electron's

ULTRA-LINEAR

DISTRIBUTED LOAD

HIGH-FIDELITY VACUUM
TUBE
AUDIO
AMPLIFIERS

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ULTRA-LINEAR OPERATION, also known as DISTRIBUTED LOAD OPERATION,
is a term when applied to single-ended or push-pull vacuum
tube audio amplifiers, that 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,
natural" and tetrode/beam power tubes as "clean, bright, sharp, punchy".

Technically speaking, the ultra-linear configuration
delivers the same 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).

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.

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 "Radiotronics" papers 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 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

One 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, however the documented
evidence has been lost. (If any reader has that information please email it to
me)

Consequently, in the absence of proof of the Australian design, the
ultra-linear design concept is attributed by documented evidence to "a
British subject" Alan.D.Blumlein in 1936 - see British
Patent 496,883 June 5, 1937. (Application No. 15620/37). See also expired
US Patent 449,4776. Also US Patent 2218902 (22 October 1940).

Blumlien'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 "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 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
Blumlien.

This ratio was later verified by other respected designers, including GEC/MOV, GE USA and
RCA

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
Blumlien 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.

Further reading on the life and times of this remarkable man is available at
http://en.wikipedia.org/wiki/Alan_Blumlein

Since the famous "Williamson" amplifier design appeared some years after that
date, 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 1952.

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.

In contrast to the above documented evidence, some researchers claim the
Ultra-Linear concept was previously used in Australia as far back as 1933.

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.

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

6550 Beam Power Tube Operating Conditions - Ultra-Linear/Distributed Load
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.

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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!!

WARNING: 6550 V KT88

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.

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 of above
Power Point presentation - 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.

Application:

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 (ie 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)

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   (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 Ultral-linear operation of vacuum tubes see
my OPTIMISED ULTRA-LINEAR © OPERATION page

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

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This page last modified 05 February 2014

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