electron's

OPTIMISED ULTRA-LINEAR
©

HI-FIDELITY ELECTRON TUBE AUDIO AMPLIFIERS

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Readers - Before reading this page, please refer to my web page on Ultra-linear high fidelity
amplifiers for comprehensive details of the traditional ultra-linear
amplifier design and attributes.

That page presents or references considerable theoretical and practical
analyses of the ultra-linear methodology. It is recommended that page should be
studied before proceeding further.
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In 1936, Alan Blumlein was
granted UK Patent 496883 covering a design concept describing a particular form
of loading configuration for power tetrodes, pentodes and beam power
tubes in electron tube amplifiers whereby the Screen Grids (Grid 2) are fed
from a tapping in the primary Plate winding, or from a separate discrete
Screen-Grid winding, of the output transformer.

Later, writing in the November 1951 edition of the US "Audio Engineering"
magazine, David Hafler and Herbert Keroes of the Acro Products Company, USA,
used the term "ULTRA-LINEAR" to describe a specific form of this mode of
operation.

This paper explains my OPTIMISED ULTRA-LINEAR ©
concept, designed to maximise the
positive benefits of ultra-linear operation and its superior performance
to conventional tetrode, pentode or beam power tube designs.
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"ULTRA-LINEAR" operation is also known as "DISTRIBUTED-LOAD"
operation- because the Screen-Grid is driving into a determined load
impedance, which also delivers power output through the output transformer.

"Ultra-linear" distributed loading may be used for either single-ended or
push-pull configurations.

"Ultra-linear" distributed loading may also be used for driver stage voltage
amplifying tubes. In this case the transformer is replaced with a system of
resistors and capacitors to produce the same load sharing and negative feedback
result.

The basic push-pull ultra-linear power output stage design looks like this:

The Screen-Grid tapping may be at any point between 0% turns (Pentode
Connection) and 100% turns (Triode Connection) - typically within the range 40
to 43% turns or 16 to 18.5% impedance for best effect - when measured from the
centre-tap to each Plate.

The basic ultra-linear configuration as shown above therefore avoids
the need for a separate Screen-Grid supply, thereby offering improved
performance at no real extra cost. In fact, it enables total elimination of the
independent "regulated screen supply" seen in professional quality tetrode,
pentode or beam power tube amplifiers, resulting in a real cost-reduction, space
saving and improved reliability.

This feature is very significant to hi-fi audio because the necessity for a
regulated Screen-Grid supply to attain high quality performance was widely
acknowledged since the 1930's.
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The Ultra-linear "Sound"

The sonic properties of the ultra-linear output stage configuration - albeit
single-ended or push-pull design - are described by reviewers as being midway
between triode and tetrode "tone".

Some audio-engineers describe the sound of triodes as "smooth, sweet, mellow,
natural" and tetrode/beam power tubes as "clean, bright, sharp, punchy".

The ultra-linear configuration also offers improved overload characteristics,
resulting in more effective power output - ie what the listener actually
hears at full power levels.

Technically speaking, the ultra-linear configuration
can deliver between 50 and 100% power output as for pentode operation of the
same tube under the same operating conditions and typically about twice the power output of triode operation
of the same tube under the same operating conditions with the same applied DC
voltages - but with substantially less harmonic distortion or intermodulation
distortion (see comparative performance graph below).

Output impedance is similar to triodes, allowing minimal or zero negative
loop feedback to be used.

Ultra-linear Circuit Characteristics:

The following graph, courtesy of GEC and AWV Radiotronics Magazine
(May 1959), shows the comparative characteristics of the KT88 beam power
tube in triode, pentode and ultra-linear connections under specified load
conditions.

These comparative relationships between the different connection
configurations should be typical for most (but not all) tetrodes,
pentodes and beam power tubes.

Note:

the relationship between output impedance and ratio of screen tap
turns.

the relationship between output power and ratio of screen taps.

the relationship between THD distortion and ratio of screen taps.

the effect of load impedance upon output power.

the power output for the KT88 in ultra-linear operation is greater
than that for pentode (tetrode) or triode operation.

Hafler and Keroes, in their US Patent 2,710,312, demonstrate the
performance of their ultra-linear concept by the following graph:

Plot A shows the tube internal impedance for the various screen taps
of 0% (tetrode connection) to 100% (triode connection). The result is expressed
as "R" in the table. It can be seen that tetrode connection has a very
high tube impedance and triode connection a very low tube impedance.

Plot B shows the "undistorted" power output in Watts for the amplifier
used in their tests - expressed as "W" in the table.

Plot C shows the level of high-level intermodulation distortion for
the amplifier used in their tests - expressed as a "percentage".

Important: Note the extremely high level of IM
distortion for push-pull triode operation (more than 30%). This explains why
commercial manufacturers of triode configured amplifiers (particularly those of
the single-ended variety) rarely - if ever - reveal the intermodulation
distortion performance.

Plot D shows the level of low-level intermodulation distortion for the
amplifier used in their tests - expressed as a "percentage"

Negative loop feedback was not used in the test amplifier.

This graph clearly demonstrates that performance
described as "ultra-linear" is available only under very specific operating
conditions that occur within a narrow band of impedance ratios.

In all other cases, the term "distributed load
operation" is more appropriate.

It is the "ultra" that makes all the
difference.

This is proven by the GE "Operation Characteristics"
graph below, that shows for a 6550 tube in push-pull ultra-linear mode, the
load impedance can range from 2,000 Ohms plate to plate to 12,000 Ohms Plate to
Plate for reasonably constant power output. This performance contrasts markedly
with modern solid-state amplifiers that, as a class, suffer from dramatic power
loss as the load increases - proportionately downwards - eg 100% at 2 Ohms, 50%
at 4 Ohms and 25% at 8 Ohms etc.

This is why solid-state amplifiers generally suffer
from a lack of bass response when driving loudspeakers, whose impedance may
typically be around 100 Ohms at low frequency. A bass boost circuit becomes
essential to offset this effect for flat audible response. See also
my "How To" page.

OUTPUT IMPEDANCE

Output Impedance is a very critical parameter for high-fidelity reproduction,
particularly when no loop negative feedback is used from the loudspeaker
circuit.

The original 1937 Blumlein Patent Application claims:

"in order to obtain a good power output the impedance of the load must be
very much less than the characteristic impedance of the valve."

"by choosing a suitable tapping point for the screening grid the
characteristic impedance of the anode and screen effectively combined can be
given any desired value between the impedance of a pentode or tetrode and the
impedance of a triode. A suitable tapping point, in practice, may be found to be
such that the voltage swing on the screen is between about a quarter and a half
the voltage swing on the anode."

"It is the chief object of the present invention to provide an improved
circuit in which the characteristic impedance of valves of the kind referred to
can be more nearly matched to their optimum loads and in which the danger of
an excessive increase of voltage on removal of the load may be substantially
avoided."

The above graphs show how Output Impedance is dramatically reduced by
ultra-linear operation of the output stage.

Output Impedance can be further reduced on a pro-rata basis by the use of
multiple pairs of output tubes.

The following article, courtesy of Amateur Radio Valve Technique,
published by The Radio Society of Great Britain 1948, explains what "Output
Impedance" is, how it can be measured, and its effect on performance.
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LOAD CHARACTERISTICS

Triode, Tetrode and Pentode operation are generally critical as to load.

However the following graph, courtesy of GE USA (4/72  6550A data
sheet), shows the excellent linearity of power output versus load impedance for
the 6550 in ultra-linear connection.

It can be readily seen that Power Output and Harmonic Distortion are
relatively independent to the load.

This is a most important feature when using loudspeakers, which are renown
for presenting a varying load to the amplifier.

For example the following graph shows a typical speaker voice-coil impedance
curve

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Note in this example the impedance rises from a nominal 7 Ohms at 100 Hz
to about 74 Ohms at 100 Hz and 11,000 Hz

Other 8 Ohms speakers show up to 100 Ohms at these frequencies

The above load curve demonstrates that markedly increased speaker impedance
at resonant frequency will not produce a spike in power output as seen in
conventional Tetrode or Pentode operation. This is a very worthy attribute for a
hi-fi audio amplifier.

This goal was the essence of Blumlein's original Patent and has clearly been
achieved.

Note: It will be seen from the above graph that the best performance is
realised with a load of about 9000 Ohms Plate to Plate. This is exactly twice
the load recommended by GEC for the KT88 under similar conditions, suggesting
the KT88 and 6550 tubes are not directly interchangeable when optimum
performance is required.

It will also be seen from the GEC graph above that the KT88 behaves very
differently under load variation than the 6550.

Recent Research

I am indebted to Rudolf  Moers, a distinguished Electrical and
Electronics Engineer located in the Netherlands, who has made available for us
his wonderful recent scientific investigation into the design theory and
practice of Ultra-linear audio amplification.

These papers are posted with permission from Linear Audio www.linearaudio.net
and their author Rudolf  Moers.

Part 1 - Power
Point presentation

Part 2 - Power
Point presentation

Parts 1 and 2 combined
presentation - pdf format

Paper - The
Ultra-Linear Power Amplifier: An adventure between triode and pentode - pdf
format

The engineering design methods developed by Mr Moers may be used to determine
theoretical plate/screen load ratios for
ultra-linear operation of power
tubes.

The detailed work of Mr. Moers may contain some surprises for the
traditionalist.

Summary:

In summary, traditional ultra-linear connection offers:

useable power output equal to or approaching that of tetrodes and pentodes
but with substantially reduced harmonic and intermodulation distortion and
improved tonal characteristics, thus making it the preferred configuration for
high-fidelity applications.

a "warmth" of tone across the full audio spectrum, not evident with the
"clinically clean" sound of unity coupling -  where the load is
distributed between plate and cathode, or the "hardness" of tetrode or pentode
configurations, or the "muddiness" (but pleasant) of triode connected tetrodes
or pentodes

simplicity in circuit design and construction

elimination of need for a separate regulated power supply to the
screen-grids

employment of a simple output transformer

constant-voltage negative feedback

high stability operation over a range of reactive and capacitive
loads.

wide-range frequency response

strong useable low-frequency power output - ie as heard through the
loudspeaker

smoother overload characteristic than tetrodes, pentodes or beam power
tubes

good dynamic range performance and transient response

higher output stage to loudspeaker coupling efficiency, resulting in a
perception of "louder" reproduction

ideal tube amplification for bass guitar amplifiers - ie high power, low
output impedance, freedom from bass resonance

Ultra-linear
operation also enables lower quality loudspeakers to be used for satisfactory
results.

IMPORTANT: Performance described as "ultra-linear"
is available only under very specific operating conditions  - usually
where the Screen load is 18.5% impedance or 43% turns of the Plate load
impedance - measured from the power supply source terminal.

In all other cases, the term "distributed load
operation" is more appropriate.

OPTIMISED ULTRA-LINEAR © HI-FI
AUDIO AMPLIFIERS

© NOTICE:    INTELLECTUAL PROPERTY
COPYRIGHT D.R.GRIMWOOD 2002 - ALL RIGHTS RESERVED. NO PART OF THIS TECHNOLOGY
MAY BE USED FOR COMMERCIAL PURPOSES IN ANY FORM WHATSOEVER INCLUDING BUT NOT
LIMITED TO PRODUCT DEVELOPMENT OR GAIN OR REWARD WITHOUT THE EXPRESS WRITTEN
PERMISSION OF THE AUTHOR.

On the available evidence, the "ultra-linear" concept appears to have
been first introduced publically by David Hafler and Herbert Keroes of the Acro
Products Company, USA in the November 1951 edition of "Audio Engineering"
magazine.

This was an outstanding development for their new company, Acro Products,
which they had founded only a year earlier in 1950.

In their "ultra-linear" concept US Patent Application in May 1952 (Serial
Number 288,908) (Granted 2710312 - June 1955), Hafler and Keroes described the
results of their obviously extensive practical research over the many years
since the original 1937 pioneering work of Alan Blumlein - UK Patent 496883.

Hafler and Keroes concluded their "Statement of Claims" with the remarkable
statement: "Whatever the true explanation, the plate transfer
characteristics of the tetrode arranged in accordance with our invention are
remarkably linear."

It is now evident from the documented record, that although patenting
Screen-Grid tap impedance ratios of 0 to 100% (Blumlein) or 5 to 30% (Hafler
and Keroes) of Plate load impedance, none of the aforementioned patents
attempted to explain the theoretical basis for those claims beyond describing
it as "a system of negative feedback". Notwithstanding the earlier work
of Blumlein, the evidence presented by the inventors to support their patent
claims was essentially empirical, supported by documented results of their
extensive and thorough verifiable experiments.

This was confirmed by the intensive investigative analysis and testing work
of F Langford
Smith and A R Chesterman of AWV Australia in mid 1955, who, despite offering
a theoretical method for calculating operating conditions, also offered no
explanation of the underlying conceptual electronic mechanism.

Furthermore, as late as the
January 1956 issue of "Wireless World"- Tetrodes with Screen Feedback
the explanation of how the ultra-linear system works was still a mystery.

The Patent itself, only focuses on two primary tube types - viz the 6L6
family (including 5881, 807 and KT66) and the 6V6 family, suggesting that in the
urgency to file the Patent claim, Hafler and Keroes limited their work to the
then popular audio tube types. However the Patent claims the invention is
suitable to any "analogous" tube type.

Fate then intervened in the evolutionary process of "ultra-linear" amplifiers
and history shows David Hafler left Acrosound in 1954 (which was the year
before their joint patent was actually granted in the name of Acro Products
Company) and joined with Ed Laurent to form the Dynaco company in 1955 - a
company now famous for its manufacture of fine amplifiers.

It has been reported that by 1955, due to its ready acceptance by the hi-fi
industry, the patented ultra-linear design - owned by Acro Products - was
licensed to more than 20 major audio manufacturers, including Dynaco, Eico,
Fisher, Harmon-Kardon, Marantz and Scott. Professional sound licensees included
Altec-Lansing, Ampex and Fairchild - an incredible endorsement by big name
brands.

Also in October 1955, a few months after the granting of Patent 288,908,
Herbert Keroes lodged a further patent application for his state of the art
design of a push-pull output transformer for ultra-linear applications (US
Patent 2791646 granted May 1957). This design was a great leap forward for
audio, being an incremental extension of his previous patents 2680218 (1950) and
2710312 (1952).

The novel features of his transformer design were sectionalised windings for
primary and secondaries, multiple impedance secondary windings and a separate
tertiary winding for the screen-grids to enable use of high-voltage, high-power
tetrode, pentode and beam power transmitting power tubes in ultra-linear
configurations.

Typical transmitting tubes have strict restrictions on the maximum applied DC
Screen-Grid voltage, which is always significantly less than the Plate DC
voltage. This new transformer design opened the way to use of high power output
tube types not previously available to ultra-linear amplifier designers.

To promote the Acro Patent 288,908 ultra-linear transformer concept, a
high-quality 100W ultra-linear amplifier design using this transformer
(Acrosound TO-350) and 6146 VHF transmitting tubes was published in the
Acrosound Catalogue of 1955. In this case the Plate voltage was 725 VDC and the
Screen Grid voltage 220 VDC.

It should be noted that these same transformer attributes had already been
covered for single-ended operation in the previous patents referenced above.

The Acrosound Ultra-linear Transformer Catalogue says:

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Joseph Marshall, writing later in "Radio-Electronics" magazine, said:

It is apparent from studying the original patents of Alan Blumlein (1936) and
Hafler and Keroes (1951) and other respected commentators, that none were quite
clear on why the ultra-linear design does what it does.

Other highly acclaimed audio engineers, like Norman Crowhurst
("Audio"Magazine 1959), and even F. Langford-Smith (Radiotron Designers Handbook
4th Edition - 1952) also appear not to have understood precisely why it is so.

D.T.N. Williamson (MO Valve Co. UK) and P.J. Walker (Quad Audio), writing in
the September 1952 edition of the Wireless World Magazine, described the Hafler
and Keroes invention as "distributed load" operation and essentially rejected
the claims made in the Acrosound Patent, declaring that both straight pentode
operation and the Quad Cathode coupled design (as used in the Quad II Amplifier)
are superior. They also described the concept solely in terms of a negative
feedback system.

Of interest is the deliberate use of the descriptor "distributed load"
by Williamson and Walker, indicating their distaste for the "ultra-linear"
label.

There may have been some commercial bias in this presentation, however that
was short-lived because their competitor Mullard Valves UK, in "World Series
Audio" publications, introduced a series of designs for "distributed load"
amplifiers using their EL34 and EL84 tubes. Importantly though, they did not
expand on the "distributed load" concept, preferring to describe it in
terms of a system of providing Plate voltage to Screen Grid feedback.

In their 1960's "Mullard EL34 World Series Valves" brochure, Mullard say: "An
interesting method for operation of high-quality domestic amplifiers is to use
two EL34's in a push-pull output stage with distributed load conditions.
The Screen Grids are connected to taps on the primary of the output transformer,
so that part of the primary load is common to the Anode and Screen Grid
circuits." This publication did not provide limiting values for "distributed
load" operation.

Later Mullard publications suggest they doggedly adhered to the
"distributed load" term until the demise of the Mullard business some
years later, without ever providing explanation of how it worked.

When the MO Valve Co/GEC introduced the KT88 in November 1956, they did not
provide recommendations for use in distributed load amplifier
applications at all - ie just triode and pentode applications.

But most importantly, from March 1959, MOV/GEC used the term
"ultra-linear" for their KT88 data sheets and amplifier designs. Not only
did they abandon their own Mr Williamson's "distributed load" concept,
but they published data, test results and graphs to demonstrate that
"ultra-linear" operation is superior to triode or pentode operation!!

Thus, as the KT88 became "the" tube to use for high powered
ultra-linear audio amplifiers, the term "distributed load" became
lost to posterity.

In November 1959, Norman Crowhurst, writing in "Audio" Magazine, described
the advantages and disadvantages of "ultra-linear" operation, expressing
his choice of terminology.

However RCA remained non-committed, describing this mode of operation in
their January 1959 RC-19 and August 1968 RC-26 Receiving Tube Manuals for types
6973, 7027 and 7868 tubes as simply "Grid #2 of each tube connected to tap on
Plate winding of output transformer, so as to supply 43/50% of the Plate signal
voltage to Grid #2 of each output tube".

The 1963 Tungsol and 1972 GE data sheets for the 6550 tube provide ratings
for "ultra-linear" modes. Full marks to GE, who at last did assign a
maximum rating for this class of service.

Neither Telefunken nor Philips (the parent company of Mullard UK), appear to
have referred to the ultra-linear or distributed load concept at
all in their tube handbooks or data sheets.

But regardless of terminology, the profound words of Hafler and Keroes in
1952  - "Whatever the true explanation" - clearly convey a level of
uncertainty in the theoretical principles that support their applied engineering
design.

For his part, Herbert Keroes, the transformer guru of the team, did not add
further ultra-linear designs to the Acrosound range and does not seem to
have pursued the subject further. Since his original TO-350 amplifier was so
good he may have felt it unnecessary to expend further effort on the concept.
After all, the Acrosound Transformer Catalogue describes this 100W amplifier as
delivering "high-fidelity power to fill a stadium". Certainly in that era, 100
watts was more than the average home audiophile would have interest in, with the
then available loudspeaker systems being typically rated at 25 Watts RMS max. In
a stereo application the home or cinema user would have 200W rms available from
this amplifier, which in that era would have been overwhelming for most typical
applications.

Ironically this development occurred at the same time users were moving away
from the use of high-voltage "top cap" tubes - eg to 6L6 instead of 807 - and
the outstanding possibilities from this development in transformer design were
essentially wasted by consumer resistance. PA and guitar amplifier manufacturers
introduced multiple parallel pairs of output tubes instead of single high
voltage pairs, enabling the tertiary Screen-Grid winding to be deleted for lower
cost.

The technology for transformers suitable for conventional tetrode, pentode
and beam power tube amplifiers, including the Acrosound sectionalised winding
design covered by US 2680218, was already well developed and documented.

These design specifications and descriptive literature clearly indicate the
ultra-linear concept had by then been fully practically developed.

Acrosound may well have been satisfied with their sales revenues, licensing
the design and/or supplying transformers to the bulk of the major hi-fi
amplifier manufacturers at that time.

In the case of David Hafler, it appears that other than producing an
ultra-linear version of the Williamson triode amplifier, and higher power
conventional ultra-linear amplifiers, there is no record of Dynaco commercially
exploiting the Ultra-linear concept further. One reason for this may have been
Herbert Keroes' ultra-linear transformer patent 2791646 - application lodged
October 1955, the same month that Dynaco was incorporated. This "Patent Pending"
would have made it extremely difficult for other manufacturers to design a
commercially competitive transformer. The actual patent was not granted until
May 1957, effectively locking up the design for two years, then for a further 14
years during the patent period.

So the patent did its job well, stifling further development until after
transistors had replaced tubes in the popular hi-fi market and the ultra-linear
technology became displaced by technological progress.

David Hafler died May 25, 2003.

So the historical record suggests that from 1955, commercial competitive
forces prevented further development of the ultra-linear concept.

Of historical interest is that the original "Cinerama" seven channel 120W rms
cinema surround sound system introduced in 1955, whilst using 6146 tubes, did
not use the ultra-linear concept, opting to stay with the tried and true pentode
connection and regulated Screen-Grid supply. One reason may have been that the
only tubes available at that time capable of delivering 120W rms were also
transmitting tubes and the 6146 was a proven reliable performer - the KT88 had
not yet appeared as an option.

One notable exception is the McIntosh
MC3500 and MI-350 duo, introduced in 1968, which combines the Acro
ultra-linear design concept with the McIntosh unity coupling configuration to
produce an outstanding audio amplifier featuring unequalled performance.

Given all the above, it follows that in the search for "why is it so?" - or
cause and effect relationships - the performance of an Ultra-Linear
amplifier might be substantially improved by giving consideration to vacuum
tube design, construction and behaviour - ie beyond thinking of the
ultra-linear concept exclusively as a transformer technology or negative
feedback system.

Research I have undertaken indicates that the conventional approach to the
design of Ultra-Linear circuits has been limited not only by commercial
forces, but also technological precedent and custom.

It is time to think outside the square. Time to rethink. Time for a new
paradigm!!

Performance results have been less than ideal - there must be a better way.

TheOPTIMISED
ULTRA-LINEAR © concept demonstrates a better
way.

Introduction to OPTIMISED ULTRA-LINEAR
©

History tells us that from the three choices of tetrode, triode or
ultra-linear configuration, the latter was the preferred configuration
for 1950's to 1960's hi-fi, because it offered the tonal "sound" of triodes,
substantially lower distortion - particularly intermodulation distortion - with
power approximating tetrodes, at a cost comparable with tetrodes.

Ultra-linear amplifier designs of that era provided commercially
competitive high quality sound. In particular, ultra-linear amplifiers
delivered excellent transient response from simple power supplies.

Ultra-linear amplifiers typically used cathode bias, which provides
excellent reliability and extends tube life so, in addition to their sonic
qualities, ultra-linear amplifiers also developed a reputation for long
tube life and reliability.

Hence, by about 1960, the commercial development of Ultra-Linear
design technology plateaued, simply because conventional Ultra-Linear
technology having equal Anode and Screen-Grid voltages offered a significant
improvement on full triode operation of tetrodes or pentodes at
virtually no increase in total cost, and there was no real incentive to develop
the concept further.

After publication of their 100W ultra-linear amplifier design using 6146 VHF
transmitting tubes and the Acrosound TO-350 transformer about 1958, the
published work of Hafler and Keroes of the Acro Products Company (Acrosound)
appears to have exhausted further development possibilities from them.

It seems most commercial manufacturers of top-end audio amplifiers preferred
the conventional ultra-linear configuration having equal applied DC voltage to
Plates and Screen-Grids, which is clearly cheaper per watt of output power. That
situation appears to have continued through to the present time.

From the outset, tubes like the KT88, KT90, EL34, EL84, 6973 and 6550 made it
practicable for commercial amplifiers to be designed without an extra
Screen-grid winding and without an extra regulated Screen-grid power supply. The
availability from current manufacture of these proven tubes ensures continuation
of this popular design.

This observation is also true of output stage configurations other than
ultra-linear.

One exception was the great Bruce de Palma, who designed a series of
hi-fidelity ultra-linear amplifiers using reduced Screen-Grid voltages -
sufficient to ensure the Screen-Grid performed its proper function of attracting
and regulating electron flow from Cathode to Anode, and to extending tube life -
but not for the purposes described herein. His ideas found little support in
mainstream commercial applications, presumably because of the extra cost and
complexity of the output transformer and necessity for a separate and extra
regulated power supply for the Screen-Grids.

In any event, despite the availability of the Acrosound T0-350 and its modern
day retro-fit equivalents, no-one appears to have ever publically argued
superior performance for ultra-linear amplifiers using a proportional
voltage separate power supply to the Screen-Grids of receiving or transmitting
tubes, and the concept has been lost to historical curiosity.

Also, what has not been fully appreciated is that when compared with true
triodes, triode connection of tetrodes and pentodes offers only compromise
performance because of the physical placement and operation of the Screen-Grid
(Grid #2) and its effect upon overall performance - for details see my papers
on Screen Grids and Triode Operation.

That is to say, even though triode connection of tetrodes and pentodes works
in practice, and has been widely used for seventy or more years, it is not a
technically ideal approach - because the Screen-Grid acts as a pseudo anode, and
in so doing ADVERSELY changes the behaviour of the electron stream within the
tube - particularly when the applied DC voltage to the Screen-grid exceeds the
design maximum rating for tetrode/pentode operation - causing unstable operation
and even loss of control over electron flow ("thermal runaway" or "dynatron"
effect (RCA).

Similarly, in relation to Screen-Grid placement and function, the same
constraints and consequences apply to Ultra-Linear operation of tetrodes and
pentodes.

An Explanation.

First a look at the basic ultra-linear circuit.

The typical AC (alternating current) power tube output circuit looks
like this:

For simplicity, the DC Direct Current power source is not shown and the
Screen-tap is set at 43%. The capacitor is assumed to have zero impedance.

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THREE separate, but inter-active, output circuits are formed - the
Cathode to Plate circuit, the Cathode to Screen circuit and the Screen to Plate
circuit.

The Cathode to Screen circuit and the Screen to Plate circuit are in series
both in the tube and in the transformer.

Each circuit is independently connected to its respective load - ie the
transformer. This configuration is very different to pentode operation where the
Screen-grid is connected to the negative AC terminal of the circuit and does not
form a power takeoff element.

The work of F Langford Smith and A R Chesterman shows that up to about 8% of
power in the system is lost from the Screen Grid circuit.

BUT because in ultra-linear configuration the transformer tap directly
connects the external two circuits to the Screen-grid INSIDE the tube an
entirely different set of operating conditions is formed when compared with
conventional pentode operation. This changes everything.

The direct external connection to the Screen-grid causes the external series
connected (via the transformer primary winding) Cathode to Screen and Screen to
Plate circuits to be now connected in parallel with the Cathode to Plate circuit
INSIDE THE ELECTRON TUBE.

It follows that the sum of their voltages at any instant in time MUST equal
the internal Cathode to Plate voltage.

Now it can be seen in this example that if the Cathode is at 0 volts AC
potential and the Plate is at 100% volts AC potential, by operation of the
voltage distribution over the transformer primary winding, the Screen-grid MUST
always have 43% (in this example) of the AC Cathode to Plate signal voltage at
its terminal.

Reference to the above diagram shows the Screen-grid to
Plate circuit inside the tube is also connected in parallel with the lower
section of the transformer. Thus the output voltage developed across the Plate
to Screen tap on the transformer - in the case of a 43% tap being 57% x the full
AC output voltage across the primary - when re-injected in opposite phase
polarity into the Screen-grid must inhibit electron flow.

That is, the design purpose and operational
effectiveness of the Screen-grid (when compared with conventional pentode mode)
is affected because the 57% applied AC voltage reduces the effective Screen-grid
voltage and therefore tube gain. For the purpose of explanation, this is not to
be confused with negative feedback.

Furthermore, the Cathode to Screen-grid AC voltage is
also fixed at 43% of the output voltage across the full transformer primary,
again influencing negatively what is happening inside the tube, but in a
different way. Another way of saying this is that the natural voltage
distribution between Cathode and Screen-grid is changed by injection of AC
signal to the Screen-grid.

In addition, the transformer load impedance has the
effect of controlling current flow within the tube between Cathode and
Screen-grid. This may explain the observations of Hafler and Keroes, as
described in US Patent 2,710,312 referenced above, that as the
Screen-grid tap increases from 0% turns to 18.5% turns the internal impedance
of the tube reduces linearly then flattens off.

In the case of a push-pull amplifier the situation is
worse because the reflected voltage from the opposite polarity tube is now out
of phase with the internal electron flow.

From this one could deduce that the ideal configuration
to maintain voltage balance in the output stage would be a 50% turns tap for
the Screen-grid. Alternatively, to maintain impedance balance in the output
stage between Plate and Screen-grid would require a 70.7% turns tap (50%
impedance) for the Screen-grid. However it should be stated that none of the
test reports quoted verifies this thought.

To overcome the above described shortcomings the core premise of myOPTIMISED ULTRA-LINEAR © design
concept is that the AC and DC voltage gradient INSIDE the tube MUST exactly
match the voltage gradient OUTSIDE the tube.

This sounds like a very simple concept but in the case
of conventional ultra-linear design there is no attempt to match the internal AC
voltage with the external AC voltage, or the internal DC voltage with the
distribution of internal resistance within the tube.

The above diagramme shows that in an ultra-linear power amplifier circuit the
output transformer becomes an auto-transformer, rigidly interconnecting
the otherwise independent three circuits described above in a fixed
relationship.

Traditionally, designers have ignored this, however
there is always AC inside the tube whilst it is conducting signal.

WHY?

Because the Cathode to Plate electron stream forms the
return circuit for the both the DC power source and AC output. There has to be a
return circuit for current to flow.

The following diagram shows how this is so:

The output transformer and power tube are in series with the applied DC
voltage from the power supply.

The conventional design paradigm is to ignore the transformer, so the DC
voltage applied to the Plate and Screen-grid of the tube is assumed to be the
total (100%).

However there is voltage loss in the transformer because the copper wire has
significant resistance.

Since the DC voltage loss is in direct proportion to the Plate Current, it
follows that as signal power output increases the Plate voltage decreases.

Similarly, the DC Screen-grid voltage will also fluctuate with signal power
output.

Now the DC voltage loss in the transformer between Plate and Screen-grid will
be in proportion to the SUM of Plate current plus Screen-grid current. It
follows that since this value is greater than Screen-grid current alone, the
Screen-grid voltage will fluctuate not in direct proportion to its own needs but
in proportion to the total current in the circuit.

Therefore as power output increases so does current flow and so does voltage
fluctuation at the Screen-grid terminal.

In a conventional transformer having the Screen-grid tap at 43% turns, the
voltage drop at the Screen-grid terminal will be 43% of the total voltage drop
across the transformer. In a transformer having a half-primary DC resistance of
250 Ohms, the DC voltage drop at 100 mA (eg KT88 - change in current from zero
signal to max signal) will be 12.5 volts DC.

Since the Screen-grid controls tube gain it follows that the signal in the
tube will be modulated by the fluctations in Screen-grid voltage. At 400 VDC
this equates to a ripple voltage of 3%. This will be nominally doubled by the
autotransformer effect on the Cathode to Plate circuit - or forced by the Plate
to be dissipated in the transformer as induction loss.

This does not seem significant but if we are chasing 0.1% THD in the output
stage then it is important.

This voltage fluctuation is in addition to the Plate voltage fluctuation.
However the tube data sheets indicate fluctuation in Plate voltage does not
affect current flow - but it does affect power output because power output =
volts x amps.

Therefore in normal operation, although the Plate to Screen voltage ratio
remains nominally constant, the Cathode to Screen voltage is not in direct
proportion to the Cathode to Plate voltage.

Thus in a conventional ultra-linear circuit there is no way the Screen-grid
DC voltage can be stable.

It can also be seen from the above analysis that the amplifier CANNOT
reproduce dynamic peak signals faithfully.

Furthermore, the AC voltage in the power circuit behaves in a similar way.

That is, in addition to voltage losses caused by AC impedance in the
transformer, there is also the mismatch between the internal and external AC
signal circuit voltages caused by the direct connection from the screen-grid to
the transformer.

Furthermore, in a push-pull amplifier both Screen-grids will be
simultaneously fluctuating in gain modulating AC voltage.

Since the output transformer rigidly connects both sides of the push-pull
amplifier in a fixed ratio relationship it follows that what happens in one
half of the transformer will affect what happens in the other half. As noted
above the transformer presents as an auto-transformer to the tube circuits so
the impedance ratios will be fixed between the three circuits described above.
Distortion in the Cathode to Screen-grid circuit - nominally 43% turns - will be
multiplied to the Screen-grid to Plate circuit section of the transformer.

When negative loop feedback is added to the amplifier from the loudspeaker
terminals an attempt is made to cancel out such distortions.

To summarise, it can be said that conventional historic ultra-linear
technology has limitations and shortcomings. These were highlighted very early
by Williamson and Walker, to name but one example.

However my OPTIMISED ULTRA-LINEAR © concept overcomes this adverse phenomenum.

HOW??

To understand the OPTIMISED ULTRA-LINEAR ©
concept it is vital to acknowledge that electric
current  flows through a conductor only when there is a CIRCUIT.

ie - if we have a positive and negative terminal
connected to a DC source with a voltage between them, no current will flow
UNLESS there is a complete CIRCUIT.

In a push-pull Ultra-Linear output stage, a vacuum tube is exposed to
two sets of operating voltages simultaneously - the applied DC voltages needed
to enable the tube to work, and the AC voltages applied to the input circuit
(Grid #1) to enable signal amplification.

The DC voltage component represents the input power - ie DC voltage
and current supplied by the power supply to the Cathode/Plate circuit, the
Grid #1 (Control Grid) bias circuit and the Grid #2 (Screen Grid) bias circuit.

The tube Cathode/Plate circuit is shunt connected across the power supply so,
assuming constant DC voltages from the power supply, the DC current
through the tube is the load on the power supply - ie the tube itself is the DC
load.

The AC voltage component is the audio signal. The loudspeaker, coupled
through the output transformer, is the AC load on the tube. Therefore, for
purposes of circuit analysis and simplicity, the primary of the output
transformer is the load on the tube.

Another way of putting it is that there are two separate CIRCUITS - AC and DC
- using the vacuum tube cathode to anode section of those circuits as a common
shared device/current path.

Therefore the vacuum tube, as a conductor, forms PART OF an AC CIRCUIT and
PART OF a DC CIRCUIT

Thus in a vacuum tube, both the AC and DC current
components are present in the tube CIRCUIT simultaneously.

Kirchhoff's Current
Law
The algebraic sum of
currents entering and leaving any point in a circuit must equal zero - ie no
matter how many paths into and out of a single point in a circuit, all the
current leaving that point must equal the current arriving at that
point.

This means that in the case of an electron tube, the electron stream
across the space, or gap, between the Cathode and Anode of the tube is used to
pass both the AC and DC currents measured in the primary of the output
transformer.

Kirchhoff's Voltage
Law
The algebraic sum of the
voltages around any closed path is zero - ie the voltage drops around any closed
circuit loop must equal the applied voltages.

This means that BOTH the DC and AC voltages present across the internal
tube gap are distributed evenly across the Cathode to Anode gap by means of the
electron stream - ie there is an AC voltage gradient across the gap and a DC
voltage gradient across the gap - simultaneously!!

However in conventional tube audio circuit design, the DC voltage within the
tube is usually considered but not the AC.

The AC component has been historically regarded as being exclusively signal -
that is a product of tube function and performance - ie the product of
variation in plate current across the load - but not so much an
inherent  function of tube operation.

That is, the AC output has been regarded as a pseudo-alternating
current, created by the variation in DC plate current across the load - eg
output transformer primary winding - magnifying (amplifying) the AC signal
voltage appearing at the Grid #1 terminal..

In fact, if AC appears across the load it must also be present in the same
magnitude elsewhere in the same circuit - ie within the output tubes
themselves.

Another way of saying this is that if AC is produced by a variation in
plate current across the load, then that AC cannot flow unless there is a
complete circuit. The output tubes provide the return circuit for the
alternating current.

Hence when the tube is operated by means of introducing a signal into the
Control Grid - Grid #1 causing a variation in Plate Current, there will always
be AC and DC currents flowing simultaneously within the tube itself.

First, if we examine the electrode construction of a typical tetrode or
pentode audio power tube we will see the Screen-Grid is always located close to
the Cathode but far from the Anode (Plate). This means that when the voltage
gradient across the electron stream in the tube is considered, the
equilibrium, or naturally distributed, voltage between the Cathode and
Screen-Grid will be proportionately small when compared with the equilibrium
voltage between the Cathode and Anode.

Careful examination and measurement of a range of tetrodes and pentodes
demonstrates that the manufacturers' maximum rated
Screen-Grid and Plate Voltages for tetrodes and pentodes are generally (but not
always) directly proportional to the actual physical distance between these tube
elements.

Another way of saying this is that if we distribute maximum rated Plate
Voltage across the measured linear distance gap between Cathode and Anode,
the manufacturers' maximum rated Screen-Grid voltage usually (but not
always) directly correlates with the ratio of the physical gap between Cathode
and Screen-Grid to that of Cathode and Anode.

Second, if we consider the DC circuits in a push-pull ultra-linear
output stage we will see that the Screen-Grids are connected to a tap in the
primary of the output transformer.

The position of this tap was determined long ago by the experts to be
optimised at 40-43% of turns, measured from the primary centre-tap terminal and
the Anode terminal of each half.

Most importantly though, and disregarding voltage drop in the transformer
windings, the DC voltage applied to the Screen-Grids is the same as that
simultaneously applied to the Anode terminals and to the centre-tap
terminal.

In other words, in a conventional ultra-linear output stage the
Screen-Grids are energised to the same DC potential as the Anodes.

Third, if we consider the AC circuit in the same push-pull
ultra-linear output stage we will see that the Screen-Grids are connected to a
tap in the primary of the output transformer - as for DC.

However because the return AC circuit to the Cathode is through the bypass
capacitor at the transformer centre-tap terminal, the centre-tap terminal of the
output transformer is effectively at AC earth potential for all frequencies in
the circuit which, in a fixed bias amplifier, results in the centre-tap of the
output transformer being effectively directly connected to the Cathode of the
tube to complete the AC CIRCUIT.

Also, in the case of a sine-wave signal in a push-pull transformer output
stage, the centre-tap equates to the mid-point axis of the output signal and is
therefore, when connected to the Cathode via the bypass capacitor, also at AC
voltage zero - ie the output stage is "balanced" about the mid-point being the
centre-tap of the output transformer being at 0 volts AC potential.

OPTIMISED ULTRA-LINEAR © OPERATION.

Now, the Anode and Cathode of the tube form the positive and negative
electrodes for both DC and AC voltages across the tube, separated only by a
physical gap.

The assembly is mounted in a vacuum dielectric so current will flow between
the electrodes only when there is an electron stream between them.

OHMS LAW

If a current flows between two points in a conductor
across which the potential difference is E, then E/I is a constant - which is
known as the resistance R - between the two points.

Hence R = E/I

In the case of a vacuum tube, its PLATE RESISTANCE (rp) is defined as
being the resistance of the path between Cathode and Plate to the flow of
alternating current. It is the quotient of a small change in Plate voltage by
the corresponding change in Plate current and is expressed in Ohms, the unit of
resistance. Thus a change of 0.1 mA (0.0001 A) is produced by a Plate voltage
variation of 1 volt, the Plate resistance is 1 divided by 0.001, or 10,000 ohms.
(ref RCA RC-14)

If we now consider the AC signal output voltage distribution across the
vacuum tube we can easily see that the AC voltage gradient across the tube will
be the same as that across the half-primary of the output transformer - because
the half-primary AC impedance is the actual load on the tube. That is to say the
transformer is connected directly across the tube terminals - one end to the
Anode, and the other to the Cathode, through the negligible impedance B+ supply
bypass capacitor.

In this case R = the resistance of the internal Cathode
to Anode gap in the tube.

It follows then that when a 43% turns transformer tap is used to feed the
Screen-Grid, the AC voltage applied to the Screen-Grid by virtue of its position
in the electron stream, is 43% of the Cathode to Anode signal voltage.

Similarly, if an alternate portion of AC signal output voltage is tapped from
the output transformer (eg 50%) then the same portion will be applied to the
Screen-Grid.

This proposition might be better understood by envisaging the tube as a
voltage-divider. The Anode and Cathodes are the positive and negative line
terminals respectively and the Screen-Grid terminal is the load tap-off point.

Therefore, in a conventional ultra-linear output stage having equal Anode and
Screen-Grid DC voltages we have a serious mismatch between the AC conditions and
the DC conditions.

ie the AC signal voltage injected to the Screen Grids via the transformer tap
is only 43% of the Cathode to Anode voltage but the applied DC voltage to the
Screen Grids via the same tap is 100% of Anode voltage.

Historically, this AC voltage has been regarded simply as "negative feedback"
contributing to improving the circuit's performance, rather than a factor in
tube behaviour.

Furthermore, in the case of DC voltage in a conventional ultra-linear output
stage , the Screen-Grid is over-excited because its applied voltage is the same
as that applied to the Anode, requiring increased negative bias voltage to Grid
#1 to offset the increased current flow in the tube.

This phenomenon is particularly evident in the KT88 beam power tetrode, which
has a physical construction conforming to the 43% ratio as described, but has
been assigned manufacturers' Plate and Screen-Grid voltage ratings inferring a
tube behaviour something quite different to that.

Now in the case of AC voltage, the actual Screen-Grid voltage is
automatically determined by the fixed position/ratio of the Screen-Grid tap on
the output transformer - typically 43% of output signal voltage - resulting in
full B+ DC voltage but only 43% of AC signal voltage being present upon the
Screen-Grid at the same time.

Although the AC voltage derived from the output transformer Screen Grid
tap is of opposite polarity or phase to the current in the tube and, when
coinciding with a 43% portion of actual tube signal voltage at the Screen-grid,
will be of equal magnitude (potential) to the voltage in the tube at that point,
it is apparent that such a condition is essential to ideal ultra-linear
operation and performance.

The effect of mismatching the feedback signal voltage with the actual voltage
in the tube is not clear but it is assumed that some of the energy would be
consumed as waste heat - an undesirable outcome. Tube instability might be
another result. Increased distortion will be a definite outcome.

Linearity will also be definitely affected because the effect of the AC
signal component upon Screen-Grid operation, and therefore tube gain, will
increase as the signal amplitude increases. In other words, as power output
increases the effects from the AC/DC mismatch will increase.

To correct these anomalies it is desirable to do three things:

1.     Apply a DC voltage to the
Screen-Grid that is within the tube rating - ie not exceed rated
maximum
Screen-Grid voltage -
to reduce stress and prevent over-excitation

2.     Optimise both DC and AC
simultaneous conditions - ie to ensure the Anode voltage cannot fall below
the
Screen-Grid voltage
during any part of the signal cycle, apply a DC voltage to the Screen-Grid that
is
consistent with and
directly proportional to the AC signal voltage ratio (eg 43% of Plate
voltage)

3.     Use a tube that has its
internal element ratio consistent with the above - ie the physical gap between
Cathode
and Screen-Grid to be
43% of the gap between Cathode and Anode.

To fulfil the above criteria we need to select a tube that is capable of
operating OPTIMALLY when its Screen-Grid is at 43% of the ACTUAL Plate voltage
for both DC and AC components.

This requirement can only be achieved by tubes having their Screen-Grid
located in a fixed physical relationship with the Cathode and Anode such that
the Screen-Grid is located at 43% of the gap between Cathode and Anode.

Importantly, Hafler and
Keroes of Acro USA (1951) determined an optimum 43% ratio for the 6L6 and
807 families of tubes - later incorporated into the modified  Williamson
ultra-linear amplifier designs.

GEC/MOV Co. (1959) later determined that this optimum 43% ratio also
applied to the KT88/TT21/TT22 families of tubes.

The following graph, courtesy of GEC and AWV Radiotronics Magazine
(June 1959), shows the comparative characteristics of the KT88 in
triode, pentode and ultra-linear connections using selected TURNS ratios for
the ultra-linear output stage configuration.

This GEC graph very clearly shows that the optimum ratio is 43% turns (19%
impedance).
.

WHY 43% ?

Up until now, nobody has provided a detailed theoretical basis or explanation
for the ratio of 43% being the optimum for most popular power tubes of the 6L6
familiy and derivatives.

The actual numerical ratio was selected by Hafler and Keroes in their Patent
2710312 and was based upon empirical evidence derived from actual testing. They
said:

18.5% impedance = 43% turns ratio.

The ratio appears to have been set on the basis of maximising power whilst
holding distortion to within 1%

It was determined to be specific to a range of identified tube types - the
6V6G being a notable exception, where they determined the optimum ratio was 5%
impedance = 22.4% turns ratio

Recent research by Rudolf  Moers, a distinguished Electrical and
Electronics Engineer located in the Netherlands, tells us very explicitly - why
it is so.

Mr. Moers has graciously made available for us his wonderful recent
scientific investigation into the design theory and practice of Ultra-linear
audio amplification.

These papers are posted with permission from Linear Audio www.linearaudio.net
and their author Rudolf  Moers.

Part 1 - Power
Point presentation

Part 2 - Power
Point presentation

Parts 1 and 2 of the
above Power Point presentations - combined in pdf format

Paper - The
Ultra-Linear Power Amplifier: An adventure between triode and pentode - pdf
format

The engineering design methods developed by Mr Moers may be used to determine
theoretical plate/screen load ratios for ultra-linear operation of power tubes.

.
.
OPTIMISED ULTRA-LINEAR © - OPTIMUM RATIO

Notwithstanding all of the work described above covering the 80 year period
from Blumlein through to the present, if we look again at the work of Hafler and
Keroes (see graph above), and later F Langford Smith and A R Chesterman, we see
a different story.

This is shown is the graph below, as published in the May 1955 AWV
Radiotronics magazine. The tapping point is expressed as "impedance". Curves 3
and 4 show distortion for each of the upper curves respectively.

In this set of tests the loads and bias were adjusted for best performance -
ie maximum power at lowest distortion.

This result shows very clearly at curves 1 and 3 that for highest power
output with lowest distortion with tube type KT66, the OPTIMUM tap is to be set
at 15.2% impedance - ie 39.0% turns.

That result correlates EXACTLY with the physical design of the 6L6 Beam
Power Tube - from which the KT66 is copied - wherein the Screen-grid is
physically located at 39% of the Cathode to Plate distance.

For a very detailed explanation of Beam Power Tube design and construction,
refer to the engineering paper BEAM
POWER TUBES by RCA tube guru Otto Schade. Read with care, this paper
provides the reader with an excellent insight into the design rationale and
theory of Beam Power Tube design. This paper is part of a set published in RCA
Electron Tubes Volume 1 (1935-1941) and Volume 2 (1942-1948).

This result clearly shows that at 39% VOLTAGE, distortion is at its minumum.

Further detail is provided in the following table, prepared from the same set
of laboratory tests.

The closest tap to the optimum was set at 15% impedance, however the optimum
is 15.2% impedance and 39% turns.

However Langford-Smith and Chesterman state that "the optimum tapping point
for highest fidelity is between 17 and 20 % impedance, and it is not at all
critical".

So if your amplifier has a conventional 43% turns transformer do not throw it
away just yet !! All is well.
.
.
NOTE: The 6V6 family of tubes is
optimised with a 5% impedance and 22.4% turns ratio. That specification is the
minimum. In some conditions of bias and applied load the ratio may be increased
to 10% impedance. Refer to AWV Radiotronics June 1955

The ratio of 15.2% impedance and 39% turns holds true for most popular tube
types and forms the basis for the OPTIMISED
ULTRA-LINEAR © technologies described in these
pages.

Obviously where tube internal physical construction
differs then the ratio will change.

Important Note: It is evident that for any given
tube type number there will be variations in physical construction between
manufacturers. For best results push-pull pairs of power tubes should be of the
same manufacture and physical appearance
.
.
.

Tube Selection:

Tube selection can be readily determined by examining tube ratings, because
the maximum rated Plate and Screen-Grid voltages are generally directly
proportional to the physical spacing of the elements within the tube.

If we want to optimise the design of our ultra-linear audio amplifier
for absolute maximum performance there are few tubes that have the exact ratio
of Cathode to Screen-Grid and Cathode to Plate voltage that we need.

HOWEVER TO BE SURE, IT IS NECESSARY TO PHYSICALLY MEASURE THE ELECTRODE
SPACING WITHIN EACH TYPE OF TUBE.

At this point in time and on available information I am able to identify only
a small selection of tubes that by their internal construction fulfil our
criteria for OPTIMISED ULTRA-LINEAR ©
OPERATION.

For audio applications the following tube types offer nearly an exact match
when standard tube tolerances are considered.

300 VDC Maximum Plate Voltage and
125 VDC Maximum Screen-Grid Voltage

5618

500 VDC Maximum Plate Voltage and
200 VDC Maximum Screen-Grid Voltage

837

600 VDC Maximum Plate Voltage and
250 VDC Maximum Screen-Grid Voltage

6146 series
6159 series
6293
6883
series
7212 series
8032 series
8552 series

8298 series
6DQ6 series (receiving tube version of the
6146)
QE05-40 series
QV06-20 series
Note: The above
set of 6146 based family of tubes have identical plate characteristics and
require a standard octal socket.. Variances relate to heater voltage, plate
dissipation, construction, pinout and ruggedised construction.

TUBES FOR HIGHER OUTPUT POWER:

For the adventurous and brave-hearted who require higher power output the
following tubes are available for operation at up to 2250 VDC CCS Plate voltage.

1000 VDC Maximum Plate Voltage and
400 VDC Maximum Screen-Grid Voltage

814

In this case I would recommend reducing the applied Plate voltage to 1000 VDC
from the maximum rated 1500 VDC, to precisely align with the actual
Plate/Screen physical tube gap ratio.

1250 VDC Maximum Plate Voltage and
600 VDC Maximum Screen-Grid Voltage

TT21 (GEC, Marconi etc)
TT22 (GEC, Marconi etc)

1500 VDC Maximum Plate Voltage and
600 VDC Maximum Screen-Grid Voltage

803

In this case I would recommend reducing the applied Plate voltage to 1500 VDC
from the maximum rated 2000 VDC, to precisely align with the actual
Plate/Screen physical tube gap ratio.

1750 VDC Maximum Plate Voltage and
750 VDC Maximum Screen-Grid Voltage

828

The 828 has a maximum rated Screen Grid voltage exactly 43% of the rated
Plate Voltage.

2250 VDC Maximum Plate Voltage and
900 VDC Maximum Screen-Grid Voltage

813

In this case I would recommend reducing the applied Screen-grid voltage 900
VDC to precisely align with the actual Plate/Screen physical tube gap ratio.

Important:

All these high-output power tubes have a top anode cap and require
insulated anode caps and wiring to prevent electric shock to users.

All these tubes get very hot and require adequate ventilation.

Plate and Screen-grid connection lead lengths must be kept as short as is
practicable to prevent RF oscillation

Excepting for the TT21 and TT22, all the above tubes are directly heated
filament types

Output transformer windings must be capable of handling the peak signal
condition Plate and Screen-grid DC currents respectively
.
.

OUTPUT TRANSFORMER

It will be evident that if the Screen-Grid is to be supplied with a DC
voltage of only 43% of the Anode voltage, then a separate winding is needed for
the Screen-Grid connection. This is called a "tertiary winding".

Such a hi-fi transformer is available in a 120 W RMS rating, being the
Acrosound TO-350 (6600 ohms PP) and Dynaco Type A-441 (4300 ohms
PP).

Equivalents are currently available from some high-quality transformer
manufacturers - eg Sowter Type 8994 and Amplimo PAT-2100-SSCR.

Also applicable is the hi-fi Dynaco A-440 (? ohms PP) transformer,
being suitable for up to 300 W RMS.

A more recent development is the Pearl SC-280 transformer, designed
for high-power hi-fidelity applications.

The design of an ultra-linear audio amplifier having a separate Screen-Grid
supply is demonstrated by the Acrosound
100W 6146 and the Bruce de Palma 300W EL34
and the McIntosh
MC3500 and MI-350 high-fidelity amplifiers.

The McIntosh MC3500
and MI-350 use four pairs of 6LQ6/6JE6B power tubes in a "balanced"
push-pull arrangement, with a regulated supply to the Screen-grids. Importantly,
the ratio of Screen-grid DC voltage to Plate DC voltage is not specified in the
McIntosh Patent.  This amplifier also incorporates push-pull driver stages
throughout and no loop negative feedback from the loudspeaker. It is not
however an "ultra-linear" amplifier in the Hafler and Keroes definition - but
it does achieve a similar effect by other means. A truly mighty amplifier.

Note however that the voltages used in these designs do not correlate with
the optimal design criteria proposed above.

In the case of the Acrosound, the Anode voltage would need to be reduced to
600 VDC, and Screen-Grid voltage to 250 VDC (with some loss of power output), to
fulfil the 43% voltage ratio at maximum rated Screen-Grid voltage.

In the case of the Bruce de Palma EL34 design having an anode voltage of
1,000 VDC, the Screen-Grid voltage would need to be reduced to 430 VDC to fulfil
the 43% voltage ratio at maximum rated Screen-Grid voltage.

In terms of applied Screen-grid voltage, the McIntosh MC3500 is already very
close (38%) to being optimised to my OPTIMISED
ULTRA-LINEAR © concept (together with other more complex output
stage refinements) but the output transformer turns ratio is not available - so
unless you have access to the level of instrumentation and expertise essential
to tweaking this complex amplifier it would be wise to leave it as manufactured
to original McIntosh specifications. In practical terms, in my humble opinion,
it is unlikely even the most critical listener would be able to discern the
difference.

OTHER OPTIONS.

If the Screen-Grid tertiary winding ratio on the output transformer is
increased to 50% of turns, then other tube types become available as
viable options for OPTIMISED ULTRA-LINEAR ©
OPERATION.

Tube Type        Plate
Volts    Screen Volts

2E24
400
200 (directly heated filament type)

2E26
400
200

802
500
250 (directly heated filament type)

807
600
300

815
400
200 (815 = 2 x 2E26 in one botlle)

1610
400
200

1624
600
300 (directly heated filament type)

1625
600
300
6BG6GA
600
300

6W6GT
300
150

The GEC graph shown above suggests that using a 50% tap, power output will be
down about 10% on the 43% tap performance, however output impedance is lower.

On the other hand, according to manufacturers' data, the above tube types
offer maximum output at 50% Screen-Grid voltage.

RULES FOR OPTIMISED ULTRA-LINEAR ©
OPERATION:

1.    Plate to Screen-Grid Voltage
Ratio

Irrespective of actual DC Cathode
to Plate voltage, the applied DC Screen-Grid voltage must always be

maintained at 39% of actual
applied DC Plate Voltage - unless the tube internal physical design and
construction

requires a different ratio - eg 6V6G

2.    Transformer Ratio

The applied DC Screen-Grid
voltage must be in approximately the same ratio to Plate Voltage as the
actual
Transformer
Screen-Grid turns-ratio tapping point used.

3.    Transformer Tapping
Point

The Transformer Screen-Grid
tapping point, when expressed as a proportion of the whole
half-primary

turns,  must match the physical gap ratio of the Cathode to Screen-Grid and
Cathode to Anode elements in the
tube type used.

4.    Ideal Tube Operation

Each type of tube has an ideal or
optimal operating point for OPTIMISED ULTRA-LINEAR ©
OPERATION.

The optimal operating point will be that portion of turns
ratio matched to the tube physical and electrical characteristics that

provides optimal performance.

This can be determined by
reference to tube handbooks/catalogues and identifying the
recommended

Screen-Grid operating voltage. This is usually constant for the full range of
recommended plate voltages.

Except where otherwise determined
by physical measurement of the tube gap ratio, to determine optimum
Plate
voltage
multiply the applied Screen-Grid voltage by 100/39 (2.564)

This will be the optimum set of
Plate/Screen voltages for OPTIMISED
ULTRA-LINEAR © operation.

It is obvious then that if a particular tube type is
preferred then its physical characteristics need to be determined before

determining transformer ratio and applied Screen-Grid DC
voltage.

eg RCA Transmitting Tube Manual TT4 indicates the type
6146 tube has a recommended Screen-grid voltage of 175 VDC

for all recommended typical operating conditions - ie
Plate voltages set between 400 and 750 VDC.

To determine the ideal Plate voltage for OPTIMISED ULTRA-LINEAR © OPERATION multiply
175 x
2.564 = 449 VDC. Therefore the optimum operating
condition for tube type 6146 is 450 VDC on the Plate and

175 VDC on the Screen-Grid.

Experiments have shown me that these voltages are very
satisfactory for the 2E26 and 6146 types of beam power tubes

- both of which have aligned Screen-grids.

Neither the 2E26 or the 6146 will work in conventional
ultra-linear mode (even at low voltages) and will self-destruct due to

excessive Screen-grid current (irrespective of Grid #1
bias voltage) - even at relatively low plate/screen voltages - ie 300

VDC.

My experiments have shown also that neither of these tubes
works satisfactorily for hi-fi in triode connection either.

However in OPTIMISED ULTRA-LINEAR © OPERATION their
performance is spectacular, providing rich,
clean, powerful sound of magnificent
tone!!

Both the 2E26 and the 6146 family
are in my opinion two of the very best hi-fi tubes available when connected as
described
above.

When used in parallel-push pull
configurations they offer high power from very little AC signal driving
voltage.

*
*
*

IMPORTANT: It may be noticed that in the above listing the ideal
operating voltage for the Screen-Grid is set at the manufacturer's
recommended Screen-Grid voltage and not the Rated Maximum.

This is because the Screen-Grid requires a certain voltage to do its job best
- ie to attract electrons from the Cathode and accelerate them towards the
Plate. It is usually low in relation to the applied Plate voltage and is
invariably less than the rated maximum Screen-grid voltage.

That voltage is determined by the tube manufacturer and is usually published
in the tube data handbooks.

Note: Before deciding what transformer primary tap ratio to use it is
essential to physically determine the actual Screen Grid to Anode space ratio.

In most cases this can be easily done without having to destroy a valuable
tube. Just measure the spacing between Cathode and Anode and of the grid
support pins at the top of the tube. In the case of beam power tubes this may
be difficult because there is usually a box-like assembly covering the Plate
structure. But a bit of ingenuity should solve the problem

If it becomes necessary to destroy a tube to examine its internals safety
precautions must be observed:

1.    Wrap the tube in a strong cloth
2.
Place the tube in a vyce and squeeze the glass bottle slowly until it implodes

or
3.    Gently hit the glass with a hammer at a point
not directly over the electrode assembly
4.    Carefully
remove the broken glass and dispose of safely
5.    Carefully
cut away the electrodes until the elements can be measured

*
*
*

RECOMMENDED
CONFIGURATION

For those who wish to build the ultimate in OPTIMISED ULTRA-LINEAR © I
would recommend the following configuration:

OUTPUT TRANSFORMER - High Fidelity

Primary winding for plates - 100% plate to plate load
impedance and comprising FOUR separate sections each having equal turns. Connect
two windings in series for each half.

Tertiary winding for screens - 50% of Plate to Plate
Primary winding TURNS and comprising TWO separate sections each having equal
turns.

The primary and tertiary sections are therefore of
equal turns.

Secondary winding - 5 separate sections or 7 separate
sections having equal turns and interleaved with the primary and tertiary
windings. Connect in SERIES to make up full loudspeaker matching
impedance.

Why 50% ratio??

Because it makes transformer design and manufacture
simple (easier to count the same number of turns for each winding during
manufacture), provides more flexible connection options, increases the level of
AC signal feedback to the Screen-grids (increasing the damping factor), reduces
the output impedance very slightly and provides better tone (than 43% turns
tap).

A slight loss of power and slight increase in
distortion may be measured compared to the conventional 43% configuration (see
graphs) but with four pairs of tubes as recommended that should not be a
problem.

The graph above shows at curves 2 and 4 that a 25%
impedance and 50% turns ratio is fine.

However to be precise the ratio should generally be 39%
turns and 39% DC voltage.

Note: As in all high-fidelity transformers, the turns
should be exactly matched as applicable.

OUTPUT TUBES - FOUR
pairs of tubes in parallel-push-pull

This has the effect of reducing the output impedance to
one quarter of that for one pair of tubes.
This
has the effect of reducing the transformer turns ratio to one sixteenth of that
for one pair of tubes.
AC signal drive voltage
is the same as for one pair of tubes enabling cathode-follower drivers to be
utilised (recommended also) - see GEC 400W amplifier circuit for a typical
configuration.
A proportionately larger final
filter cap is recommended to provide the extra current needed to handle
transient signals - eg 500 uF.

Preferred output power tube type - BEAM POWER
TETRODE TUBE with aligned grids

My picks for best results are:

2E26
6146, 6146A,
6146W, 6146WA, 7212

6146B/8298A

6159, 6159A, 6159B
(26.5V)
6293
(check ratings)
6883A/8032 (12.6V)
6883B/ 8032A/ 8552
(12.6V)
807
813
(10V filament)

All have a top anode cap.
All
are of broadcast/military quality and offer extra features (such as ruggedised
construction and reliability) compared to domestic/receiving quality
tubes.

Both the 2E26 and 6146 require a standard octal
socket.
Both the 2E26 and 6146 have relatively
large cathodes.
The 807 requires a 5 pin
socket.
The 813 requires a giant seven pin
socket.

All the above are readily available in vintage NOS or
used condition and at affordable cost.

Other Options

If no anode cap is permissible then consider the more
recently developed KT90 tube

Its manufacturer, the "Ei-RC" - Electronic tubes
factory" recommends a maximum Plate and Screen-grid voltage of 650 VDC for
conventional ultra-linear, however the plate voltage can be increased to 750 VDC
for pentode connection.

Consequently for OPTIMISED ULTRA-LINEAR © operation,the conditions may be set to up to 750 VDC Plate and
300 VDC Screen-grids.

However this high Plate voltage may lead to flashover
at the octal pin socket, hence it is recommended that the Plate voltage be
limited to 600 VDC and Screen-grid to 250 VDC

The KT90 is recommended by Ei-RC as a replacement for
6550, 6550A and KT88 however, to optimise performance, the operating conditions
must be adjusted to suit the KT90 characteristics.

Of course OPTIMISED
ULTRA-LINEAR © will work with the more popular
but lesser performing tubes such as EL34, 6L6GC, KT66, KT88 and 6550 provided
the Screen Grids are terminated to a separate tertiary winding and supplied at a
DC voltage being not more than 43% of Plate voltage.

PLATE AND SCREEN GRID VOLTAGES:

With the above described transformer arrangement DC
Screen grid voltage will be set at 50% of the DC Plate voltage.

FURTHER ENHANCEMENT:

MULTIPLE LOUDSPEAKERS

Use multiple loudspeakers connected in series. This
increases the total load to the amplifier, thereby reducing the transformer
ratio to a relatively low order, enhancing fidelity and tone.

It also has the effect of reducing total
effective loudspeaker capacitance and increasing total effective
loudspeaker inductance as presented to the amplifier. This increases stability
and linearity in a vacuum tube amplifier - but maybe not in a solid state
unit.

Using four tubes in parallel and four speakers in
series enables a transformer to be used that has only one quarter of the turns
ratio needed for a conventional single pair of tubes driving a single
loudspeaker configuration (having the same single unit impedance).

eg     8000 ohms P/P to 8 ohms
requires a transformer having a turns ratio of 31.6

2000 ohms P/P to 32 ohms requires a transformer having a turns ratio of only
7.9

Clearly if more speakers, or 16 ohm speakers instead of
8 ohms are used, then the situation can be improved proportionately.

My current output transformers are 2500 ohms P/P to 64
ohms, requiring a turns ratio of only 6.25:1
In
a conventional arrangement the ratio would be 10,000 ohms to 8 ohms, requiring a
turns ratio of 35.4:1 - nearly six times worse.

Such a configuration offers better performance from the
same standard components as used in a conventional single-pair amplifier.

Naturally, transient response is
excellent!!

NOTE: All the above comment refers to an amplifier
having no interstage negative feedback - eg from the output transformer
secondary winding to a previous stage.

*
*
*

IMPORTANT NOTICE

THE AUTHOR MAKES NO CLAIM WHATSOEVER AS TO THE VALIDITY
OR ACCURACY OF ANY STATEMENT, INFORMATION OR OPINION CONTAINED IN THESE PAGES
AND NO LIABILITY WILL BE ACCEPTED FOR ANY ERROR OR OMISSION OF ANY KIND
WHATSOEVER.

PLEASE NOTE NO WARRANTY IS EXPRESSED OR IMPLIED AS TO
THE WORKABILITY OR PERFORMANCE OF DESIGN INFORMATION DESCRIBED HEREIN.

© NOTICE:    INTELLECTUAL PROPERTY
COPYRIGHT D.R.GRIMWOOD 2002 - ALL RIGHTS RESERVED.

NO PART OF THIS TECHNOLOGY MAY BE USED FOR
COMMERCIAL PURPOSES IN ANY FORM WHATSOEVER INCLUDING BUT NOT LIMITED TO PRODUCT
DEVELOPMENT OR GAIN OR REWARD WITHOUT THE EXPRESS WRITTEN PERMISSION OF THE
AUTHOR.©

Intellectual property in the applied engineering
concepts expressed in this paper remains expressly and exclusively with the
author.

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This page last modified 26 March 2014

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