The Illustrious 6L6 tube (Part 3)

[Brian Carling]

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.
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