6L6 versions???

[Brian Carling]

There's also this.

I hope Eric doesn't mind me passing it along since I have given him
the proper credit for this fine fine article:


                                6L6 FOREVER

                              By Eric Barbour

(Note: This article does not include the full text, photos and
graphics of the original one published in VTV Issue #4.)

If you ever find yourself being harassed by a techie who insists upon
criticizing you for your interest in vacuum tubes, there is an easy
answer. He can be silenced with a simple question. Ask him if any
early type integrated circuits will still be manufactured and used in
new products in, say, the year 2030. If he's honest, the answer will
be "no". Then tell him that the first-ever beam power tube is still
selling in the millions today, and shows no sign of becoming
obsolete.....after 60 years. That should get rid of him.

As of March 1996, the mighty 6L6 is celebrating its 60th birthday.
It's still being manufactured in Russia and China. And its popularity
in guitar amps is assured for the conceivable future. Various
"experts" in the mainstream electronics industry, who relentlessly
kill old technologies and curse people who use them, can do nothing
about the 6L6 -p; it continues to 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 chips.  With varying technical success, and
with little or no financial success.

There are numerous companies making 6L6 amps today; companies such as
Fender, Mesa-Boogie, Ampeg, Peavey, Kendrick, Victoria, Soldano, THD,
Louis Electric and many others have staked some of their product lines
on the 6L6. So don't accept the mutterings about "dead technology".

1. HISTORY
In 1931, the audio outputs of radio sets were dominated by triodes
such as the UX-171 and UX-245. But even though push-pull 245s could
produce 5 watts easily, there was ongoing pressure from manufacturers
for ever-more-efficient output tubes. The pentode was the answer at
first. It originated in Europe, with the first American power types
being the Champion P-704 and Arcturus PZ. These were very early types
and had some reliability problems. They were quickly superseded by
RCA's UX-247, released June 1931. 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, and its descendant, the 42 with its
6.3 volt heater, was even more popular. The 38, 48, 59, 2A5, and 6F6
followed, as did European types such as the Mazda AC/Pen, Cossor
MP/Pen, Osram MPT4, Mullard PenA4 and numerous others.

But RCA engineers were pursuing more lofty goals: low distortion with
high efficiency. They were developing special power tetrodes, such as
the 46 (intended for Class B push-pull and giving 20 watts from a
pair) and the smaller battery-set types 49 and 52. Late 1932 saw the
48, which (unlike the 46) was intended to have its screen grid
connected only as a screen 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 ultimate father of the 6L6, and all that
came after.

This is a good place to describe the technical basics. In a triode
(Fig. 1) electrons are boiled off the cathode or filament by heat. The
electrons are attracted strongly to the positively-charged plate. But
to get to it, they must pass through the control grid in their path.
By varying the voltage on the grid, the electron stream is varied.
Simple enough.

Unfortunately, there are three problems here. First, The maximum
current that can flow at low plate voltages is limited, and this
limits the maximum power output. Second, in order to get a high
maximum plate current, the "mu" (voltage gain) needs to be low,
requiring a high grid drive voltage. The third problem is the
capacitance between the plate and grid is magnified by the gain from
the grid to the plate. This is the "Miller Effect", and it makes
triodes harder to use at high frequencies. So triodes have limits on
their frequency response and efficiency, when used as power
amplifiers. In the 1930s the problems were difficult to get around;
triodes with low capacitance were eventually designed. But at the
time, efficiency was best improved by adding another grid (Fig. 2).
This was called a "screen" grid, because it acted as an electrostatic
screen between the grid and plate, reducing the plate-grid
capacitance. This opened up the short-wave bands, because the screen
allowed greater frequency response. It also increased gain, as the
fixed voltage on the screen made the plate current less dependent on
the variations of plate voltage. The resulting "tetrode" became a
standard for RF amplifiers in radios, and the RCA 48 was about as good
as a tetrode could be made for audio. But when used for amplifying
audio, tetrodes have a problem. The secondary emission can be
attracted to the screen grid, which lowers the plate current for low
plate voltages. This is the famous "tetrode kink" (Fig. 3). It is a
source of distortion in audio, and represents some wasted energy as
well. Because of this, a third grid was added between the screen and
plate (Fig. 4). The "suppressor" grid is widely spaced and is at the
same voltage as the cathode. Thus, secondary electrons which bounce
off the plate will be repelled away from the screen and back to the
plate. The kink disappears, and we have a "pentode". Gain and
efficiency are very high, frequency response is excellent, distortion
is lowered.

Even so, the RCA engineers knew that the pentode has problems. One
obvious one is that the screen and control grids are wound with
different wire spacing. So, some electrons will pass through the
spaces in the control grid, only to strike (or be defected in a
useless direction by) a screen-grid wire directly in that space. That
electron is wasted energy, and does not reach the load. The electrons
that strike the screen just heat it up. A similar interaction can
happen with the screen and suppressor, but 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 extreme case can
make them emit electrons, causing the tube's plate current to run
away.

In England, studies on secondary emissions showed that by spacing the
plate 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 several ways. First, they wound the control grid and
the screen grid with the same spacing. The wires were aligned, so very
few electrons would strike the screen. Second, the suppressor grid was
replaced with a pair of "beam plates" on either side of the grid
structure. This assured that the only electrons reaching the plate
were in the area where the critical plate distance was right, insuring
the "virtual suppressor" was effective. The result was extremely high
efficiency, high linearity and lowered grid heating. The first
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
using it in their audio output stages, essentially eliminating the
triodes, such as 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; ham-radio operators found that it could give usable power in
a transmitter, even at shortwave frequencies, and at far lower cost
than previous tubes or the official transmitting version of the 6L6,
the 807. The cost of public address amplifiers was affected by the new
tube, as it was now practical to get 25 watts without using four 2A3s
or expensive larger triodes like the 50 or 300B. Only two 6L6s were
now needed, at a fraction of the cost.

2. TYPES
The 6L6 gave birth to a vast array of beam tubes (Figure 6). The 6V6,
25L6, and others were immediate developments, which gave lower power
for small radios at lower cost. The 807 was the beginning of a series
of beam tubes intended for radio transmitters, some of which are
usable beyond 500 MHz. The 807 was the direct ancestor of the famous
6146 transmitting tube. The major VHF push-pull tetrodes of World War
II, the 815 and 829 were based on the 6L6. The 6550 was a high-power
audio tube based on classic beam tetrode principles. The first
American television horizontal amplifier or "sweep" 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 sweep tubes of the
1970s, such as the 6LQ6 and 6KG6/EL509.

To this day, new tubes are being developed that are descended from the
6L6. The KT90, KT99 and KT100 are examples. These recent audio tubes
are derived 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
who were afraid to replace their own radio tubes because of the danger
of injuring their hands on broken glass. The steel envelope was more
expensive to manufacture and had real problems dissipating heat, so
the fad was virtually over by 1940. The metal 6L6 and its premium
version, 1614, were often used in early jukebox amps and in many
Zenith radio chassis, not to mention PA amps.

A few maniacal radio hams found that a metal 6L6 could be operated in
a bath of transformer oil, allowing it to dissipate 150 watts for
short periods. The glass 6L6G, appearing in 1937, proved more popular
with the conservative audio industry. It was common in nearly all WWII
jukeboxes, and became nearly universal in PA amps right through the
1940s. Although the G version had the same ratings as the metal style,
it took over the market.

During World War II, improvements were made in the glass envelopes,
and 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
a straight-sided S-12 envelope. These all had the same maximum ratings
as the original 6L6G.

After the war, an escalation in power ratings began. This had been
prefigured 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.
It 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
amps. In 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. In the early 1960s, the 8417 was developed.

The 5881, introduced by Tung-Sol, was intended as a smaller 6L6
version for use in military and industrial equipment. Millions of
5881s were plugged into servo amplifiers in aircraft such as the B-52
bomber, so this must be a rugged and reliable tube. It was standard
equipment in some home hi-f amplifiers, such as the classic Heathkit
W-3 and W-4 series, Fisher 70A, Pilot AA-410 and many others. Fender's
Bassman was equipped with 5881s, and this guitar amp (like many later
models) is very demanding of its power 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
the 3 variations on this tube. General Electric tried to make a
super-6L6 in the mid-50s, and the result was the 7581. You can easily
recognize a real GE 7581 by its pinkish flesh-colored base, which is
virtually unique. It was the standard tube in the classic Harman-
Kardon Citation 5 amplifier, but was rarely used otherwise due to its
high cost. Tube manuals sometimes give the 7581 as an exact
replacement for the KT66, although it is mechanically quite different.
Still, it has become a valuable tube due to its ability to 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
similarly. All used 0.9 amps at 6.3 volts on the filament, except the
KT66 which used 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 list them
in this magazine. I could go into the 6AR6, or the Bendix Red Bank
6384 (covered in a separate article), or variations with different
filament voltages like the lower-power 25L6. There are numerous
variations of the 6V6, there are Western Electric types like the 350B,
there are numerous transmitting types, there are hundreds of sweep
tubes. There are miniatures like the 6AQ5 and 7189. There are the
late-50s audio types like 7591, 7868, 7355. Those will have to wait
for future articles.

As I said, the major applications of these tubes were in PA amplifiers
and 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
large guitar amps, starting with the Dual Professional in 1947.
Fender's large amps of the late 1950s, including the Showman, Bassman,
Pro, and Twin models, became the essence of American rock. Indeed, the
1959 Bassman and 1960 Twin are among the most copied electronic
gadgets in history, with a variety of new "boutique" manufacturers
producing their own versions. If you include the 6V6-powered Deluxe
models in that short list, then the old Fender designs are the
undisputed standards.

In 1972, the late Tom Ruberto of Sylvania developed a special version
of their standard 6L6GC, for Fender. This type had extra mica spacers
and was designed to hang upside-down, as well as being designed to
tolerate 500 volts on the plate and screen. This was the first STR
(special test requirement) 6L6. It became a standard, so much so that
"STR", long after the 1988 shutdown of the Sylvania tube factory, is a
standard term used to describe 6L6GCs with this large cylindrical
envelope. GE even introduced their own version, and both had numerous
guitar amps designed around them. 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 such amps continue to be
used, although the STR tubes are no longer being made 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
with some types is usually their design limitations, not design flaws.
Older tubes often had surface treatments on their mica insulators
which reduced manufacturing costs, while allowing some leakage current
to reach their control grids. Such tubes are limited in plate-voltage
capability. And supertubes like the KT66 usually have gold-plated
grids to prevent grid emission, which can also destroy the tube. Since
I have tried out many tubes for this magazine (primarily with an eye
toward high-fidelity use), it's worth looking at the 6L6 types closely
to also determine what vintage-guitar-amp users need.

3. TESTS
As with previous tube tests in past issues of VTV, I used a special
single- ended test amp to examine the distortion characteristics of a
large cross- section of old 6L6 types, as well as a few
current-production items. The driver was a 6EM7 and the output load
was a One Electron UBT-1 with the

8-ohm test load connected to the 4-ohm tap, thus presenting 3200 ohms
to the tube's plate. This test has been most revealing in the past,
and the 6L6s were even more unexpected in their behavior. As in the
past, distortion is almost all second-harmonic and was measured at 1
watt into an 8-ohm load. Each tube was biased to 50 milliamps, a
typical value for 6L6s, then tested. All the types were run at 300
volts triode connection, then types that were rated to accept 500
volts on plate were run again at 500v, 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
is because those particular tubes are NOT rated by their manufacturers
for 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 a 6L6GC even though it is not intended for more than 400v on the
plate. (A 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 they usually start to creak (and, sometimes, try to
self-destruct due to grid emission or leakage currents). So I
definitely do not recommend these types 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 was revealed that the MOV KT66,
Mullard EL37, Sylvania GC and the rare 350B (a Western Electric type)
are superior to other 6L6 types in peak output. If the application
demands maximum peak output (and money is no object), these tubes are
best. Be prepared to pay more than $150 for each KT66, EL37 or 350B.
NOS usually brings such prices, but good used tubes are acceptable.
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
amplification 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
especially sought- after, and the list reflects this. On the other
hand, for guitar the tubes preferred are usually the short GE 6L6GC,
the "STR" 6L6GCs made by Sylvania and GE, and the various 5881s,
6L6WGBs, 7581As and 7027As. In this case, distortion is OK (and
sometimes deliberately sought by the user) but physical ruggedness is
more critical. This is why the metal types and the old 6L6G, GA, and
GB are less sought-after. The latter are in demand, but mostly by
radio collectors and juke-box owners who want to use original tubes.
For applications like these, where the plate voltage is below 350
volts, the current Russian 6P3S works just fine and is outrageously
inexpensive.

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
heat-sink, neither of which is practical in typical audio amps. The
more extreme collectors of Macintosh hi-f equipment usually insist
that their MC-30s be equipped with 1614s, the original equipment in
these amps.

For true obscurity, the Sylvania 5932 is worth looking at. It is a
special super-rugged 6L6 replacement for military equipment. It came
in three versions; two had a conventional single structure. The other
version is 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 5932, and the prices charged for it reflect the demand
(high). Its distortion and power output were only average, similar to
Tung-Sol 5881s.

All of the tubes listed here are pin-compatible replacements for any
6L6 type, except the 7027 and 7027A. Sockets must be rewired to use
them in 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 for 6L6s, 7027s have become very scarce. There were few
things that used them as original equipment. They are very tough and
are popular in Fender amps that have been rewired appropriately.
Purists tend to scoff, as 7027s are quite different from 6L6GCs and
the like; but they do work fine with just a socket rewiring and
rebiasing.

The 6L6 is not often seen in high-end hi-f amplifiers. There are some
old amps out there, however, and they can be kept going with the
Russian 5881. It is unpopular in guitar amps, even though it's rugged
and inexpensive. 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. The 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 Russian 5881.

I conducted casual listening tests at the VTV office; they tended to
back up the distortion tests above. The old 6L6GCs tended toward a
warm, "romantic" sound with greater "darkness" and much more
distorted, fat bass. The metal 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 5881, but the same kind of clarity. Old
5881s were mostly made by Tung-Sol, and sounded warm, slightly nasal,
with good bass. The KT66s and EL37s were outstanding hi-f tubes, more
like triodes in character and very detailed.

Two examples of the "skinny" Shuguang 6L6GC are listed here. These
look remarkably like the Russian 6P3S, but are slightly different. The
Chinese version has four square holes in its top mica spacer, rather
than the two in the Russian tube's spacer. The Chinese ones also look
less well-made and use the same ugly brown refractory cement (to hold
bases on) that is seen in other Chinese octal tubes. These, like the
6P3S, are not really 6L6GCs and should not be used at more than 400v.
During test at 500v pentode, they creaked and groaned alarmingly. Note
that their distortion was much higher than in the Russian ones.
Obviously these tubes were made with Russian tooling, but are much
poorer quality. There is a new "Coke-bottle" shaped 6L6 from Shuguang,
with a brown base and optional blue glass; it is too new to appear
here and will be reported on later. All of the NOS tubes are out of
production, leaving only the Russian 6P3S, 5881, and the Chinese
types. The Russian tubes are old Soviet commercial and military types,
not originally intended for export. Svetlana is going to introduce a
new 5881 of its own soon, and we will report on it in a future issue
of VTV.

4. OUTRO
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
6L6s. To claim that this market will soon dry up and be replaced by
transistors is simply prevaricative. Although no 6L6 type is being
produced in America or Europe at the present time, there are a few
popular ones from Russia and China which own the market. The Shuguang
types, including a new 6L6GC with a blue glass envelope, are
consistent sellers; and although they are very clean-sounding tubes,
the Russian-made 5881s are Soldano's favorites and are used widely.
They will likely be available for years, if not decades, to come. Add
in the soon-to-come Svetlana 5881 and a rumored 6L6GC-"STR" which may
be produced in California soon, and the 6L6 looks good for another 60
years.

Side-Bar:
The British Connection
In England in 1931, J.H. Owens Harries discovered that if the electron
flow in a tetrode was confined to beams, and that the distance from
screen grid to plate was kept at a critical distance, secondary
emission from the plate would be suppressed, just as in a pentode1,3.
This discovery enabled the 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 then came out with a series of beam
tetrodes, with their most famous being the KT66. KT stood for
"Kinkless Tetrode" -p; since it eliminated the kink in the transfer
curve that happened with regular tetrodes. The KT66 was meant to be a
plug-in replacement for the 6L6, but had superior characteristics. It
was introduced in 1937.

Due to: the head start the British had in developing component-type
high fidelity systems, the transfer of British RADAR technology to
American during WW II, and the common language, Americans in the late
1940s looked to England for ideas in hi-fi design. Williamson's
seminal Wireless World articles gave British hi-fi a tremendous boost
in America. The marketing efforts of the British Industries Corp
(B.I.C.) brought the best of British hi-fi components to America
during the 1950s. The net result of this was the inclusion of
"foreign" tubes, such as the KT66, KT88, EL34, GZ-34, etc. into
American hi-fi and even guitar amp designs.

Footnotes:
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.


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AM International #1024, TENTEN #13582. GRID FM19. Using a SWAN 250 on 6m,
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