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DISTRIBUTED LOAD
HIGH-FIDELITY VACUUM TUBE
AUDIO AMPLIFIERS
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Readers please note that in November 2026 the history section "How It All
Began" of this page was revised in response to new information becoming
available to me by chance. In 2030 ultra-linear technology will have been with
us for a century.
ULTRA-LINEAR
OPERATION, also known as DISTRIBUTED LOAD OPERATION, is a term when applied
to single-ended or push-pull vacuum tube audio amplifiers,
describes the particular output stage configuration whereby the Screen Grids
(Grid 2) of Tetrodes, Pentodes or Beam Power Tubes are fed from a tapping in
the output transformer (single-ended) or each half of the primary of the output
transformer (push-pull) - typically 43% turns or 18.5% impedance when measured
from the centre-tap, instead of from a DC supply either independent of, or
common to, the anodes.
The sonic
properties of the ultra-linear output stage configuration - albeit single-ended
or push-pull design - are midway between triode and tetrode/beam power tube
"tone".
Some
audio-engineers describe the sound of triodes as "smooth, sweet, mellow,
musical, natural" and tetrode/beam power tubes as "clean, bright,
sharp, dynamic, punchy". The tonal properties of Ultra-linear mode are
somewhere in between, losing some of the purity of triodes and some of the
dynamic response of beam power tubes.
Technically
speaking, the ultra-linear configuration is
capable of delivering the same power output as for pentode operation of the
same tube under optimum 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).
The ultra-linear
configuration also offers improved overload characteristics, resulting in more effective
power output – i.e. what the listener actually hears at higher power levels.
Output
impedance is similar to triodes, allowing minimal or zero negative loop
feedback to be used.
The ultra-linear amplifier concept is
beautifully described by David Hafler and Herbert Keroes in their 1952 US
Patent Application 2710312.
The
following extract is included for convenience:
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Three years later in 1955, after granting of US Patent
2710312, and supported by further research, verification and validation, Hafler
and Keroes published a comprehensive booklet to promote their Acrosound product
range of ultra-linear amplifiers and transformers.
This rare
booklet has been made available to us from the private library of Dr. Andre G.
Routh, who has generously provided a copy for our study.
To view
click here: "Theory and Operation of the Ultra-Linear
Circuit"
Note the
theoretical analysis for determining screen tap position as expounded in pages
15-17. The extensive "Radiotronics"
papers on ultra-linear amplifiers are also referenced and linked below in this
web page.
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The following illustrations show the conventional Ultra-Linear configuration in
three typical arrangements.
The first
shows the Ultra-Linear output stage with Cathode Bias having a common bypassed
Cathode Resistor.
Superior results will be achieved by the use of a separate bypassed Cathode
Bias Resistor to each tube.
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This next illustration shows Fixed Bias Ultra-Linear
operation, with each power tube having an individually adjustable bias supply
for improved balance in the output stage - essential for large power tubes:
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Because
the Screen-Grids are supplied with DC from taps on the output transformer, the ultra-linear
configuration therefore avoids the need for a separate Screen-Grid supply.
The third
illustration shows a variant of the Acrosound Ultra-Linear design, also
by Hafler and Keroes, which features a separate and lesser voltage supply to
the Screen-Grids for improved performance and cooler operation.
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The ultra-linear output configuration is also
suitable to single-ended output stages, however only push-pull
amplification is discussed in this paper.
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HOW IT ALL BEGAN
The 1920's and 30's were decades of massive research and development in
the field of radio communications.
ELON
UNIVERSITY reports an excerpt from Janna
Quitney Anderson’s book “Imagining the Internet:
Personalities, Predictions, Perspectives,” published by Rowman &
Littlefield in 2005:
"In the boom of the 1920s, people rushed to buy
radios, and business and social structures adapted to the new medium.
Universities began to offer radio-based courses; churches began broadcasting their services;
newspapers created tie-ins with radio broadcasts.
In the case of radio, as with all breakthroughs in the development
of human communication tools from the telegraph and onward, the truth is that
many inventors made contributions to its creation, refinement and successful networking and distribution.
The development of a vast array of programming choices in
the 1930s brought the “Golden Age of Radio,” and by 1939 nearly 80 percent of
the United States population owned a radio." (end quote)
Researchers and investors across
the whole world devoted extensive resources into the development and
commercialisation of this new technology that developed before, during and
after WWI – and continues to this day.
Audio and electronic audio
reproduction was also a major focus because radio and audio electronics are
inextricably interdependent. Radio broadcasts of voice and music depended upon
being able to hear them. Audio reproduction in that era was of very poor
quality, offering an open-ended goal towards high-fidelity. Audio engineering
was a new field and competition intense.
One
respected authority, F Langford Smith of Radiotron
Designers Handbook fame, attributes the original design concept to a pair
of Australia inventors, R. Lackey and R.R. Chilton of the Australian Radio
College in 1933, however the documented evidence has been lost. (If any reader
has that information please email it to me)
Google AI
reports:
Historical
Context & Documentation
·
The 1933
Student Notes: Lackey and Chilton developed the topology—which feeds a
portion of the output signal back to the vacuum tube's screen grid via an
output transformer primary tap— and
distributed it as student notes at
the Australian Radio College in Sydney in 1933. [1]
·
Loss of
Original Records: The earliest physical records and original notes from 1933
were destroyed in a fire. Because no surviving 1933 manuscript was publicly
available, formal patent priority later
went to Alan Blumlein (British Patent
496,883 in 1937) and later to Hafler and Keroes (1951/1955). [1,
2]
·
1955 Radiotronics
Study:
R. R. Chilton later partnered with renowned Australian audio engineer Fritz
Langford-Smith (author of the Radiotron Designer's Handbook) to
publish an exhaustive three-part
technical investigation titled "Ultra-linear Amplifiers" in
the May, June, and July 1955 issues of the Australian journal Radiotronics.
In that paper, Langford-Smith and Chilton formally documented the 1933 origin and appealed to former students for any
surviving copies of the 1933 notes. [1,
2]
·
·
However, contrary to the above
record, Pat Hawker's
Technical Tips page in the Radio
Communications magazine of July 1994 – Page 55,
refers to the 1931
EKCO RS3 domestic wireless receiver design incorporating the ultra-linear concept – but not
identified as such in that era. The new design was simply described as "pentode screen connection to
transformer primary" – a concise and obvious description
reportedly designed by Ekco’s in-house designer, J.
K. White.
Pat
Hawker's "Technical Tips" report is authenticated by a feature
article in the Wireless
World magazine of August 26 1931 (page 210), also reproduced at the Classical
Radio Gallery
Wireless
World describes the ECKO design as "an
arrangement which produces the necessary tone control correction required with
the pentode output valve".
The
article to the Wireless World of August 26 1931 opens up a whole new view of history.
This irrefutable evidence suggests that Blumlein's patent was invalid because
the design had been in the public domain six years prior to his claimed
ultra-linear invention - Application Date: June 5, 1937. No.
15620/37 by its commercial
exploitation and published information in the Wireless
World magazine of August 26 1931 and service information.
Consequently, the ultra-linear design
concept can now be confidently attributed by documented evidence to the E.K.
COLE LTD. (EKCO) UK Design Team in 1931 – specifically J.K. White.
Service
Data for the EKCO RS3 is available at https://elektrotanya.com/ekco_rs3_ac_radio_1945_sm.pdf/download.html
and https://classicradiogallery.com/radiopages/schematics/Ekco_RS3_schem.pdf
Notably
the ultra linear configuration in the RS3 was not continued into the next EKCO
model SH25.
This may
explain why White's ultra-linear design disappeared into obscurity. In a
competitive environment the extra cost of tapping the output transformer and
additional wiring would not have been justifiable in a small domestic receiver.
If it was used again in the EKCO range please email details.
Six years
later in 1937, "a British subject" Alan.D.Blumlein - see British Patent 496,883 June 5, 1937. (Application No.
15620/37) successfully applied to patent the design. See also expired US Patent
449,4776. Also US Patent 2218902 (22 October 1940).
Further
reading on the life and times of this remarkable man are available at
http://en.wikipedia.org/wiki/Alan_Blumlein
Blumlein's
Patent specified an optimum feedback tapping ratio being between 25 and 50% of
plate output voltage to the Screen Grid, however he did not use either of the
terms "Ultra-Linear" or "Distributed Load" in
his Patent.
The much
later detailed work of D. Hafler and H. I. Keroes of Acro USA
(1951) demonstrated the optimum feedback voltage ratio to the Screen
Grid for a range of defined tube types.
To our
benefit, they published a paper on the topic in the November 1951 edition of
the US "Audio Engineering" magazine.
Specifically,
the 6L6, 807and KT66 families prefer a Screen-grid load of 18.5% Plate load
impedance. (43% turns or 43% Plate Voltage).
Interestingly,
they also claim the 6V6 family prefers a Screen-grid load of 5% Plate load
impedance. (22.5% turns or 22.5% Plate Voltage). This is understandable because the 6V6 tube
has a rated screen grid voltage close to the rated plate voltage – an unusual
configuration, but optimal to reduce costs for audio amplifiers because the B+
can be common to both Plate and Screen tube elements in common domestic
applications.
This
concept is fully described in US Patent 2710312,
dated June 7, 1955. History shows Hafler and Keros successfully
patented in the USA a further development of the general concept that had
already been patented by Blumlein in the UK, had run most of its full term,
then expired (maybe not quite, because the H and K application was lodged May
20, 1952). Their claims included a level of impedance loading to the Screen
Grids of between 5% and 26% Plate Impedance - a much lower range to that of
Blumlein.
This ratio
was later verified by other respected designers, including GEC/MOV, GE USA, RCA and AWV.
See also
the thoughts of Norman Crohurst.
Of
particular interest is the Australian work of F Langford-Smith (of Radiotron
Designers Handbook fame) in 1955, which investigates the Ultra-linear
concept in detail for a range of tube types.
See AWV
Radiotronics Magazine May and July 1955
It is
worth noting that the original Blumlein Patent would have remained in force until
1953, so not much would have - or could have - happened regarding practical
implementation of this technology in commercial products by other than Blumlein
until after that date.
However,
long before Blumlein's patent expired he was killed in the year of 1942 in an
aircraft crash whilst testing a new British radar design. Thus we can only
speculate as to what might have been had he survived. It would be reasonably
safe to assume that hi-fi audio would have been allocated a very low priority
during the years of WWII during which Britain was struggling to survive.
During
that period pre-war triodes were used in most UK hi-fi equipment. It was not
until 1949 that the triode connected Williamson and 1953 Peter Walker's
distributed load Quad amplifiers using KT66 tetrodes were released.
Since the
famous "Williamson" amplifier design appeared some years after the
1932 EKCO RS3, it is reasonable to assume the "ultra-linear" concept
was either safely locked away, or rejected by the gurus of the time in favour
of triode mode. In fact, Williamson not only rejected the term but openly
condemned the concept altogether - see Williamson
and Walker - Wireless World - September 1952.
Williamson and Walker asserted:
"Circuits
with Distributed Loads.-Articles 2, 3 have recently been published in the
United States claiming the superiority of a so-called “ ultra-linear ” output
circuit in which the output valves are
used as tetrodes, with negative feedback applied non-linearly by connecting the
screens to a tap on the primary of the output transformer.
It
is stated that the performance is audibly improved over that of triodes with similar
degrees of negative feedback.
The
present writers do not believe this claim. The circuitry which forms the basis
of these American claims for “ultra-linearity” and higher efficiency has, in
fact, been familiar in this country for
several years, and the technique has been further developed and used in a
commercially produced high-quality amplifier". (quote)
Both
authors had something to lose from the Acrosound competition from across the
Atlantic. Neither design could compete on power and price.
Despite
their caustic comments Acrosound
responded with an ultralinear version of the Williamson amp or https://www.hifiengine.com/manual_library/acrosound/ultra-linear-transformers.shtml
This
little bit of nationalistic one-upsmanship was a little premature because just
two years later the impressive PYE HF25
"Provost" Hi-fi ultra-linear amplifier and matching HF25A
"Proctus" Preamplifier were produced in the UK – the existence of which I discovered
only in late 2026 by chance, resulting in this web page being revised and
updated.
This gem
has been hidden away from the hi-fi history world for 75 years.
Pye say it
was released in 1954.
Full
details, specs, manuals and photos are at https://drive.google.com/file/d/1QUrnefr0Fch9ynUbHbTucwvIvpprY_b2/view
The design
of this amplifier is conventional for its time but component quality appears to
be of a high standard. Claimed frequency response is flat from 2 Hz to 160 kHz.
THD less than 0.3% at 25 Watts rms. The design uses Cathode Bias so is safe and
reliable, but would not compete with the Acrosound fixed bias system for
dynamic response.
Both
amplifier and preamplifier incorporate some novel features.
DIY Design Note: Due to practical
limitations with the capacitors of the time the filter caps have a low value by
modern standards (C13 and C14 - 2 x 64 uF series connected = 32 uF effective) ,
however . Dynamic performance of this
amplifier could be improved by using a 100uF 500VDCW electrolytic filter cap AFTER the filter choke to
replace C13 and C14. The first cap of 8 uF (C15)
could be safely increased to potentially 40 uF depending upon circuit component
design characteristics – refer to the Philips GZ32 data sheet for ratings. Note
though that increasing C15 will
reduce the DC B+ output voltage. Is a
trade-off. A 440VAC polypropylene motor start capacitor would be a worthwhile
replacement investment for each of C15 and the set of C13 + C14 instead
of electrolytics.
Of
important significance is that PYE did not use either term "ultra-linear"
or "distributed load" to describe the amplifier, describing it as
"high fidelity".
A little
earlier, Pye introduced its "Black Box" portable record player unit
in 1953. The early versions used several conventional single-ended and
push-pull amplifier designs. Then in Late 1954 the MKII model was released
using an ultra-linear output stage and advertised as an
"ultra-linear" amplifier. Performance details and the schematic are
published at https://www.hifinews.com/content/pye-black-box-record-player
Thus the term "Ultra-Linear" may be
rightfully attributed to Hafler and Keroes - an opinion supported by the
"Radiotron Designers Handbook 4th Edition. This is the term used in their 1955
US Patent 2710312 (Application date 20 May, 1952) and is supported in that
Patent by technical justification for use of the term.
The
question is - "How many ways is it possible to configure a 4 electrode
device in an electronic circuit?"
Of course the
answer is that someone, somewhere, at sometime will figure it out – and they
did.
The famous
Williamson triode-connected beam power tube amplifier was later modified
by Hafler and Keroes to incorporate the ultra-linear design concept.
Details are provided in their paper "Ultra-linear operation of the
Williamson Amplifier" by Hafler and Keroes, published in the June 1952
edition of "Audio Engineering" magazine. It seems then that they had
the last word, by improving Williamson's design despite his dismissal of the
concept.. Numerous examples of this particular design are now available on the
internet.
The Ultra-linear
configuration in audio amplifiers is also known as "DISTRIBUTED LOAD"
operation.
It appears
this term was intoduced by Willamson and Walker of the UK, who objected to the
term "ultra-linear".
Of course
this term is also technically correct, because the taps on the output
transformer primary do in fact present a push-pull reactive load to the Screen
Grids, enabling them to contribute to useable power output. This is explained
in detail in the Hafler and Keroes Patent.
However
importantly, Mullard describe the "distributed load" configuration as
a system of negative feedback.
The term
"distributed load" was adopted by Mullard as corporate policy. Their
use of this term was published in the May and June 1955 editions of "Wireless
World" in their article "Design for a 20 Watt High Quality
Audio Amplifier".
It is
interesting to note that this design appeared from Mullard, notwithstanding Williamson's
previous condemnation of the concept.
I am
unable to verify the specific designer of the 5/20 amplifier but later credits
go to Mullard Applications Research Laboratories Engineers Mssrs R. S.
Babbs, D. H. W. Busby, P. F. Dalloso, C. Hardcastle, J. C. Latham and W. A.
Ferguson. (No mention of Williamson and Walker). It is assumed the commercial
pragmatists won the argument.
Mullard
continued to use this term through until at least the 1960's when their
compendium of audio amplifier and pre-amplifier circuits "Mullard
Circuits for Audio Amplifiers" was published as a second edition
reprint in 1962.
The
following extract from that publication provides an overview of Mullard's
design concept.
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For an excellent technical introduction to the ultra-linear
design concept see http://www.aikenamps.com/UL.pdf
More
technical detail, single-ended ultra-linear circuits and non-power tube ultra-linear
applications is also available from Glass Audio - "Tube CAD
Journal" Vol. 2 No. 1 - January 2000
For further
views and technical details see also http://www.vintageradio.me.uk/amplifier/10watt.htm
Important Note: Performance described as
"ultra-linear" is available only under very specific operating
conditions - and in the case of popular commonly used audio tubes, usually
where the Screen load is 18.5% impedance or 43% turns of the Plate load
impedance - measured from the power supply source (B+) terminal.
More detailed information is provided below.
In other cases, the term "distributed load
operation" may be more appropriate.
Tube
Operating Conditions:
The
attached graphs, courtesy of GE USA, show the marked difference in operating
conditions for pentode, triode and ultra-linear operation of the 6550 Beam
Power Tube.
6550 Beam Power Tube Operating Conditions - Pentode Connection
6550 Beam Power Tube Operating Conditions - Triode Connection
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Benefits:
The ultra-linear
configuration ensures constant stage specific voltage feedback between Plate
and Screen grid in the output stage, thus reducing output impedance and
distortion, whilst improving linearity and frequency response under variably
reactive loudspeaker load conditions.
An
important advantage is that being single stage-specific, ultra-linear
operation avoids the problems of time-delay and phase-shift commonly associated
with cascaded stage amplifiers using negative feedback from the loudspeaker for
the purposes of reducing distortion and increasing loudspeaker damping.
Ultra-linear output stages
provide automatic constant ratio stage-circuit specific feedback, free from the
adverse effects of conventional multi-stage feedback systems.
Ultra-linear output stage power
output is dependent upon the proportion of Plate load that is applied to the
Screen-Grids - typically in the range 75% to 100% that of tetrode/pentode
connection at the same plate voltage - but is still twice triode connection for
the same tube type under the same operating conditions.
Ultra-linear tone or
"sound" approximates that of triodes.
Ultra-linear operation is very
forgiving of circuit design and generally provides an acceptable quality sound
from average quality output transformers. Note however that the esteemed Norman
Crowhurst, in a November 1959 "Audio" Magazine article
entitled "Puzzled About Amplifiers", indicates that a
high-quality output transformer is essential for ultra-linear operation,
to ensure high-fidelity performance over the entire audio frequency range.
To this
end, Herbert Keroes of Acrosound developed a special transformer for ultra-linear
configuration output stages. The main purpose of this transformer is to enable
the use of transmitting tubes, where the Screen-grids must operate at a
significantly reduced DC voltage to that on the Plates. Full specifications are
provided in US Patent 2791646
(May 7 1957).
A set of
high-fidelity amplifier designs was published in the Acrosound Ultra-Linear
Transformer Catalogue, published a little later.
The
flagship of the range - the TO350 - offers 100W from a pair of 6146 tubes. This
is an exceptional amplifier by any standard.
Design
Concept:
A good
example of a typical conventional ultra-linear circuit is shown in the GEC KT88 100W
Amplifier.
A more
highly developed design is shown in the GEC KT88 400W Amplifier.
The
conventional ultra-linear configuration is arranged such that the screen
grids have maximum DC voltage applied to them (in practice slightly higher than
their corresponding plate/anode voltage because of voltage drop in the primary
winding) - thus maximising power output and efficiency, whilst simultaneously
receiving an AC negative feedback signal from the output transformer - thus
minimising output impedance and distortion.
The
magnitude of the AC feedback signal is directly proportional to the percentage
turns ration of screen tap to full winding.
The screen
tap may be positioned anywhere from 0% (pentode connection) to 100% (triode
connection), however performance and tonal qualities change relative to the
ratio of the screen tap - see GEC graph below.
In
practice, research by the inventors of this method of amplifier configuration
suggests that the ideal for most tube types is in the range 40 to 50% turns of
each half primary, measured from the primary centre tap.
Further
research by Mullard UK for the EL34/6CA7 and EL84/6BQ5, GEC/MOV Valve Co. for
the KT88, GE USA for the 6550, and RCA for the 6973 (6CZ5 hi-fi), 7027A
(6L6GC), 7591, 7868 (6L6 family), also recommends 43% turns (or 43% of plate
signal voltage), or 18.5% impedance.
An example
of EL34/6CA7 operation with cathode bias is shown in the following graph -
courtesy Amperex Electronic Corporation
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Screen-Grid Operation:
One very
important feature of the ultra-linear configuration - different to
normal Tetrode/Pentode operation where both Screen-Grids are at nominal AC
earth potential - is that when one Screen-Grid in a push-pull pair is driven
positive, the opposite Screen-Grid is driven negative by the turns ratio of the
output transformer acting about the centre-tap of the transformer, which is at
nominal AC earth. This is not a problem because the opposite Plate is also
being driven negatively anyway by the action of the push-pull driver/phase
splitter.
The
opposite applies when the alternating signal reverses polarity.
In Class B
operation, the output transformer operates as an auto-transformer, so the
opposite Plate and Screen-Grid are still driven to opposite polarity (together
in a constant ratio to each other), even though they are not conducting.
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Balanced
Amplifier
It should
be noted that in the case of a push-pull amplifier, the DC negative bias
voltage (even if it is at 0 VDC) applied to the Control Grid #1 is located at
the centreline axis of the balanced input signal. This means the inputs
are floating and the centre-axis is earthed. That is to say, the input Grids
are being driven in a push-pull manner about a common centre-point or axis - eg
as seen with a centre-tapped push-pull driver transformer.
Thus in a
conventional push-pull Tetrode or Pentode amplifier, any alternations of the
balanced input signal to the Control Grids will proportionately increase or
decrease current flow in both tubes of the push-pull pair in response to the
input alternating waveform shape.
Now in
each tube of a push-pull pair, the negative terminal (Cathode) of each tube is
AC earthed - even in Cathode bias.
The load
on each tube is connected between the Plate and the transformer centre-tap, so
the negative terminal of the load - which is at the output transformer
centre-tap - is also effectively AC earthed.
But most
importantly to ultra-linear operation, in a push-pull amplifier output stage,
Screen-Grid behaviour will be similar to that of Grid #1 - ie the Screen-grids
will behave as a balanced amplifier - balanced about the centre-tap of
the output transformer which, as previously noted, is effectively AC earthed -
ie variations in Screen-Grid voltage will produce proportionate variations in
electron flow in both output tubes simultaneously.
In
conventional Tetrode and Pentode operation, when the DC applied voltage to the
Screen-grids is constant, the AC voltage appearing at the balanced
Screen-Grid terminals will the product of the electron flow within each tube
and will always be a voltage determined by the natural AC voltage gradient
applied internally across each tube.
Also in a
conventional Tetrode/Pentode output stage, the AC signal voltage
appearing at the Screen-Grids is diverted to AC earth at the Screen-Grid
terminals via the filter capacitor and is lost as heat. Hence the output
voltage appearing at the Screen-Grid terminals is of no consequence to the
sound produced by the amplifier - ie it is not reproduced in the output
transformer or loudspeaker.
However,
in the case of the Screen-Grids in ultra-linear push-pull amplifier
configuration this is not so, because the Screen-Grids are located at about 43%
of the lineal distance between Cathode and Plate. Thus the AC signal voltage
appearing at each Screen-Grid as a result of linear voltage gradient between
the Cathode and Plate within each tube will thereby be about 43% of the Cathode
to Plate signal voltage.
The
Screen-Grids are connected to the load so they will contribute to the sound
produced by the amplifier.
Class A
and Class B Ultra-Linear Operation
In some
transmitters, it has been the practice to drive or control the output power
tube by means of the Screen-Grid, rather than the Control Grid.
This
method offers some benefits to RF situations but is relevant to the explanation
of ultra-linear audio amplifiers.
Because
the Screen-Grid is located much further into the physical Cathode to Plate
distance - ie typically nearly centrally between them - it follows that a
substantially higher AC signal voltage must be applied to the Screen-Grid if
that element is to control the electron flow in the tube.
Operation
will be the same as for Grid #1 but at a higher AC voltage.
It follows
that if the DC Screen-Grid voltage controls electron flow within the tube and
it is varied by means of a superimposed AC voltage, then the Plate Current will
vary in response - as for conventional Grid #1 operation.
But what
if the Screen-Grid voltage is applied in 180 degrees directly opposite phase to
the electron stream within the tube - as is the case for ultra-linear
operation?
Obviously the
electron stream permitted by the AC signal as applied to Grid #1 is offset by
the opposing signal voltage applied to Grid #2 via the tapping on the output
transformer - because they are both in the same circuit at the same time.
Consequently
behaviour of the tube in response to the controlling signal voltages applied to
BOTH Grid #1 and Grid #2 simultaneously, will be different to that of either a
Triode or Tetrode.
This is
what the above GE graphs demonstrate - ie a change in the operating characteristics
of the basic tube, verifying the claim of Hafler and Keroes that they had
produced a "virtual" tube, intermediate between a triode and a
tetrode.
The
situation is however very different between Class A and Class B operation.
In the
case of Class A ultra-linear operation, the Screen-Grid of one tube will be
driven AC positive but the Screen-Grid of the opposite tube will be driven to
an equal, but opposite polarity, voltage by the output transformer balanced
output about the AC earthed centre-tap. This will have the effect on the second
tube of reducing the effective Screen-Grid control voltage, thereby reducing
voltage gain and therefore power output - regardless of the shape of the input
waveform to that tube.
But also
note that Grid #1 of both tubes is also controlling current in them.
Hence in
Class A ultra-linear operation, it is necessary to consider the effects upon
Cathode current in BOTH output tubes by BOTH Control Grids and BOTH
Screen-Grids.
However in
the case of Class B ultra-linear operation, although the second tube will be
similarly driven to reduce its gain, there is no signal in it at that moment in
time because its Control Grid #1 is at cutoff bias during the relevant
half-cycle of signal input so zero or near zero current is flowing in that
tube.
Thus a
Class A ultra-linear amplifier will have completely different behaviour to a
Class B ultra-linear amplifier.
Distributed
Load Operation
On the
other hand, that portion of the output transformer primary winding between the
Screen-Grid tapping and the centre-tap is not subject to cancelling out by out
of phase signal in the power circuit.
This
portion of the winding - usually 18-19% Plate to Plate load impedance -
therefore imposes a load directly onto the Screen-Grids.
Thus a
portion of the power output is delivered through this Screen to Screen winding.
However,
although the winding and its corresponding load impedance is presented to the
Screen-Grids, they are incapable of delivering much power because the
Screen-Grids will be aligned in the electron stream such that the small
diameter grid wires are not directly in the electron stream - so because of
space-charge effects cannot attract electrons.
Furthermore,
the Screen-Grids are charged to opposite polarity to the electron stream during
half of each signal alternation cycle, which further detracts from their
electron collecting capability.
For the
record, in their US Patent 2,710,312 Hafler and Keroes state: "It
should be noted that power is transferred to the Screen only over part of the
signal cycle, i.e. when the absolute value of Plate potential falls below the
absolute value of Screen potential. This transfer has the effect of linearizing
the Plate characteristics."
Power
Output
Ultra-linear operation
typically (for the popular audio tube types) delivers 100% power output
compared with the same tubes in tetrode or pentode connection at the same
plate voltage and bias system (GEC) - but sometimes less for other tube
types.
For
example, Mullard quote the power output for the EL34 tube as being the
same for pentode and ultra-linear connection with 20% turns screen-taps, but
for minimum distortion the screen-taps increase to 43% turns, which provides a
power reduction of 15% (but distortion is halved).
Note:
Worthy of note is the KT88, which GEC (MO Valve Co./Genalex) claim produces the
same power in ultra-linear connection as in Pentode connection at 43%
turns. This may be due to the applied Screen-Grid voltage used in ultra-linear
operation being twice the recommended value as that for Tetrode operation.
Importantly, the electrode structure of the original GEC-MOV KT88 is internally
physically identical to the GEC TT21 and TT22 RF transmitting beam power tubes,
so the GEC-MOV made KT88 is therefore capable of operating at its rated 600 VDC
maximum Screen-Grid DC operating voltage continuously without distress.
The TT21
or TT22, which have a top anode cap and intended for professional broadcast
use, may be used in lieu of the KT88 as a direct electrical substitute. Grid to
plate capacitance is reduced, which should produce superior audio performance
over the KT88.
One of the
challenges to the home constructor is that manufacturers' tube manuals and data
sheets often quote "typical operation" for fixed bias pentode
connection but cathode-bias for ultra-linear connection.
Ultra-linear
Circuit Characteristics:
The
following graph, courtesy of GEC and AWV Radiotronics Magazine (May
1959), shows the comparative characteristics of the KT88 in triode, pentode and
ultra-linear connections. 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 ultra-linear operation is greater
than that for pentode or triode operation.
This graph shows that for
ultra-linear operation the original GEC/MOV KT88 is unsurpassed!!
- Although having identical general specifications the
6550 Beam Power Tube - its USA cousin, has substantially lower
electrical ratings for pentode, triode and ultra-linear
connections - so be warned!!
- Although the 6550 and KT88 are often specified in
tube handbooks and amplifier schematics as equivalent substitutes for each
other it may be wise to check the ratings first before making a change
from KT88 to 6550.
- Physically, some brands of 6550 tubes have a
parallel glass envelope smaller in diameter than the KT88 so there may be
inadequate space to instal the KT88 or to provide adequate natural air
cooling without major modification. The reverse change is electrically not
a problem.
- Although the KT88 is derived from the TT21 and
TT22 transmitting tube stable, the origin of the 6550 is not clear.
- Some users have reported detailed examination of
certain Russian made KT88 and 6550 tubes suggests they are physically
identical but branded for use as either KT88 or 6550 thus it is wise to
check the manufacturer's specifications before installation in any high voltage
circuit.
WARNING:
- The GEC/MOV KT88 octal socket centre base spiggot
is made from a brittle plastic moulding material and tends to break off if
the tube is not inserted and removed very carefully from its socket. This
tube is big and heavy in relation to its octal base so is unsuited to
horizontal or inverted mounting unless retained by a suitable device.
- The GEC/MOV KT88 was typically made with one or
two base pins omitted (presumably to save on cost) that has the effect of
increasing stress on the remaining pins whilst providing a less secure
installation.
- Do not use the KT88 in inverted mounting without
a retaining device.
EL34/6CA7 Ultra-linear Circuit Characteristics:
The
following graph by Mullard UK shows performance of the EL34/6CA7 valve.
Note the
comparative "linear" performance for power out v distortion over a
wide range of loads and similar operating conditions.
Further
information is provided by the following table ex Mullard UK, that shows
comparative performance between the Mullard EL34 and EL84 valves in various
output stage configurations.
I am
indebted to Rudolf Moers, a distinguished Electrical and Electronics
Engineer located in the Netherlands, who has made available for us his recent
extensive scientific investigation into the design theory and practice of
Ultra-linear audio amplification.
These
papers were added in February 2014 with kind permission from Linear Audio http://www.linearaudio.net
and their author Rudolf Moers.
Part 1 - Power Point presentation
Rudolf
Moers has published a new book offering refreshingly different guidance for the
DIY designer, blending solid theory with practical application and a new
detailed approach to ultra-linear amplifier design.
Fundamental Amplifier Techniques With Electron Tubes - Second Edition –
Rudolph Moers 2018.
Application of Ultra-linear tube audio amplification:
In
practice, for tube types other than KT88, the real loss of useable output power
from the ultra-linear connection is actually significantly less than any
power differential measured with resistive loads might suggest (i.e. a 1.5 db
reduction in loudness produced by the loudspeaker), because the ultra-linear
connection produces a higher coupling efficiency between the amplifier and
loudspeaker than tetrodes or pentodes - ie is more triode like - thus
approximating an equivalent "loudness" to pentode connection.
This
phenomenom is particularly true of low frequency reproduction, suggesting that ultra-linear
connection is superior for double bass violin, bass guitar and general
hi-fidelity reproduction down to about 40 Hz - which is the lowest musical
frequency normally reproduced in popular music.
The 6L6
family of tubes can be used for ultra-linear connection, but only safely
and reliably at reduced voltages not exceeding rated screen-grid voltage. More
suitable 6L6 style types include 5881 and 7027A - see manufacturers' data
sheets for typical circuit values.
Ultra-linear operation
typically delivers about two to three times the power output compared with the
same tubes in triode connection at the same plate voltage.
Most
importantly, inter-modulation distortion is substantially lower with ultra-linear
connection compared with pentode or triode connection for the same tubes.
Note: In
the case of hi-fi systems, percussive instruments such as the bass drum,
tympany, harpsicord, guitar and the piano present typically
short-duration/transient low fundamental frequency signals rich in harmonics.
Thus they are less demanding to reproduce than the electronic organ, pipe organ
or bass guitar, which produce an essentially long-duration signal, approximate
to a sine wave, in the lower register.
Triode
connection is still the preferred option for seriously loud organ music because
of the lower frequencies to be reproduced, the need for "boom" free
bass around the loudspeaker resonant frequency and, most importantly, a need
for consistent gain (flat response) throughout the musical scale to ensure all
notes are reproduced with equal loudness when they are recorded that way.
However triodes deliver substantially higher intermodulation distortion than
ultra-linear operation so adequate power headroom is essential to ensure clear
mid to upper range reproduction simultaneously with sustained lower register
signals.
A major
advantage of ultra-linear connection over tetrodes and pentodes is the
improvement in tone for recorded music and public address reproduction. The
tone is more triode like, being smooth and mellow (but "clearer" than
triodes), compared with the harsh tone of tetrodes and muddy tone (read
"less definition") of pentodes.
Guitar
Amplifiers:
Ultra-linear operation is not
usually suitable for lead guitar amplifiers because it lacks the
"crispness", "harshness" or "bite" in the sound
commonly provided by beam power tetrodes and pentodes, however some jazz and
country music guitarists may find it preferable where a smooth, natural mellow
tone is sought.
However, ultra-linear
operation is superior to tetrode/pentode connection for bass guitar
applications because low frequency power delivered to the loudspeaker is
substantially greater - ie it is "louder" - due to lower output
impedance and improved coupling to the loudspeaker.
It also
has a "deeper" tone, suggesting improved sub-harmonic performance.
Summary:
In
summary, ultra-linear connection offers:
- useable power output 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, not evident with
the "clinically clean" sound of unity coupling, where the
load is distributed between plate and cathode.
- simplicity in circuit design and construction
- elimination of need for a separate 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
- strong useable low-frequency power output - ie as
heard through the loudspeaker
- good dynamic range performance and transient
response
Ultra-linear operation also enables lower quality loudspeakers to
be used for satisfactory results.
Note that
in high power applications - ie more than 100W RMS, transmitting TRIODE
tubes such as 805, 809, 810, 811, 812, 833, 845, 8000, 8005, etc may be a more
economical and practical solution than trying ultra-linear configuration with
tetrodes/pentodes because of simplified wiring, output transformer and power
supply requirements.
TUBES
FOR ULTRA-LINEAR OPERATION
The "Ultra-linear"
configuration avoids the conventional conflict between plate and screen
voltages by creating a voltage divider network through the output transformer
primary to AC earth (transformer centre-tap), ensuring the screen voltage
tracks and thus always remains, both below and proportional to the plate signal
voltage. By this configuration, the screen is intended to be prevented from
exceeding its power dissipation rating.
This
statement is subject to the Screen Grid always being operated within its Rated
DC Screen Grid voltage and at a DC voltage less than that of the Plate.
Applied
plate (B+) voltage for ultra-linear connection should never exceed Grid
2 rated voltage, so standard ultra-linear configuration is only suited
to tubes designed for audio applications having a Grid 2 rating approximating either
the plate voltage rating (or actual applied plate voltage if less than rated
maximum).
Important:
Reference to tube data sheets will show that few output tubes have ratings
remotely matching this requirement.
Important:
The use of poor quality output transformers having a high DC resistance in the
primary windings may establish a situation whereby the actual DC Plate voltage
drop across the transformer primary winding is high, causing the DC Screen-Grid
voltage to be higher than the Plate at high signal levels.
Unfortunately,
only a small number of tube types are thereby suitable for ultra-linear
operation, because in ultra-linear mode the screen grid is operating at
or above the plate voltage - a dangerous operating region for any tube.
Only a few
tube types were recommended by their manufacturers as being suitable for ultra-linear
connection, the most notable being EL34/6CA7, EL84/6BQ5, KT88, 6550, 7027 and
8417 - see manufacturers' data sheets for typical circuit values.
In a
typical output tube, the Screen Grid is the ANODE, or positive electrode. It is
designed to accelerate electron flow from Cathode to Plate, but is structured
in such a way that most electrons pass through it and on to the Plate for
collection.
Excessive
screen grid voltage attracts excessive electrons to it, thereby resulting in
excess grid current, excessive grid power input/dissipation, overheating and
melting. The fused screen grid wires may short-circuit B+ to earth, damaging
the output transformer and/or power supply components.
Unfortunately,
most audio tetrodes, pentodes and beam power tubes are designed such that the
Screen Grid may be operated only up to a maximum DC voltage that is well below
the Plate voltage - typically 150 to 300 volts.
Plate
current is very much controlled by the Screen Grid, thus when the Screen Grid
is made ineffective (ie over-active) by application of excessive voltage, the
Plate Current is likely to exceed the tube ratings and also melt the plate.
Another
way of saying this is that if the Screen Grid voltage is excessive, the
capability of Grid #1 (Control Grid) to control electron flow in the tube is
diminished - or lost altogether.
Operation
of typical power tubes (having a screen grid voltage rating substantially lower
than the rated plate voltage) in ultra-linear connection is likely to
result in loss of control over electron flow by the screen, resulting in
thermal runaway or dynatron action - resulting in self-destruction of the tube.
Fire is a constant risk.
For
example, tubes designed for RF power service typically have a plate voltage
rating many times higher than their corresponding screen grid voltage
rating. This class of tube (eg 807, 2E26, 6146, 4CX series) has the
screen grid physically positioned close to the control grid (Grid 1) and tend
to self-oscillate or suffer thermal runaway when the screen grid
voltage is higher than their rated screen-grid voltage, which is always
the case with conventional ultra-linear service, thus rendering them
unsuitable for ultra-linear service.
WARNING: When using tubes
fitted with a plate top cap (anode cap) in ultra-linear configuration,
consider also the risk of self-oscillation and/or parasitic oscillations from
the combination of long leads from the plate top caps and screen grids to
the output transformer - particulary significant when using multiple tubes to
obtain higher power. Some of this lead length may be avoided by chassis layout
however in the case of top cap style tubes the screen connection is always
under the chassis, thus ultimately requiring connection in some way from top to
bottom of the tube through the output transformer.
High
Power Ouput:
Where more
power is needed it is preferable to use a larger tube that is known to be more
suited for ultra-linear circuitry, such as the KT88, KT90 or 813.
Another
useful option is to run multiple pairs of tubes in parallel push-pull, such as
the arrangement used in the GEC 400W KT88
Amplifier. It goes without saying that normal precautions against
instability and parasitic oscillations are essential in layout, lead dress, use
of grid stopper resistors wired directly to the pins, and keeping inputs away
from outputs. The output transformer must be of high quality with low leakage capacitance
and low leakage inductance between windings. Tubes should be mounted
close together to minimise inter-connecting lead length. Grid #1 circuit
resistance must be held within the manufacturer's ratings. A low-impedance
driver, such as a cathode-follower or
transformer is recommended.
For high
power applications where high plate voltages are needed, success may be
achieved by adding a separate winding for each screen grid to the output
transformer, to enable the screens to be AC coupled to the plates thus
providing ultra-linear operation, but separated from the DC plate supply, thus
enabling the screens to be supplied within their rated voltage from an
independent supply.
The ACROSOUND 100W Ultra-Linear Amplifier using
6146 tubes and the TO-350 Transformer is an example of this excellent and
innovative design configuration.
One of the
benefits of this configuration is that the Screen-Grid supply can be
independent to the Plate supply and therefore better regulation can be
incorporated into the Screen Grid circuit. Noting the Screen-Grid is the ANODE
in a Tetrode or Pentode tube, a regulated supply will deliver improved
transient response and a "brilliance" to the reproduction not
available with a common B+ power supply as is the case in conventional
Ultra-Linear operation.
Bruce
DePalma, one of the few Gurus of modern hi-fi amplifier design, presents an
interesting and comprehensive commentary on the core design philosophies
supporting this approach in his excellent Design Paper - "Analog Audio Power
Amplifier Design"
Bruce has
developed designs that enable both Ultra-Linear and low Screen-Grid voltage
technologies to be successfully integrated in such a way that extremely hi-fi
performance results.
BIAS
Grid 1
bias for ultra-linear operation is normally higher than that for
tetrode/pentode connection, so output stage sensitivity is reduced. Higher
output voltage from driver stages is therefore needed.
Grid-stopper
resistors to Grid #1 and Grid #2 are still required for ultra-linear
operation.
Important:
The technique of using a silicon rectifier diode on each Screen Grid in series
with the Grid-stopper resistor, as described in my "OPTIMISED ULTRA-LINEAR ©" page, is
very helpful in ultra-linear connection. AC signal output voltage from
the Screen-Grids is prevented from conducting through to the output
transformer, which means that theultra-linearoutput stage operates as a
negative feedback system only - ie Plate voltage is fed back to the Screen
Grids via the transformer taps but not the other way around. This method
ensures all the electron flow goes to the Plates, with all the advantages described
previously.
Note:
This configuration has opposite polarity as when zener diodes are installed to
reduce screen-grid voltage, so therefore the benefits described are not
relevant to the zener diode technique.
Important
- When using a high B+ voltage:
To ensure
Plate Dissipation remains within manufacturer's rating at both zero and maximum
signal, it may be essential to use Class B operation - thus introducing further
complexity into the circuit design and perhaps offsetting much of the benefit
offered from the ultra-linear configuration. It may be more prudent to
use a tube having a higher rating.
RECOMMENDED
TUBES
The
following tubes are known to be suitable for conventional ultra-linear
operation having nominally equal Plate and Screen-grid DC supply volts:
TUBE
TYPE MAX.
ULTRA-LINEAR CONNECTION PLATE TO CATHODE VOLTS
6AQ5A 285
6BM8
300
6BQ5/EL84
300
6BW6
285
6CA7
500
6CM5/PL36 250
6CZ5
285
6L6GB
270
6L6GC
400
6V6GT
285
5881
400
6146A, B, W
etc
250 (Top cap style)
6550 450 (Note: GE USA
"design maximum" rating. Some brands may not tolerate this voltage)
6973 300
7027A
450
7189A
400
7581 450
7591 400 (50% turns recommended
by RCA)
7868 400 (50% turns recommended
by RCA)
807/5B-255M
300 (Top cap style)
8417 500
EL34
425
EL84/6BQ5 300
KT66
400
8417
500
KT88 600
(Note: GEC UK "design maximum" rating. Some brands may not tolerate
this voltage)
KT90
600 ("Absolute Maximum" rating)
KT94
600 ("Absolute Maximum" rating)
KT100 600 ("Absolute
Maximum" rating)
KT120 600
813
1100 (This tube is an excellent option for serious audiophiles but
has a top cap and requires a centre-tapped 10 V AC/DC filament supply - 5A per
tube. External wiring must be screened to prevent RF induction and parasitic
oscillations. Adequate ventilation is essential)
In my
humble opinion, the most suitable candidates for ultra-linear connection are:
6CM5/PL36
(250 VDC)
6AQ5/6V6GT (275 VDC)
6CZ5/6973 (285
VDC)
5881/6L6GC/7027A/7591/7868 (400 VDC)
KT88/KT90/KT94/KT100/KT120 (600 VDC) (may need to reduce to
450/500 VDC with non GEC manufacture)
813
(1100 VDC)
All are
well proven fine quality beam power tubes - each famous in its own right.
Other
options are the EL34 (425 VDC) or 6CA7 (500
VDC) or EL84/6BQ5 (300 VDC) pentodes,
however in my opinion sound from these tubes is not as clean as those above.
All of this family of tubes "sounds" similar because of their
generally identical electrode construction.
If you
have had successful experiences with other tube types that are useful for
ultra-linear connection please email your comments.
For
further information regarding Ultra-linear operation of vacuum tubes see
my OPTIMISED ULTRA-LINEAR ©
OPERATION page
© NOTICE:
INTELLECTUAL PROPERTY COPYRIGHT © D.R.GRIMWOOD 2002
- ALL RIGHTS RESERVED.
This page last
modified 14 November 2026
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is located at http://www.oestex.com/tubes/ul.htm