Reproduced from VTV and 

http://newsgroups.derkeiler.com/Archive/Alt/alt.guitar.amps/2006-05/msg00640.html


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 fnd 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 frst-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 fnancial

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-effcient output tubes. The pentode was the answer at frst.

It

originated in Europe, with the frst American power types being the

Champion

P-704 and Arcturus PZ. These were very early types and had some

reliability

problems. They were quickly superceded by RCA's UX-247, released June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

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

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 flament 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

magnifed

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 effciency, when used as power amplifers.

In the 1930s the problems were diffcult to get around; triodes with

low

capacitance were eventually designed. But at the time, effciency 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 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

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 effciency 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 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

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 amplifers 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

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.

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 amplifers 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

amplifers, 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 fesh-colored base, which is virtually unique. It

was

the standard tube in the classic Harman- Kardon Citation 5 amplifer,

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 flament, 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

flament

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 amplifers

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 frst 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 faws. 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-fdelity

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 defnitely 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

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

especially

sought- after, and the list refects 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 fne 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 McIntosh 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 refect 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 fne with

just a

socket rewiring and rebiasing.


The 6L6 is not often seen in high-end hi-f amplifers. 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 offce; 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 keptat a critical distance, secondary emission from the

plate

would be supressed, 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 headstart 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.