The Illustrious 6L6 tube (Part 2)

[Brian Carling]

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.
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