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Topic 5 of 7

Output transformers

The output transformer is the heart of a valve amplifier.   The right
pic shows a stereo Leak amplifier.   The two output trannys are rotated
90deg to the power mains tranny to avoid picking up induced hum from the mains
tranny external magnetic field.   Output trannys contribute most of the
distortion in a valve amp and have a limited bandwidth.

Valve amp manufacturers purchase trannys from transformer winding companies.
Transformer winding is an extension of the fitting and turning industry.
Few people working in transformer winding companies have a knowledge of
electronics or valve amps.   Most valve amp manufacturers have a basic
academic understanding of output trannys, but often have a limited or incorrect
understanding of the physics of transformers or how they are made.

Academic formulas for calculating the design of
output trannys is available in many text books and web sites.   There is no
need for this page to repeat academic information that is readily available.
This page gives an overview of the physics that governs the performance
of output trannys and helps give a perspective that academic text often
omit.

Output trannys are the highest cost and most labour intensive part of a valve
amp to make.   Each output tranny, depending on size, takes approx 1 to 2
hours to wind, assemble with laminations, heated in an oven, dipped in resin and
baked overnight, cleaned, painted, external leads connected, dress caps fitted,
packaged and sent to a valve amp manufacturer.   A tedious task to say the
least.   The right pic is a simple analogy of an output tranny.

B+  historically represents battery.   B+
is the supply Voltage to the CT Center Tap of the primary windings of the output
tranny.   560V DC supply is connected to the CT Center Tap of the Primary
winding.   The amplified audio from the 2 output valves, push pull is in
opposite phase 180deg.   300VAC + 300VAC = 600V AC is across the primary
winding.

Depending on the tranny design there may be 2,000 turns of wire in the
Primary and 90 turns for the Secondary winding.   22:1 step down ratio.
600V AC / 22 = 27V AC into an 8&Omega speaker is 90 Watts.

The primary and secondary windings are isolated.   The iron core is
magnetised by the AC current in the primary winding and the AC magnetism is
passed into the secondary winding generating an output Voltage to drive the
speaker.
The turns ratio between the windings adjusts the secondary Voltage.   Only
AC can pass through a tranny.   It is not possible for DC to pass
through a tranny.

Copper mass   The total mass of copper wire is
divided 50% 50% approx between primary and secondary.   Often the primary
has slightly more mass.   In the example tranny the primary has 2,000 turns
of thin wire, whereas the secondary has 90 turns of thick wire.

E I and C core.   The majority of power and
output trannys in valve amps are E I construction.   The right pic shows
the thin E I mild steel laminations that are stacked together to assemble the
core.   The tranny next to the E I is a C core which can be used for the
same size bobbin.   Bobbins can be open or have a center division.
The C core is approx 20% more efficient compared to the best quality grain
orientated E I laminations.   The square shape of the E I laminations is
not an efficient conductor of magnetism.   A % of the induced magnetism is
spewed outside of the core and lost.   A further % is lost as eddy currents
within the laminations which causes the laminations to get hot.   E I mains
power trannys are noticeably hot when touched.   C cores are an efficient
conductor of magnetism, but C cores do not have an aesthetic square shape and
are more difficult to mount onto a chassis.   C cores are mostly used in
industrial electronics and were approx twice the cost of E I laminations.
Today, C cores are only slightly more expensive than E I laminations and it
makes no sense not to use the superior quality C core in all output trannys,
including power trannys.

It is essential to fill the window space with copper wire.   Once the
number of turns is calculated, then the thickest wire is selected to fill the
window space.   Any space not filled with copper wire causes magnetic
energy being lost, described as leakage Inductance, resulting in reduced
performance.

Mains power tranny   The primary is wound on the
bobbin first, then the secondaries are wound on the outside of the primary.
There are required safety standards for the minimum thickness of
insulation placed between the primary and secondary windings.   A mains
tranny only functions at a fixed primary Voltage and frequency (120V AC 60Hz) or
(240V AC 50Hz) depending on the standard of the country.

Output trannys function over a wide range of
frequency and Voltage.   The primary and secondary windings must be split
up and inter-leaved.   Secondary - Primary - Secondary - Primary etc.
Interleaving the primary and secondary enables the tranny to achieve a
high frequency response.   As a general rule, the greater the numbers of
primary and secondary interleaving the better the high frequency response.
A physicaly large transformer has increased capacitance between windings
which causess phase shift and restricts the high frequency response.   A
smaller tranny with less turns favours the high frequency response.

Total turns   Increasing the total number of
turns in an output tranny increases the Inductance (magnetic efficiency) which
improves the bass response.   But, increasing total turns requires a
smaller wire gauge which increases DC resistance and reduces the power output of
the tranny.   Calculating turns ratio, maximum number of turns and
interleaving is a balancing act.

Traditional approach is to minimise total number of
turns and core mass, restricting the bass response in favour of improving high
frequency response.   The argument for justifying the approach of
minimising core size and total number of turns also minimises cross capacitance
keeping phase shift to a minimum which allows a greater amount of Negative
feedback to be applied hereby improving the pseudo academic figurers of lower
distortion.   Valve amps were part of the vinyl era where deep bass caused
the needle to jump off the record.   Most HiFi valve amps had a rumble
filter to reduce sub-bass.   The majority of speakers were approx 6dB to
10dB more efficient and larger than most speakers boxes today.   A few
Watts was very loud.

Inductance.   The real reason for minimising the
output tranny size was/is to minimise the cost of making the tranny.   An
inductance of 20 Henries is often insufficient.   40 Henries or higher is
required for an output tranny to deliver full power below 40Hz.   The
mathematical Inductance can be academically calculated, but in the real world,
the physics of Inductance in output trannys is not a fixed value.   For a
full range output tranny, the aim is to get as much Inductance as possible
regardless of what the academic number ends up being.

At bass frequencies the speaker Fs (Fundamental resonance) raises to approx
20 to 30Ω.   Valve amps naturally have a high output Impedance and the
output Voltage will automatically rise in an attempt to maintain constant power
to the speaker.   As the output Voltage rises, especially at high power, a
tranny without sufficient Inductance at bass frequencies can easily saturate
creating intense distortion.   To prevent core saturation, the primary
turns should be the maximum number and the core mass must be as large as
possible for a given chassis size.

In the past when only valve technology existed, materials were expensive in
comparison to labour.   An output tranny for a 100 Watt valve amp was
approx $10 to $20 in quantity.   $5 for materials, $5 for labour and little
profit.   The majority of output trannys were the minimum size, made from
the least expensive materials.   Only a few Hi-end valve amps had output
trannys that were made with the best quality materials.

Paper layering   Early trannys for valve amps
mostly used the less expensive, lower Voltage rating enameled copper wire and
thick impregnated paper insulation between each layer of winding.   Paper
layering enabled the tranny to be wound at high speed by automated machines,
reducing labour and keeping costs under control.   Poor insulation of the
paper and poor enamel coating on the wire enabled flashover to occur between
windings.

Extreme high Voltages are generated across the primary winding if the speaker
is disconnected while music is playing.   Flashover between windings was a
major problem with valve guitar amps.

A disproportional % of window space is consumed by paper layer insulation
which limits the total number of turns to enable the transformer have sufficient
inductance to obtain a good bass response.   With a restricted window space
the wire has to be thinner which causes further losses in the DC resistance of
the wire.   The space occupied by thick paper insulation causes leakage
inductance, reducing efficiency of the tranny.

Modern materials   High Voltage thin Polyester
mylar, Kapton and Nomax insulation including high Voltage hardened enamel wire
(used in electric motors) was readily available from the mid 1960s.   Using
these higher quality insulating materials does not require paper layering
between each layer of winding.   The above pics of modern mains and output
tranny windings shows the thin high Voltage insulation is only used between
primary and secondary windings, minimising the space occupied by the insulation,
enabling the maximum mass of copper wire to fill the window space.

But very few valve amp manufacturers were/are aware of the physics of output
tranny design, therefore the higher quality insulations and high Voltage enamel
coated wire including C cores were rarely used.   Output trannys were made
to a budget and when solid-state technology arrived the era of valve technology
came to a close.

Today, an output tranny for a 100 Watt valve amp is approx $50 to $100 for
materials, $100+ labour, 100%+ profit.   End price approx $200 to $600.
But because of alchemistic beliefs in imagined magical perceptions of
bygone brand names and components, many output trannys today are still made
using the same methods and low cost materials as in the past.

Output Tranny types

In most output trannys the windings are layer wound across the bobbin.
Interleaving the primary and secondary windings enables the tranny to achieve a
high frequency response.   The primary windings are connected in series.
The secondary windings are mostly connected in parallel.   The
secondary windings often have taps to adjust the output Impedance for 8Ω or
4&Omega speakers.

A cheaply made output tranny, used in many tube guitar amps, will have 2
primaries with a single secondary between the primaries (P1, S, P2,) or have 3
secondaries interleaved between the 2 primaries
(S, P1, S, P2, S,) as shown
in the right pic.

A better output tranny will have 4 primaries with 3 secondaries interleaved
between the primaries
(P1, S, P2, S, P3, S, P4,) as shown in the right
pic.

Some high quality output trannys may have as many as 8 primaries interleaved
with 7 to 9 secondaries.   The greater the number of primary and secondary
windings, the better will be the high frequency response, but the longer the
time it will take to wind the bobbin.

The right pic is a circuit showing the series connections of 4 primary
windings.   P1 is the first winding on the inside of the bobbin whereas P4
is the last winding on the outside of the bobbin.   P1 and P4 are referred
to as outer windings, because the other windings are in-between.

Cross connection   The primary windings are
alternately cross connected in series around the secondaries (outer P1 - inner
P3 - inner P2 - outer P4).   Alternate cross connecting the primary
windings enables the physics of the tranny to be closely balanced and the
induced magnetic current to be distributed equally into the secondary
windings.

The aim is to balance (as close as possible) the primary secondary windings
(equally) on both sides of the Center Tap CT.   However, balancing can not
be perfect, because the inner and outer diameters (circumference) of the
windings are different.   Wire length and DC resistance of P4 is greater
than P1.   Increasing the number of primary and secondary interleaving has
been the traditional approach to improve the balance and performance of an
output tranny.

Fully balanced windings can be achieved by taking a lateral
approach to how the bobbin is wound.   2 Primaries and 3 secondaries are
wound on each half of a split bobbin.   This gives a total of 4 primaries
and 6 secondaries.   The primary windings are series cross connected on
opposite sides of the split bobbin
(P1, P2, CT, P3, P4).
Very few output trannys are constructed using this lateral approach.

Why this lateral approach of using a split bobbin to achieve a fully
ballanced output tranny was/is not seen by the majority of transformer winding
companies is a mystery.   The answer is possibly tradition and conformity
and that very few electronic designers, especially audio valve amps, directly
participate in the design and construction of transformers.

27kg bass tranny.   The right pic is a 27kg
(60lb) bass output tranny, fully balanced split bobbin.   Yes you read
correctly 27kg is possibly the largest output tranny ever constructed.
This output tranny was constructed for experimental reasons.   The
Inductance was higher than the test equipment could accurately measure.
The tranny is capable of delivering 1,000 Watts below 10Hz.   However, the
practical application is for 4 KT88s in push pull parallel 200 Watts.
These large output trannys can be made available to valve enthusiasts who wish
to have the ultimate active sub-bass valve amplifier.

Pi windings   The Greek letter Π is the symbol
for transformer windings that are rotated 90deg in a bobbin.   Π windings
are sandwich assembled.   The below pic is an experimental valve amp with
Pi wound output trannys.   The final design had 8 Primaries sandwiched
between 9 Secondaries.   Pi wound trannys are 100% symmetrically balanced
and have a high frequency response x 2 greater than a conventional layer wound
output tranny.

Pi winding is commonly used in ferrite core trannys that transfer energy at
very high frequencies, as seen in switch mode power supplies and Rf amplifiers.
Pi winding is also the perfect method to construct an audio output
tranny.   Pi winding output trannys are not known to have been used in
audio valve amps.   A possible reason is the high tooling and assembly
costs.   Also, the marketing of audiophile products, particularly tube
amps, is driven by brand names, romantic nostalgia and reviews, not by
engineering or performance.

Toroidal output trannys are now being used in some tube amps.
In the past, toroidal trannys were x 2 more expensive than EI trannys
because of the high cost of toroidal winding machines.   Today, labour is
expensive by comparison to the cost of a toroidal winding machine.   Many
transformer winding companies now have toroidal machines.   E I and
Toroidal are now approx the same cost to construct.

Toroidal trannys have superior magnetic coupling between windings.
But, a toroidal core can easily saturate at low frequencies creating intolerable
distortion.   Therefore, the mass of the toroidal core has to be larger
than a conventional E I tranny (for the same power) to avoid any chance of
saturating the core.   Toroidal cores are un-forgiving of any DC or AC
audio imbalance in the primary windings.   Both output valves must be
perfectly biased with exactly the same current and have exactly the same gain
when driven.   Providing these conditions are met, a toroidal tranny will
outperform a conventional EI tranny.

4 way active valve tower.   The below pic is a 4
way active valve tower using toroidal output trannys as in the above pic.
There are 5 100 Watt Ultra linear amps using KT88s.   The 2 amps at the
bottom of the chassis are paralleled to give 200 Watts.   The chassis is
divided down the center.   One side of the chassis (left side of pic) is
the power supply.   The power supply also uses toroidal core trannys.
The other side of the chassis (right side of pic) are the toroidal output
trannys.

Right pic shows output trannys and active crossover →

Hi-frequency tranny (5kHz - 50kHz)→
Below are the 2 mid range
output trannys   The large bass tranny (at base of chassis) is
capable of 5Hz ↓

The 3 small toroid chokes mounted above the large bass output tranny
are for filtering the B+
← Left pic shows the power supply
opposite side of chassis.

↑ The small top toroid power tranny (+-20V and +-100V) is for the
electronic crossover and driver circuits of KT88 output valves.

Below the top small power toroid is the power toroid and toroid choke
for the 12.6V DC filaments heater supply.

← The 2 large toroid trannys create the 560V B+ supply.   The B+
supply is constructed from 3 X 188V supplies in series.

↓ Power turn on management.

Total mass 60Kg approx.

The 4 way active crossover and driver stages are solid state to insure a
perfect distortion free symmetrical signal is delivered to the output valves.
Each KT88 has dual LED monitoring to enable the quiescent current through
the KT88s to be calibrated.

Output Tranny application

Valve amps require perfectly matched output valves in order to give the best
performance.   This is similar to having balanced pistons in a vehicle
engine.   It is essential that the quiescent current through the output
valves be calibrated exactly.   A 10R resistor is placed in series with
each Cathode.   The -bias Voltage to the Grids is adjusted so 500mV appears
across each 10R resistor.   500mV across 10R is 50mA.   The valves are
then driven to full power and the current through the valves will increase to
approx 150mA.   Both valves must be perfectly equal in both quiescent and
full power condition.   Any imbalance of current between the valves will
result in the iron core of the tranny being partially DC magnetised.   The
Inductance will be reduced, the bass response will be diminished and the core
will readily saturate at bass frequencies.

Tetrode   Many early HiFi valve amps and almost
all guitar valve amps are configured in Tetrode operation.   In tetrode
operation the Screens of the output valves are connected to a second B+ filtered
supply Voltage from the power supply.

The primary of the output tranny is connected to the Anodes of the power
valves.   Tranny designed for Tetrode operation only need 3 wires from the
primary.   CT Center Tap to the B+ supply and 2 wires to the Anodes.
Trannys designed for tetrode operation (guitar amps) are often made as cheaply
as possible.   However there are exceptions.

Ultra linear   Output trannys designed for Ultra
linear application are often made from the highest quality materials and have
multiple primary secondary interleaving.   The screens of the output valves
are connected to the primary windings.   Ultra linear provides the same
high power as Tetrode configuration with superior power and inter-modulation
performance than a Triode configuration.   The power supply for Ultra
linear amps must be of high regulation, smooth and ripple free.

The original academic text stated that the screens should be connected to a
42% tap position of the primary winding, to achieve maximum power.   The
original text only refers to graphs and did not take into consideration how an
output tranny is wound.   It is difficult to mechanically achieve perfect
symmetry with 42% tap positions on both sides of the Center tap.   However,
50% can easily be achieved.   If a trade off has to be made, the mechanical
symmetry of the primary windings, must come first.   Also, the theoretical
difference between 42% and 50% is too small for any auditory or measured
difference to be detected.

Output Tranny parameters

The distortion figurers and frequency response of Valve amps are often
specified at 1 Watt.   Most output trannys can easily achieve a bandwidth
of 20Hz to 20kHz at 1 Watt.   But this bandwidth is rarely achieved at full
power.   The low frequency response at full power is directly proportional
to the number of primary turns and mass of the iron core.

Above 2kHz the iron core has little effect.   The high frequencies
response is directly dependant on the interleaving of the primary and secondary
windings.   However if the tranny has a large core and a large number of
primary turns enabling it to achieve sub-bass at full power, then the large area
of the windings causes increased capacitance between primary and secondaries
which restricts the high frequency response.   Therefore a small core with
less turns favours the high frequency response.

The bobbin and core in the right pic is for a 100 Watt output tranny, 1.5in (one
and a half inch) core with an approx 2 inch stack.   Mass approx 3kg.
The majority of output trannys of this size have approx 2,000 primary
turns.   With skilled winding technique and compacting the wire tightly,
the primary turns can be increased to 2,500 or 3,000.   The total DC
resistance of the primary should not exceed 100R,   0.36mm wire or greater
can be used.   The parallel secondary windings can be 1.1mm or 1.2mm.
Secondary DC resistance approx 0.1R.

There is no one way to wind the perfect tranny.   There are a few highly
skilled people who wind trannys and will have various techniques for achieving
the objective.   The inductance of the tranny will vary depending on core
mass and quality, and number of primary windings.   The aim is for the
inductance to be as high as possible.   The below pic is a complied list of
basic formulae for those who wish make output trannys.

Academic formulas for calculating output tranny
design are available in many text books and web sites.   Those who make
output trannys will already be overwhelmed by the math and there is no need for
this page to repeat academic information that is readily available.   This
page is intended to give an overview of the physics that governs the performance
of output trannys, from a perspective that academic text often omit.

Impedance ratio

"The Impedance ratio is the square of the turns ratio"

Understanding Impedance ratio between primary and secondary is difficult for
those who are technical and almost impossible to understand for those who are
not.   Hopefully this explanation will make it easier for anyone who is
interested.

Academic text states that a pair of KT88s in push pull Class AB, from a B+
560V supply Voltage will deliver 100 Watts.   Under this condition the
output tranny primary Impedance is stated as 4k5Ω. The Impedance of 4k5Ω is an
academic figure and represents the highest plate to plate Impedance that will
enable 100 Watts to be obtained.   (Plate to Plate simply means Anode to
Anode).   This academic figure of 4k5Ω assumes 100% transformer efficiency
with zero losses.

A pair of KT88s will rarely achieve 100 Watts in the real world. The average
is 80 to 90 Watts.   Also the plate to plate Impedance can vary between 3kΩ
to 5kΩ with little change to the available power.

Our sample output tranny has 4 primary windings (600 turns per winding) a
total of 2,400 turns.

4k5Ω plate to plate / 8Ω speaker = 562.5.     √562.5 = 23.7 turns
ratio.
2,400 primary turns / 23.7 = 101 secondary turns.

This academic process requires the formula to be repeated over and over for
each change in calculation.   Another way to view this process is through
the math related to the output tranny physics.   The Voltage ratio between
primary and secondary is the same as the turns ratio.

A general rule for push pull Class AB valve amplifiers.   The RMS
Voltage across the Primary winding is   1.1  x  B+ supply Voltage
(at full power).

1.1 x 560V DC = 616V AC
100 Watts into an 8Ω speaker = 28V RMS
616 /
28 = 22 turns ratio.
2,400 primary turns / 22 = 109 secondary turns.

There is a small difference of 8 secondary turns between the two
calculations.   This second formula does not look at the plate to plate
Impedance.   Therefore, which calculation is more practical to use ?
By reversing the last calculation of a 22:1 turns ratio, by the first formula,
we can see what the reflected plate to plate Impedance has been reduced to -

22 x 22 x 8Ω = 3,872Ω plate to plate Impedance.

A difference in turns ratio (23.7:1) (22:1) results in a small difference of
8 secondary turns.   This small difference causes a large change to the
academic reflected plate to plate Impedance.

The right pic shows that an 8Ω speaker has an average Impedance well above 8Ω.
Two and three way HiFi speaker boxes attempt to keep the overall
impedance as close to 8Ω as possible.   Also the leads connectors add extra
resistance.

Output trannys are not 100% efficient.   Core losses, leakage
Inductance, DC resistance of the copper wire, all contribute to increasing the
overall reflected plate to plate Impedance, higher than the academic figure.
Therefore the second formula closely represents real world conditions.
However it is wise to be conversant with both approaches and cross check
each method.

End of Topic 5

Written by John Burnett

Created: 30-Apr-2010

Valve Amps

Overview

Valve amp Basics

Pre-amp Driver

Output Stage

Output Transformers

Power-supply

Valve V Solid-state

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