The illustrious 6L6 tube (Part 1)

[Brian Carling]

                                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 They were quickly superseded by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.
****************************************************
*** 73 from Radio AF4K/G3XLQ Gaithersburg, MD USA  *
**  E-mail to:  bry at mnsinc.com                     *
*** See the interesting ham radio resources at:    *
**  http://www.mnsinc.com/bry/                     *
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AM International #1024, TENTEN #13582. GRID FM19
Rigs: Valiant, DX-60/HG-10, Eldico TR-75, Millen 90810
FT-840, TM-261, Ameco TX-62, Gonset Communicator III
HTX-202...TEN-TEN #13582, DXCC #17,763 Bicentennial WAS

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