1.
Intro
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 transistors 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--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".
2. 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 They were quickly superceded by RCA's UX-247, released June 1931. Suddenly you
could get 2.5 watts out of a single tube, at only 250 volts! Millions of radios used the
247, and its descendant, the 42 with its 6.3 volt heater, was even more popular. The 38,
48, 59 and 2A5 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 two problems here. First, the
capacitance between the plate and grid is magnified by signal feeding back from the plate.
This is the "Miller Effect", and it limits the high-frequency response of any
given triode. The second problem is due to the electrons as they reach the plate. Some
aren't absorbed, but bounce off and wander around. These "secondary emissions"
may return to the plate, or they may enter the grid, or they may just be wasted against
other parts of the tube. 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. It was a peculiar and transitional
tube, with a huge cathode and a set of small Barkhausen plates on the inner surface of the
plate to improve electrical performance.
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 deflected 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 grids, which in an extreme case can make them emit electrons,
causing the tube's plate current to run away.
The solution was well-described in O.
H. Schade's classic article in the PROCEEDINGS OF THE IRE, February 1938. Schade and his
fellow RCA engineers made two blindingly simple changes which rocked the electronic world.
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.
Any electrons that flew past the siderods would be forced back into a nicely-ordered set
of beams that passed from cathode to plate with little interference. 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, thus elbowing out old tetrodes like the 46 and 48,
pentodes like the 6K6 and 6F6, and triodes like the 45 and 2A3. 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. The cost of PA
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.
Excerpts from original article in Summer 1996 Vacuum Tube Valley
Magazine. This back issue is available - see home page.
1.
THE GOLDEN AGE OF HI-FI STEREO
In 1960, the missile age was in full glory and the Golden Age of hi-fi Stereo was nearing
it's peak. Production of vacuum tubes and related equipment was at its all-time peak.
Also, this was the era of Popular Electronics and Popular Mechanics. American males
enjoyed hands-on craftsmanship and building their own electronic equipment and tools. The
electronic kit business was at near record levels and the transistor was still an oddity
used by the telephone company and the government Just as the
competition for more horsepower was on in Detroit in the early Sixties, the race for more
watts was running at the local hi-fi shop. With the advent of inefficient speakers like
the AR-3 acoustic suspension speaker in 1959 and the larger, concert hall speakers like
the B-310 Bozak Concert Grand (which actually came out in 1951), more watts were needed
for more realism. In the post-war mono era, 10 to 30 watts was more than adequate for the
popular, large horn-type speakers (read: Altec, JBL, Klipsch, Tannoy, etc) with 12 or 15
inch woofers and compression-type horn high frequency drivers. On many of these systems, a
two watt triode amp is more than enough to drive you out of the room with volume. However,
inefficient speakers like the AR-3 needed at least 35 to 40 watts continuous to get any
volume. Even the famous KLH Model Nine Electrostats needed at least 40 watts continuous
for any volume. This inefficiency trend still continues today with most speakers being no
more efficient than 87-91 dbA one meter @ one watt. But that is another issue to be
covered at a later date.
2. THE COMPETITION
About the same time, a number of hi-fi manufacturers took on the challenge to develop and
market tube stereo "super power" amplifiers in kit and/or assembled form.
Acrosound developed the UL-120, a 120 watt stereo power amplifier kit using Ultra-Linear
T0-600 output transformers driven by push-pull KT77s. A similar amplifier kit was sold
under the Radio Shack label as the HK-210. In late 1960, EICO brought out the HF-89, a
beefy 100 watt stereo amplifier kit with push-pull EL-34s. Lafayette introduced the KT-550
in late 1960. The KT-550 was actually designed by Stewart Hegeman and featured push-pull
7027As generating 50 watts per channel continuous. H.H. Scott came into the fray with
their powerful LK-150 - 65 watt per channel amplifier kit using pentode-connected Tung-Sol
6550s and huge, in-house wound output transformers. Finally, McIntosh unveiled their
legendary MC275, a 150 watt chrome stereo powerhouse that is still a demand item and
status symbol in today's crowded audio amplifier market. Clearly, the race for high power
amplification was on!
3. DESIGN CONSIDERATIONS
Hegeman's approach to designing the Citation line was from a professional recording
engineer's perspective. When you listen to recorded music for a living, listener fatigue
becomes a major concern. Hegeman felt that distortion and frequency response were main
factors in amplifier design and superior performance. He believed lower distortion, wide
bandwidth and multiple feedback loops were essential for realism and to reduce listener
fatigue. The amplifier must have minimal distortion to reduce the overall distortion
generated from the cutting head, cartridge and speakers. "Distortion", he said,
"is a deviation from the original. It includes harmonic, transient and
intermodulation distortion components as well as phase response, restricted dynamic range
and restricted distribution patterns from microphones and musical instruments".
Frequency response of the amplification system was another major
design consideration. The musical response bandwidth must extend considerably beyond the
hearing characteristics of the human ear in order to provide satisfactory reproduction.
Thus, the concept of "wide band" amplification further developed. Hegeman felt
that amplifier performance below the 20 cycle range is very important to a tight and
clearly defined low end. Conversely, an amplifier which has a frequency response beyond
100,000 cycles without evidence of ringing or instability when hooked to a reactive load
can offer clean, transparent tone in the higher frequencies with outstanding instrument
separation.
4. A NEW APPROACH TO FEEDBACK
In the Fifties, the use of feedback in amplifiers was controversial in some amplifier
designs. Typical amps back then used a "single loop" feedback circuit from the
voice coil terminals to the cathode of the input tube to smooth the frequency response and
lower distortion. This approach limits usable feedback to 20 - 26 db. This method can
reduce distortion components by a factor of up to 20 to 1.
Hegeman felt that a "multiple loop" method to increase the
overall feedback was the answer to providing lower distortion without sacrificing
stability. Multiple loops become additive if their ratio is adjusted to the relative
degree of distortion produced. Thus, if one stage has twice the distortion of another, it
should have twice as much feedback around it. The three feedback loops employed in the
Citation II include: one from each 12BY7 driver tube plate to its own grid, one from each
KT88 output tube plate to the opposite driver grid and one from the secondary of the
output transformer to the cathode of the 12BY7 input stage. With this design, 32 db of
overall feedback was achieved in the Citation II with unconditional stability.
5. CIRCUITRY IN THE CITATION II
The power amplifier consists of two identical 60 watt continuous rated power amps on one
chassis with a shared power supply. The power supply is a low-resistance voltage doubler
type with silicon rectifiers, more than adequate capacitance mounted under the chassis and
a filter choke for B+ filtering. Basically, the amplifier design employs a
distributed-load (Ultra-Linear) output circuit using KT88 power pentodes operating in
push-pull with fixed bias. Still thought by many as controversial, 12BY7A video-output
pentodes were used in all low-level stages for extremely wide frequency response and
minimal distortion. To test the wide frequency range, pulse amplifier techniques were
applied to the 12BY7A video pentodes feeding into a low impedance load which provide a
flat frequency and phase response beyond the capability of the output transformer.
Speaking of output transformers, the Citation II's were outstanding!
They were huge, well-potted units that had extremely wide response characteristics.
Leakage inductance in these transformers was kept to an absolute minimum and the
distributed capacitance of the primary halves were carefully balanced against each other
to maintain natural resonances of the unit well above 200,000 cycles. The massive design
utilized the highest grade core materials available which, lowered the effect of core
distortion to a region well below the limit of human hearing. With feedback, the Citation
II transformers were capable of high frequency response up to 270,000 cycles!
There appear to be three distinct variants of these units: the early
version had cloth tubing to guide the wire out of the bottom of the transformer, the next
version had rubberized cloth wiring for the same purpose and the last version had the wire
going through rubber grommets on the bottom of the can. Apparently, the first versions of
this transformer did not have the performance of later versions according to local amp
builders and transformer experts. Freed Transformer Corporation of New York was the sole
manufacturer of the output transformer and the part number is FT-3273671A. Freed also made
the power transformer for this unit.
Excerpts from original article in Summer 1996 Vacuum Tube Valley
Magazine. This back issue is available - see home page. |
A Tube
Manual Fragmenta
or
A Comparison of Various Tube Manuals
1. Intro
Tube manuals are a necessary part of the tube audio enthusiast's library. They provide a
wealth of information that helps one to appreciate his equipment and are a necessity if
one wants to design or build his own equipment. They also can be useful when
troubleshooting; especially when one does not have a schematic. The problem is that there
are a lot of tube manuals around. While some tyros want as many manuals as they can find,
many of us are limited by space, or other considerations, and just want a practical number
that will satisfy our needs. In this article, the author will review a number of different
manuals in the hope that this exercise will enable the reader to be able to intelligently
choose the manuals that will be the most useful for his needs and, perhaps, save some
space and money as well.
2. RCA
The first company that we will explore is RCA. Their manuals seem to be the most commonly
available and, therefore, merit the closest consideration. They are the ones that one is
most likely to find at bookstores, swap-meets and through the mail. RCA tube manuals come
in many different types.
According to Barry Nadel, who has made
a study of the RCA-Cunningham series, the series went from RC-11 to RC-30 (1975). There
were separate RCA Radiotron and Cunningham manuals published prior to 1932. A single
example, R-10 is known to the author. It is postulated that the "RC" designation
delineates a combination of the Radiotron and the Cunningham manuals. The "RC"
series of softbound RCA tube manuals started about the same time as did the beginning of
High Fidelity, 1932, and continued to the end of the tube era. Each of these manuals
contains a wealth of material, much of which is repeated from manual to manual. One need
not own the entire series to obtain the information he needs. As older tubes became
obsolete (in the eyes of the manufacturer, not necessarily the tube enthusiast), they
would be relegated to the back of the manual and less information would be given about
them. Therefore, it behooves the enthusiast to acquire at least some of the older numbers
to be sure to have all the necessary information. For this article, we will examine RC-11,
RC-15, RC-20, RC-23, and RC-29 Receiving Tube Manuals. RC-11 manual was printed in 1933.
Herein, one will find the 2A3, the 2A5, the 250 and even the '01A discussed in detail. The
manual starts with a discussion of vacuum tubes. The end of the book has circuit diagrams
showing "typical" examples of various circuits, as well as tabular tube
information, base diagrams and obsolete tube information. This format was followed
throughout the series. Manual RC-15, from 1947, is almost twice as thick as RC-11. While
there is great detail for the 2A3 and the 6L6, the 10 and 50 tubes are discussed only
briefly. There is a nice discussion of resistance coupled amplifiers, along with the usual
circuit diagrams and tabular information. This manual was printed at the start of the
"Golden Age" of High Fidelity. RC-20 was printed in 1960, towards the end of the
"Golden Age", when stereo was becoming fully established. It has much
interesting information for the High Fidelity amplifier designer and appreciation. Various
circuits used in Hi-Fi applications are discussed in detail. The 2A3 and 6L6 are still
mentioned in detail, but most of the early triodes are barely mentioned. Few industrial
numbers are mentioned (5581, 6973, 7025, 7189, 7199 are), and only one foreign-numbered
type is mentioned. RC-23 (1964) is essentially the same as RC-20, except the 2A3 is
obsolete and there are more of the industrial types. RC-26 (1968), is heavily weighted
toward the (then) latest tubes and only has older tubes very briefly mentioned in the
back. RC-29 (1973) is similar to RC-26, except that there is more emphasis on the
industrial and foreign tubes. As one can see, possession of a smattering of these manuals
can give the enthusiast much of the tube information that he needs.
Excerpts from original article in Summer 1996 Vacuum Tube Valley
Magazine. This back issue is available - see home page. |
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