The Magic of Triodes Demystified
The Single Ended Triode Conundrum - What's all the fuss?
This article is an attempt to explain a little of the valve audio world,
and the Single Ended Triode (SET) field in particular. It endeavours
to bring a little background to the confusing jargon behind the resurgence
of an anachronistic, old and inefficient style of amplifier which was
last widely used in the late 1940s.
To a human being, music is a complicated collection of different sounds,
assembled in a way that tickles the mind. In electrical terms, music
is a stream of complex positive and negative waveforms swaying back and
forth across a zero, (usually ground), point. This is a gross simplification
of the phenomenon; AC and DC are the root concepts. DC comes
naturally from batteries, and has invariant polarity. AC comes from
the mains, and music sources, (microphones, speakers, CD players, turntables,
tape players, etc.), exhibit constantly changing polarity. This
means the two wires which come out of these sources cannot be described
as positive or negative, merely as HOT and GROUND.
An amplifier of any kind is a device for magnifying very weak signals
to a strength suitable for driving a load. It can do this in a myriad
of ways. In the audio world, we must amplify the very low voltages
that issue from music sources so that they can drive the speaker.
Typically these vary from about 0.2 millivolt, (that's small - an Ortofon
moving coil cartridge), to about 2 volts; (a CD player). For any
kind of reasonable power, an 8W speaker needs around 40 lusty volts for
25 Watts; a four ohm speaker needs about 28 volts for the same power
- but more current. And there's the rub...
Speakers are in fact less voltage animals than current animals.
A magnet will move a cone based solely on the current passing through
the voice coil. Thus we must also amplify the tiny currents produced
by our music sources. So the amp must deliver higher voltage, and
most notably higher current. An important aspect of amplifier design
is keeping the voltage and current in step; they must be amplified
in the correct ratio, largely determined by the speaker impedance.
All amplifying devices have the capacity to amplify both parameters,
although the valve is far better at voltage amplification than current
amplification, at which the transistor excels. Thus it is fair to
say that valves amplify small voltages to large voltages very well, and
transistors are outstanding at amplifying small currents to large currents.
This is, of course, the principle behind my hybrid amplifiers; giving
the right job to the best man.
Valves come in an enormous range; TV valves, oscilloscope displays,
magic eyes, large transmitter valves, radar klystrons, magnetrons in microwave
ovens, signal valves in low level voltage amplification, and output valves
like the 845 in essentially current amplification roles. Like the
internal combustion engine, they can be engineered for almost any application.
But the output stages of valve amplifiers are extremely problematical
because valves on their own are singularly unable to pass the currents
necessary to directly drive loudspeakers. This is the reason that
95% of valve amplifiers use large output transformers; they are the means
by which the high voltages and low currents of the valve amplifier can
be converted to low voltages and high currents required to drive the speaker.
Valves, in their simplest form, were created by a couple of researchers
in England and the US at the turn of the century. The Englishman
Childs formulated much of the maths of the thermionic phenomenon,
while the American Lee De Forest created and patented the vacuum valve
triode in 1904. He took one of Edison's electric lamps, added an
outer cylindrical plate, (the anode), around the filament, and interposed
a simple wire mesh, now called a grid, between them. He found that
by controlling the voltage on the grid he could change the current flowing
between the hot filament and the cylindrical plate. Thus was created
the first amplifier; De Forest found that if he moved the grid voltage
up and down by five volts, the voltage appearing across a resistor attached
between the anode and a high voltage source moved up and down by about
twenty volts. This three terminal device, the triode, was demonstrably
an amplifier, giving both voltage and current amplification. Very
soon after Gugliermo Marconi was able to effect his radio technology,
which by about 1908 was widely adopted by ships at sea for Morse code
telephony. You may recall in 'Titanic' the use of radio; it was
a modern marvel in 1912.
Triodes have limited voltage gain, and in the quest for more performance,
the tetrode, (four electrodes), and the pentode, (five electrodes), were
developed in the years from about 1925. Sadly however, the increased
performance is at the heart of an engineering compromise, and most audiophiles
shun the pentode for its sonics. This is perhaps an unfortunate
attitude, because some outstanding amplifiers have been implemented with
pentodes, particularly in Britain. Examples of legendary pentodes
are the KT66, the now obsolete KT88, and the 6550.
The symmetrical music signal confers a strong advantage for the amplification
process; it may be split in two, each half sent off to two output devices,
and then recombined at the output for driving a speaker. The advantage
is that each device, (valve or transistor), can work just half the time;
this arrangement is unsurprisingly called 'push-pull' and it works very
well. However, each half of the music signal is amplified by a different
device, and unless they are precisely matched, the way in which they amplify
can differ sufficiently to be just audible. But on the plus side,
push-pull topology cancels certain forms of distortion, and offers high
power at relatively low cost. And when implemented with transistors,
it has resulted in massively powerful amplifiers of up to 2000 Watts.
In the valve world, the push-pull topology permits use of a relatively
small, inexpensive transformer; these engineering considerations have
made it the darling of the engineers and accountants who run the industry
and push pull are quite the norm in almost all of the transistor world
and most of the valve world.
For those who pursue musical nirvana, this finally brings us to the Single
Ended Triode (SET) phenomenon. This topology uses just one device,
almost always a triode, to amplify the whole music signal.
'The valve is operated at roughly one half its maximum intermittent
anode current rating, which usually relates to approximately 90% of its
maximum continuous anode dissipation. For example, both the 211
and 845 valves are rated at 100 Watts, and both are biased to approximately
90mA at just under 1000 volts. In operation the music signal moves
this current above and below 90mA by equal amounts such that average dissipation
remains almost constant. Only a small portion of the energy expended
appears at the speaker; the majority, (close to 80%), is wasted
as heat. However, the SET is the electronic equivalent of the open
hearth fire, and the romance and charisma is equally unmistakable.'
This is hardly romantic in engineering terms, and it is incredibly inefficient,
rivaling steam power. It is also the very oldest topology; a single
device operating across the whole musical cycle so that it is never entirely
switched off, and never saturated. A great deal of heat is produced,
and the transformer compromises are horrific, but for some reason not
fully understood the sound quality is palpably better. The majority
of engineers dismiss the topology as an archaic oddity; but when they
listen, and particularly when they compare with push-pull, they usually
agree that the SET is almost a spiritual experience. Perhaps the
most telling descriptors of the SET are 'realism' and 'involvement'.
The single-ended topology can be readily implemented with solid-state
devices. The same orders of improvement over conventional solid-state
push-pull designs are observed. However, the fundamental nature
of transistors versus valves is still discernible to the cognoscenti,
and by and large single-ended amplifiers are implemented with triodes.
My own Glass Harmony design is a hybrid, using a valve voltage amplifier
and solid-state devices for the single-ended output stage. The sonic
characteristics truly lie midway between the best solid-state amplifiers
and the SET, giving a sense of fast dynamics and power to a sweet, triode
Hugh R. Dean
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