1954) 30 W 500 V 450 V
7027 (1958?) 35W 600V 500 V
7581 (1956) 30 W 500 V 450 V
7581A (1960) 35 W 500 V 450 V
All of these were pluggable into any 6L6 socket, and biased very
All used 0.9 amps at 6.3 volts on the flament, except the KT66 which
1.25 amps and the EL37 which used 1.4 amps.
There were so many variations of this form that we can't get space to
them in this magazine. I could go into the 6AR6, or the Bendix Red
6384 (covered in a separate article), or variations with different
voltages like the lower-power 25L6. There are numerous variations of
6V6, there are Western Electric types like the 350B, there are
transmitting types, there are hundreds of sweep tubes. There are
like the 6AQ5 and 7189. There are the late-50s audio types like 7591,
7355. Those will have to wait for future articles.
As I said, the major applications of these tubes were in PA amplifers
radio outputs, jukeboxes, and some early hi-f amps. But the future and
longevity of the 6L6 were assured when Leo Fender put them in his
guitar amps, starting with the Dual Professional in 1947. Fender's
amps of the late 1950s, including the Showman, Bassman, Pro, and Twin
models, became the essence of American rock. Indeed, the 1959 Bassman
1960 Twin are among the most copied electronic gadgets in history,
variety of new "boutique" manufacturers producing their own versions.
you include the 6V6-powered Deluxe models in that short list, then the
Fender designs are the undisputed standards.
In 1972, the late Tom Ruberto of Sylvania developed a special version
their standard 6L6GC, for Fender. This type had extra mica spacers and
designed to hang upside-down, as well as being designed to tolerate
volts on the plate and screen. This was the frst STR (special test
requirement) 6L6. It became a standard, so much so that "STR", long
the 1988 shutdown of the Sylvania tube factory, is a standard term
describe 6L6GCs with this large cylindrical envelope. GE even
their own version, and both had numerous guitar amps designed around
I once repaired a guitar amp made by Acoustic, circa 1979. It had four
6L6GC-STRs, and put 750 volts on them. The owners of this model don't
realize that they have a dangerous beast there. Unfortunately, many
amps continue to be used, although the STR tubes are no longer being
and are getting expensive.
Because of the chaos of 6L6 types and the often-brutal conditions they
endure in music amps, testing becomes even more important. The problem
some types is usually their design limitations, not design faws. Older
tubes often had surface treatments on their mica insulators which
manufacturing costs, while allowing some leakage current to reach
control grids. Such tubes are limited in plate-voltage capability. And
supertubes like the KT66 usually have gold-plated grids to prevent
emission, which can also destroy the tube. Since I have tried out many
tubes for this magazine (primarily with an eye toward high-fdelity
it's worth looking at the 6L6 types closely to also determine what
vintage-guitar-amp users need.
As with previous tube tests in past issues of VTV, I used a special
ended test amp to examine the distortion characteristics of a large
section of old 6L6 types, as well as a few current-production items.
driver was a 6EM7 and the output load was a One Electron UBT-1 with
8-ohm test load connected to the 4-ohm tap, thus presenting 3200 ohms
the tube's plate. This test has been most revealing in the past, and
6L6s were even more unexpected in their behavior. As in the past,
distortion is almost all second-harmonic and was measured at 1 watt
8-ohm load. Each tube was biased to 50 milliamps, a typical value for
then tested. All the types were run at 300 volts triode connection,
types that were rated to accept 500 volts on plate were run again at
with 300 volts on the screen.
These lists only show types for which I was able to obtain multiple
samples. The 5932s came in 3 styles, I tested one of each and combined
them; they weren't much different electrically. Only one WE 350B was
tested; it warmed up very slowly but gave excellent results.
6L6 TYPES WITH MULTIPLE SAMPLES:
1. Triode 300v average distortion
1614 metal RCA .61% 4 samples
6L6 metal RCA .62 4
KT66 MOV .63 4
6P3S Russian .64 12
6L6GC Sylvania short .72 4
EL37 Mullard .78 4
5881 Sovtek Russia .85 4
6L6G RCA .85 3
5932 Syl JAN .91 2
6L6GC China .93 2
6L6WGB Philips short .93 8
6L6WGB GE Canada .96 9
7027A RCA .97 4
5881/6L6WGB TungSol .98 18
7581A Philips 1985 1.06 2
7581A GE pink base 1.06 2
6L6GC GE short 1.18 3
2. Pentode 500v (screen 300v) average distortion
KT66 MOV .88% 4 samples
EL37 Mullard .91 4
6L6GC Sylvania short .95 3
5881 Sovtek 1989 .97 4
6L6WGB GE Canada 1.07 6
5881/6L6WGB TungSol 1.08 17
6L6GC China 1.08 2
6L6WGB Phil/Syl short 1.12 10
7027A RCA 1.14 4
6L6GC Sylvania STR 1.16 2
5932 Syl JAN 1.16 2
7581A GE pink base 1.19 2
7581A Philips 1985 1.22 2
6L6GC GE short 1.25 2
Many tubes that appear on the 300v list are not on the 500v list. This
because those particular tubes are NOT rated by their manufacturers
operation at 500 volts on the plate. This includes the metal 6L6s and
1614s, the 6L6G, GA, GB, and the Russian 6P3S, which is often sold as
6L6GC even though it is not intended for more than 400v on the plate.
true GC should be rated for 500v.) We respect the intentions of the
original manufacturers. So, too, should users stick to the published
ratings. I have tried to put 500v on the older types and on 6P3Ss, and
usually start to creak (and, sometimes, try to self-destruct due to
emission or leakage currents). So I defnitely do not recommend these
for guitar amps, which often have plate voltages of 450v or more.
The peak-power tests are not listed here, but we will summarize: it
revealed that the MOV KT66, Mullard EL37, Sylvania GC and the rare
Western Electric type) are superior to other 6L6 types in peak output.
the application demands maximum peak output (and money is no object),
tubes are best. Be prepared to pay more than $150 for each KT66, EL37
350B. NOS usually brings such prices, but good used tubes are
Make sure your NOS dealer warranties that the used tube is healthy!
In using this list, keep in mind that the needs of hi-f and guitar
amplifcation do not necessarily match. It is typical for hi-f users to
prefer tubes from the top of the list; the KT66 and EL37 are
sought- after, and the list refects this. On the other hand, for
tubes preferred are usually the short GE 6L6GC, the "STR" 6L6GCs made
Sylvania and GE, and the various 5881s, 6L6WGBs, 7581As and 7027As. In
case, distortion is OK (and sometimes deliberately sought by the user)
physical ruggedness is more critical. This is why the metal types and
old 6L6G, GA, and GB are less sought-after. The latter are in demand,
mostly by radio collectors and juke-box owners who want to use
tubes. For applications like these, where the plate voltage is below
volts, the current Russian 6P3S works just fne and is outrageously
Metal 6L6s (including the 1614) are low in distortion, but tend to be
microphonic and have dissipation problems. A power tube with a metal
envelope really should be cooled by forced air or attached to a
neither of which is practical in typical audio amps. The more extreme
collectors of McIntosh hi-f equipment usually insist that their MC-30s
equipped with 1614s, the original equipment in these amps.
For true obscurity, the Sylvania 5932 is worth looking at. It is a
super-rugged 6L6 replacement for military equipment. It came in three
versions; two had a conventional single structure. The other version
unique-it has a pair of smaller oval structures connected in parallel.
There is an underground following in the guitar world for the 2-plate
and the prices charged for it refect the demand (high). Its distortion
power output were only average, similar to Tung-Sol 5881s.
All of the tubes listed here are pin-compatible replacements for any
type, except the 7027 and 7027A. Sockets must be rewired to use them
place of 6L6s. A good tube-amp technician can do this at a reasonable
price. Because of the manic market for NOS types that can substitute
6L6s, 7027s have become very scarce. There were few things that used
as original equipment. They are very tough and are popular in Fender
that have been rewired appropriately. Purists tend to scoff, as 7027s
quite different from 6L6GCs and the like; but they do work fne with
socket rewiring and rebiasing.
The 6L6 is not often seen in high-end hi-f amplifers. There are some
amps out there, however, and they can be kept going with the Russian
It is unpopular in guitar amps, even though it's rugged and
Guitarists tend to dislike Russian 6P3Ss and 5881s because they sound
"bland". A shame, they're good hi-f tubes but rarely used for that.
Golden Tube Audio SE-40 single-ended amp and various VTL push-pull
amplifiers are among the few contemporary high-end amps that use the
I conducted casual listening tests at the VTV offce; they tended to
the distortion tests above. The old 6L6GCs tended toward a warm,
sound with greater "darkness" and much more distorted, fat bass. The
types and Russian 5881s were more "dry" and clean, as were 6L6Gs and
Sylvania GCs. The 6P3S has a slightly wetter sound than the Russian
but the same kind of clarity. Old 5881s were mostly made by Tung-Sol,
sounded warm, slightly nasal, with good bass. The KT66s and EL37s were
outstanding hi-f tubes, more like triodes in character and very
Two examples of the "skinny" Shuguang 6L6GC are listed here. These
remarkably like the Russian 6P3S, but are slightly different. The
version has four square holes in its top mica spacer, rather than the
in the Russian tube's spacer. The Chinese ones also look less
use the same ugly brown refractory cement (to hold bases on) that is
in other Chinese octal tubes. These, like the 6P3S, are not really
and should not be used at more than 400v. During test at 500v pentode,
creaked and groaned alarmingly. Note that their distortion was much
than in the Russian ones. Obviously these tubes were made with Russian
tooling, but are much poorer quality. There is a new "Coke-bottle"
6L6 from Shuguang, with a brown base and optional blue glass; it is
to appear here and will be reported on later.
All of the NOS tubes are out of production, leaving only the Russian
5881, and the Chinese types. The Russian tubes are old Soviet
and military types, not originally intended for export. Svetlana is
to introduce a new 5881 of its own soon, and we will report on it in a
future issue of VTV.
It is estimated that more than 2 million tube guitar amps exist in the
world today. Of that number, probably more than 40% use push-pull
claim that this market will soon dry up and be replaced by transistors
simply prevaricative. Although no 6L6 type is being produced in
Europe at the present time, there are a few popular ones from Russia
China which own the market. The Shuguang types, including a new 6L6GC
a blue glass envelope, are consistent sellers; and although they are
clean-sounding tubes, the Russian-made 5881s are Soldano's favorites
are used widely. They will likely be available for years, if not
to come. Add in the soon-to-come Svetlana 5881 and a rumored
which may be produced in California soon, and the 6L6 looks good for
another 60 years.
The British Connection
In England in 1931, J.H. Owens Harries discovered that if the electron
in a tetrode was confined to beams, and that the distance from screen
to plate was keptat a critical distance, secondary emission from the
would be supressed, just as in a pentode1,3. This discovery enabled
British General Electric Co. (GEC, unrelated to the American General
Electric) to bring out a sensitive, high-power output tube without
infringing on the pentode patents held by Philips and Mullard. GEC
came out with a series of beam tetrodes, with their most famous being
KT66. KT stood for "Kinkless Tetrode" ?p; since it eliminated the kink
the transfer curve that happened with regular tetrodes. The KT66 was
to be a plug-in replacement for the 6L6, but had superior
It was introduced in 1937.
Due to: the headstart the British had in developing component-type
fidelity systems, the transfer of British RADAR technology to American
during WW II, and the common language, Americans in the late 1940s
to England for ideas in hi-fi design. Williamson's seminal Wireless
articles gave British hi-fi a tremendous boost in America. The
efforts of the British Industries Corp (B.I.C.) brought the best of
hi-fi components to America during the 1950s. The net result of this
the inclusion of "foreign" tubes, such as the KT66, KT88, EL34, GZ-34,
into American hi-fi and even guitar amp designs.
1 ?p; Harries, Secondary Electron Radiation, Electronics, Sept. 1944.
2 ?p; Schade, O.H., Proc. of the IRE, Feb. 1938.
3 ?p; Harries, British patents 380,429 and 385,968, 1931. and Wireless
Eng., vol. 13, pp. 190-199, April 1936.