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MUDL ARK
A 205
a u d i o
s y s t e m s
A 20-watt/channel class A2 triode amplifier
Over the years, we have had lots of requests to design and publish
a valve stereo amplifier. We have always resisted, partly because
of the huge expense of a valve amplifier and the comparatively
poor performance relative to well-designed solid-state amplifiers.
But now we present a modern valve amplifier, designed by David
Whitby. It uses quite a few novel techniques to keep the cost down
while obtaining good performance. We’ll let David tell the story . . .
Pt.1: By DAVID WHITBY
V
ALVE AMPLIFIERS are undergoing
a resurgence today and those who
would like to own one are often confused by all the technical and marketing jargon (eg, which type, brand, model, circuit, valve types, etc are best?).
Many people are understandably put
off by the fact that they may have to sell
the family car to get the best model and
the special speakers and super cables,
etc that they are told you may need to
make it all work properly.
Take heart – the Mudlark A205 is a
modern stylish design which works
well with medium-efficiency speakers
and comes as an affordably-priced kit
or as a fully built-up amplifier.
The Mudlark A205 comes in two
parts, the main amplifier and the
separate power supply box. The main
chassis has four large output valves
and two smaller driver valves. These
are placed either side of a central
rectangular housing which accommo30 Silicon Chip
dates the filter and plate load chokes
and output transformers.
The chassis itself is a rugged
aluminium extrusion while the front
panel is made of polished black Bakelite with the labelling screen-printed
in white for good legibility. The two
vertical Perspex panels at the top of
the amplifier are edge-lit with blue
LEDs. Combined with the soft reddish
glow from the filaments of the valves,
this really adds to the appearance of
the amplifier.
The front panel is quite simple, with
just a toggle switch for the power and
two push-on/push-off switches for
program selection and negative feedback on/off. Typical program sources
which can be used include CD, tuner
or tape deck.
The overall styling is a combination
of retro and modern which should
have a very high PAF (partner acceptance factor).
Why Mudlark? The Mudlark A205 is
really the big brother of the Australianbuilt Mudlark A2-1 (pictured in this
article), which was chosen as one of
the highlights and also the “cutest
thing” at the 2003 HiFi and AV Show
in London. It has now been sold in
Europe, the UK and the USA. The
A2-1 is a small 2-channel valve circuit
teamed with a solid-state subwoofer
amplifier, while the A205 is a bigger
full-range valve system.
The A2 designation stands for class
A2 operation. This refers to class-A
which involves positive drive to the
grids of the output valves on positive
signal peaks for increased efficiency
and higher power (sometimes called
“extended” class-A in valve literature).
Valve sound
Simply put, a valve amplifier will
have a particular tonal quality, desiliconchip.com.au
The front panel controls are
simple and include a power
switch, a volume control, a
negative feedback (in/out)
switch and a source selector
switch.
pendent on the circuit configuration
and valve types and even the brand of
valve. This will be due to factors such
as the relative levels of remnant second harmonic versus third harmonic
components. Even very minute levels
and differences can alter the perceived
richness, sharpness, smoothness, etc.
The generally lower damping factor of
a valve amplifier (compared to the very
high damping factor of solid-state amplifiers) also shifts the tonal spectrum,
producing the “warmer” sound often
attributed to valve amplifiers.
All of this is hard to measure
and quantify and is quite subjective but the bottom line is that
valve enthusiasts find the sound
attractive and enjoy experimenting with the circuits and loudspeaker combinations.
The measured performance of
valve amplifiers usually looks worse
than for a solid-state amplifier but
siliconchip.com.au
The rear panel has RCA sockets for the source inputs plus the left & right
channel speaker outputs.
August 2005 31
Fig.1 (left): the Mudlark is a 3-stage
single-ended amplifier operating
in class-A2. Its two output valves
are connected in parallel to give
increased current drive, doubling
the power available from a single
valve. The circuit is unconventional
because of the emitter follower
used to drive the grids of the
output valves and in the use of a
choke as the plate load instead of a
conventional output transformer.
don’t let this put you off as the type of
distortion and not just the total (THD)
has a real influence on the final sound
character.
A valve stereo amplifier need not
cost the earth, as you will see with this
amplifier. The aim with the A205 was
to get the looks and sound of a good
valve amplifier with enough power
to drive medium to high-sensitivity
speakers.
Design features
Rather than going for a push-pull
design, the chosen mode for the A205
is single ended triode. This early
mode of valve amplifier operation is
becoming quite common with audiophile designs today and although it
is harder to get high power and low
distortion than with a push-pull design, its particular sound character has
found many followers. The residual
distortion components of single-ended
designs are mainly second harmonic
(harmonious!) and a smaller amount
of third harmonic, allowing operation
with low or zero negative feedback.
Most single-ended circuits of the
1940s to 1960s used pentode valves
(eg, radios or “radiograms”), which
had their own “tone” and often boomy
bass – liked by many but hardly hifi.
Very few people have experienced
single-ended triode circuits with good
transformers and high quality modern
signal sources. I prefer triode operation
for the output valves but have used
a pentode in the output valve driver
circuit in a configuration which has
very low distortion.
Modern rugged Russian valves from
the 6L6 family have been chosen for
the A205.These are beam power valves
and are used as power triodes by connecting the screen grid to the plate
(anode) in the circuit. Of all the reasonably priced power valves available
32 Silicon Chip
siliconchip.com.au
that I have tried, I find the modern 6L6
family of valves to be a good choice for
sound, reliability and availability. Big
power triodes like the 300B are good
but they can be really expensive and I
think most people will be happy with
the 6L6 triode sound, their long life
and low replacement cost.
The actual types supplied with the
kit are Russian-made Sovtek 6L6WXT
or Electro-Harmonix 6L6GC/EH or the
5881. The latter is a ruggedised version
of the 6L6 and it has a slightly different
sound than the other valves. All these
valves are made in the millions every
year for famous name guitar and audio
amplifiers around the world.
The pre/driver valve is the 6GW8/
ECL86 or 14GW8/PCL86 triodepentode which combines one half
of a 12AX7 type triode with a 9-watt
power pentode. Both valve types are
identical except for heater voltage and
this is catered for by a link on the main
PC board.
The 6GW8 was used in huge numbers of sound systems in the 1960s,
while the 14GW8/PCL86 was used as
the sound section in millions of black
and white TV sets in Europe, the UK
and the USA. The 14GW8/PCL86 is
available in large quantities and this is
the one supplied in the kit. The 6GW8
is available as NOS (new old stock)
and is more expensive.
These valves were designed when
miniature valve design was well advanced and are excellent as a combined pre/driver and output valve
for small amplifiers and for driving
larger power valves in higher-powered
systems. In the A205, these valves are
operated at far below their maximum
ratings, resulting in good linearity and
long life.
Transformer solutions
The power transformer in a typical valve amplifier is usually big and
costly, with multiple windings for
HT (high tension), heaters, bias, etc.
By contrast, the Mudlark uses two
standard toroidal transformers interconnected in a unique way to provide
all the necessary voltages. This results
in big cost savings without loss of
performance.
Just as expensive as the power transformer in a typical high performance
valve amplifier is the output transformer, especially so for a single-ended
design. The high plate current in the
primary winding of single-ended
siliconchip.com.au
Fig.2: this set of plate current curves for the 6L6 family of valves shows
the additional characteristic for positive grid voltage (red curve).
transformers means a 20-watt unit
can weigh several kilograms and cost
$200+ each.
To overcome this, we have used an
output configuration that has become
known as “parafeed”. This uses a
relatively low-cost high impedance
choke as the load for the plates of the
output valves and a capacitor couples
the audio signal to the primary of the
output transformer. As a result, there
is zero DC in the primary winding, so
the output transformer can be much
smaller and lower in cost without
sacrificing quality.
Circuit description
Let’s have a look at the amplifier
section first – see Fig.1.
The audio signals are switched by
a 2-way push-on/push-off switch and
fed directly to a 50kW dual-ganged
(log) potentiometer. From there, the
signal is fed via a 22kW 1W resistor to
the grid of the triode section of valve
V1 (14GW8, etc). The output signal
The Mudlark A205
is the big brother of
the Mudlark A2-1,
a small 2-channel
valve amplifier
teamed with a solidstate sub-woofer
amplifier. The
“A2” designation
stands for class-A2
operation, which
involves positive
drive to the grids of
the output valves
on positive signal
peaks for increased
efficiency and
higher power.
August 2005 33
The power supply box houses
two large toroid transformers,
which are connected back to
back. Their outputs are fed to
an octal socket and connected
to the amplifier board via a
1.5-metre cable.
from the plate of this valve is coupled
via a 22nF 400V polyester capacitor
and an attenuator consisting of 470kW
and 220kW resistors to the grid of the
pentode section.
The pentode section is supplied
with a screened grid voltage of 120V.
This is below the lowest voltage swing
of the anode and under these conditions, the driver stage contributes very
little distortion to the amplifier.
The output impedance of the pentode driver is 13.5kW, as set by the two
27kW 3W paralleled plate resistors.
Fig.3: want to bypass that dreadful
transistor? Here’s how to do it. The
result will be a small reduction in
maximum power output.
34 Silicon Chip
This is suitable to drive the grids of
the 6L6 valves directly (class-A) but
improved performance (class-A2) is
provided by the use of a BF469 transistor as an emitter follower. This is
important because, depending on the
signal amplitude, the grid is driven positive and does draw grid current but only
on the highest positive peaks.
Incidentally, this is a no-no for some
valves but the 6L6 is characterised for
positive grid current, as can be seen
in Fig.2. In practice, with average listening levels and reasonably efficient
speakers, class A2 is seldom reached
but is available if needed. It allows
higher peak power for more headroom,
which also translates into higher RMS
power if measured with a sinewave
input signal.
Two valves are used in the output
stage. They are connected in parallel
and have a common 56W cathode resistor, bypassed by a 220mF capacitor. The
plate current for the two valves passes
through a 9H (nine Henry) choke –
essentially three standard compact
fluorescent ballasts connected in series
– and this provides a high AC impedance but low DC resistance to supply
the HT to the plates. The output signal
is then coupled to the primary of the
output transformer via a 10mF 400V
polypropylene capacitor.
As result, since there is no DC in
the primary of the output transformer,
it can be a small and relatively lowcost hifi 100V line transformer. This
is my version of the “parafeed” mode
referred to earlier and it gives substantial cost reductions while giving good
performance.
The bias on the output valves is a
mixture of negative grid bias, provided
via the emitter follower, and cathode
bias provided by the common 56W
cathode resistor. The latter provides
a degree of automatic adjustment of
the plate current to compensate for
changes in plate voltage and valve
aging.
For those not familiar with valve
operation, they can be likened to Nchannel FETs, in which the operating
current is set by the negative gatesource voltage. In a valve, the plate
current is set by the grid-cathode
voltage, called the “bias”.
By the way, for those people who
don’t like the idea of a transistor
being used in a valve amplifier, it is
possible to keep the circuit “pure”
siliconchip.com.au
siliconchip.com.au
August 2005 35
Fig.4: this is the circuit for the power supply. It incorporates a delay circuit to prevent HT being applied to the output valve plates
until their cathodes are at the correct operating temperature.
Here’s a sneak preview of the PC board inside the amplifier case. We’ll show you how to build it next month.
and bypass the transistor, as shown
in Fig.3. This does cause a reduction
in maximum power of less than 3dB.
This is noticeable only at very high
output but otherwise the character of
the amplifier is not changed.
While we’re on the subject of bias,
the pentode section of the first stage
operates with cathode bias, provided
by the 220W 1W resistor. This is not
bypassed by a capacitor, so we have
cathode current feedback, otherwise
known as “cathode degeneration”.
This serves to lower the stage gain and
slightly improve linearity.
The triode section of the first stage
also employs cathode bias. However, in
this case, the 680W resistor is bypassed
by the 220mF and 470nF capacitors, so
the full gain of the triode is obtained,
although to set the required gain and
reduce noise, some of it is lost in the
following resistive attenuator. Note
that the 680W resistor also carries the
cathode current from the triode section
in the other channel (V4).
36 Silicon Chip
The triode grid also has a small degree of negative feedback applied from
the secondary of the output transformer, via a 220kW resistor to the junction
of the 100kW and 10kW grid resistors.
Switch S2, on the front panel, allows
the negative feedback to be cancelled,
which gives a gain increase of about
+6dB and a corresponding increase
in harmonic distortion. The switch is
included for those people who may
prefer to use the amplifier without any
overall feedback.
The overall gain of the driver is set
at 20 and the drive voltage capability
is in excess of 120V peak-peak.
Two output valves were used in
parallel to get the required power.
With both channels operating, power
output is 17-20 watts per channel (3440 watts in total) over a 220-245VAC
mains voltage range.
Power supply
Most of the power supply is housed
in or above the main chassis while the
two toroidal transformers are mounted
in a separate box. The power supply
circuit is shown in Fig.4.
The power supply looks fairly complicated but that is mainly as a result
of the delay circuit which stops the
high voltage (HT) being applied to the
valves before their cathodes have been
heated by the filament current. Without this delay, the HT could initially
rise to quite high values, which could
damage some of the filter capacitors.
It could also cause what is known
as “cathode stripping” when higher
than normal HT voltages are applied
to the plate before the correct cathode
temperature is reached and material
is stripped from the cathode, thereby
reducing valve life. The extra complication is well worth it for extended
valve and component life.
In essence, the two 160VA toroidal
transformers, T1 & T2, are connected
back-to-back. Transformer T1 runs
conventionally, supplying 25VAC
to the 30VAC centre-tapped secondsiliconchip.com.au
ary winding of transformer T2. With
25VAC applied in this way and coupling losses due to reverse operation,
T2 produces 150VAC at full load on
what would normally be its primary
winding. This feeds a voltage doubler
rectifier circuit consisting of diodes D1
& D2 and four 270mF capacitors.
The DC from the voltage doubler
is further smoothed by the LC filters
consisting of 3H (three Henry) chokes
and 100mF 400V capacitors. The
resulting output is about 400V DC.
Incidentally, the 3H choke used here
is the same compact ballast
choke as used for the valve
plate chokes. These chokes
are of extremely high quality
and low noise. Better still,
they are Australian-made and
are a bargain when compared
to hard-to-get and costly EI
lamination chokes.
If you have looked closely
at the valve circuit of Fig.1,
you will have noted that the
supply voltages to the plate
chokes for the output valves
are + 480V (nominal). How so?
The extra 100V comes from a diode
pump involving diodes D4, D5, D6 &
D7 and two 1000mF 63V capacitors.
The 100V DC is stored in four 1000mF
63V capacitors which are connected as
two series-connected pairs, to obtain
sufficient voltage rating. This diode
pump circuit also supplies +60V,
from the junction of diodes D5 & D6
and the associated 1000mF capacitor.
The +60V is the positive supply rail
for the emitter follower Q1 (and Q2 in
the other channel).
Another diode pump circuit, involving diodes D8-D11, provides the
-27V and -120V rails for the emitter
followers (Q1 & Q2).
HT delay & protection circuit
As noted previously, the circuit provides a delay so that HT is not applied
to the plates of the output valves until
their cathodes have been warmed up.
This delay is provided by a 555 timer
IC, connected as a conventional monostable to drive relay 1. This switches
one of the 150VAC secondary feed
wires from transformer T2 and thus
stops the abovementioned voltage
doubler rectifier from working.
Transistor Q3 monitors the AC current drawn by the filaments (heaters)
of the four output valves, which are
connected in series across the 25VAC
siliconchip.com.au
Although not visible here, the
two Perspex panels at the front
are edge-lit with blue LEDs for
a really fancy appearance.
supply from the secondary winding of
transformer T1. If one of the valves is
removed from its socket, no filament
current can flow and so Q3 turns off.
This removes the negative supply to
the 555 and the relay, which then
switches off the HT rail.
Note that there is a link provision
in the circuit so that the filaments
(heaters) of the driver stages can be
25VAC for two 14GW8 filaments in
series or 12.5VAC if two 6GW8s are
used. Normally, 14GW8 valves will
be supplied in the kits.
All of the DC and output chokes and
the output transformers are mounted
under the long central cover on top of
the chassis. They are connected to the
main PC board by a short cable and octal plug which makes dismantling and
removing the board relatively easy.
Next month’s article will provide
the full assembly details and the perSC
formance measurements.
Where To Buy A Kit
The price for the full kit (including finished metalwork, panels and all components) is $870.00 (including GST) or with a fully assembled and tested PC board
$970.00 (including GST). All metalwork and panels in the kit are fully finished and
no drilling, cutting or punching is required.
Kits will be available from September 2005 and we strongly recommend that
you order before mid-August to receive your kit in the September shipment.
The A205 will also be available fully built and tested. Please contact Gless
Audio for prices. Valves and other components will also be available.
Kits and fully built amplifiers will be available from: Gless Audio, 26 Park St,
Seaford, Vic 3198. Phone (03) 9776 8703; email sales<at>glessaudio.com
August 2005 37
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