Elliott Sound Products

Valves vs. Transistors (Part
II)

Valves vs. Transistors (Part II) - What are the
differences?
Copyright © 2004 - Rod Elliott (ESP)
Page Created
15 Aug 2012

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Valves Index
Main
Index

Contents

1.0
Introduction

2.0
Obvious Differences

2.1
Less Obvious Differences

2.2
Valve Output Stage Efficiency

2.3
Amplifier Tests

2.4
Internal (Limiting) Impedance

2.5
Overall Differences

3.0
Conclusion

1.0 - Introduction

Valves (vacuum tubes) vs. transistors (yes, again
) ... this somewhat simplified article looks at the primary differences between
the two. I have included some measured data from a valve amp that may help
understanding the differences in real terms. Apart from the obvious differences
set out below, there are some important considerations that for reasons I do not
understand, seem to have been completely ignored for the most part. These will
be covered in detail (together with the measurements) later in this article.

There are differences - not only between valved and transistorised
amplifiers, but between different valve (or transistor) amps amongst their
peers. However, the fact remains that when similarly specified amplifiers are
compared using a double blind test (DBT) methodology, it is rare that listeners
are able to pick the difference with statistically significant reliability.

While this could be because of 'test stress' as commonly complained of by
many subjectivists (and assuming you actually believe it happens), it is
generally more likely that the differences are inaudible. I do not propose to
discuss the audibility or otherwise of various distortion mechanisms here. I
will just describe, in simple and measurable terms, the primary differences
between vacuum tube amps and 'solid state' versions of similar
specifications.

2.0 - Obvious Differences

Well apart from the physical differences between valves and semiconductors,
there are several differentiating features of each that must be understood.
While some of these are of prime importance, others are merely a curiosity ...
they are certainly different, but there is nothing to suggest that these
differences can be translated into audibility. This is not to say that some
people won't try to do exactly that, but their results should be subjected to
intense scrutiny to determine whether the 'facts' are real or imagined.

The following list (and most of this discussion) is based on using valves in
power amplifiers, rather than line level (preamplifier) stages. The analysis
that follows compares like with like (insofar as possible), and assumes
push-pull, Class-AB amplifiers, driving resistive and loudspeaker loads. While
other topologies may give (marginally) different results in some cases, the
general (and overwhelming) factors remain much the same.

Firstly, we shall look at the physical characteristics of the amplifying
devices themselves ...

Parameter

Vacuum Tube

Semiconductor

Insulation materials

Vacuum / Mica

Semiconductor junctions/ oxides

Insulation linearity

Excellent

Fair to good

Operating voltage

High

Low *

Operating current

Low

High

Device gain

Low

High

Device linearity

Fair to very good

Fair to very good

Output transformer needed

Yes

No

Heat Dissipation (quiescent)

High

Low

Manufacturing Cost

High

Low

Running costs

Relatively high

Low

Table 2.1 - Fundamental Device
Differences

*  While there are many transistors that can operate at voltages
that will stress most valves, these are uncommon in audio amplifiers

While the above is not an exhaustive list, it gives a reasonable overview of
the primary differences between the devices. By necessity, the list is not as
detailed as it might be, and in some cases I have made assumptions that are
reasonable for audio amplifiers. For example, many valves have extremely high
amplification factors, but generally have poor linearity as a result. While some
high gain valves are used in audio, they are not common in modern equipment.
Likewise device linearity. Both valves and semiconductors range from being
highly non-linear to having very good linearity (before feedback is
applied).

2.1 - Less Obvious Differences

One of the most commonly known aspects of valve amplifier behaviour is the
'soft' clipping characterised by virtually all vacuum tube amps. Where a
transistor amp clips with sharp edges and an almost perfectly symmetrical
waveform, a valve amplifier typically does not. In addition, a valve amp at very
low volume has vanishing low distortion levels, and the distortion increases
steadily with amplitude. When the onset of clipping is reached, the valve amp
will already be showing perhaps 1% distortion or more, where a transistor amp
will typically register its typically low (< 0.1%) distortion, which
increases very rapidly as the signal clips.

This is one reason that a valve amp may seem to sound louder than an
equivalently specified transistor amp, because clipping is not as harsh, and
more clipping can be tolerated before the sound becomes objectionable.
However, this is a very minor point, and although it is valid, it is the
least of the reported differences in sound quality. We also need to consider the
very best of the hi-fi valve amps that appeared just before they all started to
vanish (Quad, McIntosh, Audio Research, etc.). Many of these amps used complex
output transformers to minimise phase shift, and as a result used a
comparatively high amount of negative feedback. As a result, these amps had very
low distortion, low output impedance, and clipping behaviour that was almost
indistinguishable from a transistor amp!

Of far greater importance (and ignored by a great many listeners and
reviewers) is output impedance. Where a typical transistor amplifier will have
an output impedance of < 0.1 Ohm (100 milli-Ohms), a valve amplifier may have
an output impedance approaching (or exceeding) the load impedance. Six Ohms is
not at all uncommon for a push-pull amplifier with no feedback (assuming a
transformer tapping for nominal 8 ohm loads), and even a valve amp with an
ultra-linear output transformer and having the maximum feedback obtainable
without incurring stability problems will have an output impedance that is
rarely below 1 Ohm.

By modern standards, this is very high, and it will change the sound
of nearly all loudspeakers. The accuracy of the speaker will almost certainly be
reduced, but the subjective sound is usually 'better' - at least in the short
term. Longer term listening may reveal that the bass is under damped and perhaps
a little 'boomy', while treble response may be accentuated. It is rare that the
accentuation will be highly objectionable, but it certainly will not be natural.
Crossover performance is also compromised, but the degree depends on a great
many factors that are impossible to predict without detailed testing of the
specific loudspeaker system. Naturally, a speaker that was designed using valve
amplifiers will sound completely wrong with a transistor amp, but such
loudspeakers are in the minority.

One of the main differences (and the least reported of all) is a direct
result of the relative 'efficiency' of the output devices. Bipolar transistors
conduct very hard when turned on fully, and there may be as little as 1 Volt
lost across the output device. MOSFETs are a little different, losing perhaps 3
to 7 Volts when turned on and supplying appreciable current. Valves can 'lose'
as much as 33% of their supply voltage when turned on and supplying the nominal
load impedance.

2.2 - Valve Output Stage Efficiency

This is a major difference between the two different amp types, and even a
fully conducting (turned on hard) valve may have many volts between cathode and
anode. The effects when driving a (resistive) dummy load are effectively
invisible - the two different amp types will appear to be rather similar. The
real difference is immediately apparent when a real-world loudspeaker is the
load, and the full impact of the relatively low maximum conduction efficiency of
the valve is understood. The magnitude of this difference depends on the output
stage topology, with triodes having the greatest differential, and pentodes the
smallest.

What does this mean during listening tests? It means that the amplifier will
be able to provide a higher voltage swing into higher than normal load
impedances - this is something that a solid state amplifier cannot do, as its
voltage swing is determined almost entirely by the supply voltage.

The saturation voltage of a vacuum tube (i.e. the valve is turned on as hard
as possible) is determined by the grid voltage and anode current. The valve will
also be subject to current saturation (based on the type of cathode and its
temperature), and while pure theory indicates that this region should be avoided
to prevent cathode damage, in reality it cannot be avoided and always has some
influence. While it is always possible to decrease saturation voltage by making
the grid positive with respect to the cathode, this will cause grid current to
flow. This mode of operation is often referred to as Class-AB1 and
although it provides more power, this is at the expense of distortion and in
some cases, transient recovery (also known as 'blocking').

2.3 - Amplifier Tests

The valve amplifier used for these tests was borrowed, and is reasonably
representative of a hi-fi power amp (that's what its owner uses it for, and that
was the design intent). While I do have another at my disposal, it is not
suitable for hi-fi use. The amp used is a 100W Class-AB design, using
ultra-linear (50% taps) operation and having no global feedback. Not
surprisingly, distortion is somewhat higher than one would expect from a circuit
using feedback, but the subjective sound quality is still very good. While
triode operation would reduce distortion, it is extremely difficult to achieve
this power level with triodes, unless the number of output valves is increased -
this comes at a very high cost when valves are used (in this case, KT88s).

With a rather massive 20 Ohms output impedance, the amplifier will sound very
different from any solid state 'equivalent'. The output impedance is made up
from a number of internal impedances - the plate 'resistance' of the valve
itself, primary and secondary winding resistance in the output transformer, as
well as any other resistances that may be in series with the valve itself
(cathode resistors, plate 'stopper' resistances, etc.)

Analysing the peak output swing at the speaker terminals, we find that at the
onset of clipping, there is 25V RMS across an 8 ohm load - a power of 78W. With
no load at all, the output voltage swing can be increased to 40V RMS before
clipping - a very significant difference. Only a part of that can be attributed
to the power supply, which, like all unregulated power supplies, collapses under
load. We can examine this closely to see what has happened ...

With no load (or very light loading), the peak output voltage swing is almost
exactly 10% of the supply (which is not loaded down, so remains at 584 Volts).
When the load is increased (using an 8 Ohm load), the supply collapses by about
3.5%, but the output voltage swing is reduced to 6.3% of the available supply
voltage. If the output valves were able to provide the same output swing with
full load and no load, the peak loaded output voltage would be 54 Volts (38V RMS
or 186W). The following table shows the operating conditions under all of the
above possibilities.

Output Loaded

Output Unloaded

Supply Voltage (DC)

564 V

584 V

Output Voltage ¹ (Peak)

35.4 V

54.6 V

Output Voltage (RMS)

25 V

38.6 V

Internal Voltage Drop ² (Peak)

19.2 V

0 V

Output Power ³

78 W

186 W

Table 2.3.1 - Loaded Vs. Unloaded
Performance

Notes:
1   Output voltage is normalised to
that which would be developed with a loaded supply voltage of 564V. The
reasons for this will (hopefully) become clear later.
2   The
internal voltage drop is referred to the output - internal voltage drop is a
lot higher (approximately 10 times the externally measured value in this
case)
3   The figure quoted for unloaded output power is what the
amplifier would be capable of if it did not have a high internal
impedance. The supply voltage is assumed to be the same as for the loaded
case (i.e. 564V), and zero losses are included.

2.4 - Internal (Limiting) Impedance

The above data are interesting. From this, it is possible to calculate the
internal resistance (power limiting impedance) of the amplifier, since we know
the unloaded and loaded voltages (this is quite different from output impedance,
although internal impedance does form a part of the final output impedance
because there is no feedback). In the following drawing, I included a cathode
resistor (Rk). This is not always used, but if present it's another limiting
impedance. It doesn't matter a great deal if it's bypassed or not, because any
voltage across the resistor(s) will always be unavailable to the maximum peak
voltage swing.

Figure 2.4.1 - Valve Output Stage Limiting Impedances

A simple resistance calculation is all that is really necessary here, since
the impedance curve is flat enough over the frequency range of interest that
reactance does not need to be considered. We know that Ohm's law will give us
the resistance, so we may do the following calculations ...

I = V / R   (Current = Volts / Resistance)
I =
35.4 / 8   (Peak voltage divided by load impedance)

This gives us the current in the load, so we can calculate the internal
resistance of the amplifier. Yes, I know that this can be done in one formula,
but it will be a lot harder for you to remember it, so I shall do it the long
(but simple) way.

Now that we know the peak current (4.4 Amps), we can work out the internal
resistance. We know that we 'lose' 19.2V (peak) inside the amp, so again using
Ohm's law ...

R = 19.2 / 4.4 = 4.36 Ohms (at transformer secondary)

This is the amplifier's internal impedance, and is the cause of the losses
measured when the amp was driven with full load and no load. Note that the
effective supply voltage was taken as 564V (the loaded supply voltage), so
supply loading is factored out of the equation. While I am sure that anyone who
tried hard enough will find fault with the methods I used, this is not an exact
science. The variation in real load impedances will cause variations that exceed
the errors made here by a wide margin.

Note that the internal impedance calculated is not the same as output
impedance. Output impedance is determined by overall feedback, and can be lower
or higher than the internal impedance. The internal impedance is a
current-limiting effect within the output valves themselves, aided by the
resistance of the transformer primary winding. Any valve can be forced to have a
relatively low saturation voltage if it is allowed to draw control grid current
(Class-AB¹), but this class of operation is restricted to applications where
higher than normal distortion is allowable.

The purpose of this article is to demonstrate a trend - an ability that most
valve amplifiers have that is not duplicated by solid state amplifiers. This
'ability' is to provide a greater voltage swing into a normal loudspeaker load
than the amplifier power rating would indicate, thereby making the valve amp
seem louder or more 'effortless' than its solid state rivals.

It is important to understand that the above is not a
function of the short-term capability of the power supply. This was
exploited with amplifier ratings some time ago, and the term 'dynamic
headroom' was used to provide a short term power rating. As a figure of
merit, dynamic headroom has none - a poorly designed or inadequate
supply would give 'better' dynamic headroom figures than a regulated or
very robust supply. Fortunately, the term seems to have fallen by the
wayside.

The internal impedance essentially consists of a combination of things. The
combined winding resistance (primary and secondary) of the output transformer,
the internal impedance (determined by valve saturation voltage) of the output
valves, plus any additional resistance that may be added to the circuit (cathode
resistors for example). While the supply impedance also contributes part of the
impedance, this is a function of the power supply - not the amplifier itself. I
want to keep these separate, as they are separate issues and are unrelated.

I stated earlier that the internal impedance is not directly related to
output impedance. This remains true, but when no feedback is used the two would
appear to add together. Thus, one would expect the actual output impedance to be
significantly higher than theory might indicate. As described above, the output
impedance is 20 Ohms, and the internal (limiting) impedance is 4.36 Ohms. While
feedback could be used to reduce the output impedance dramatically, it will
have no effect whatsoever on the internal impedance of the amplifier.

I have referred to this internal impedance as a 'limiting impedance'
deliberately - it only applies at the limits of valve conduction, when the valve
is turned on as hard as possible. The valve is able to turn on harder when the
current through it is lower. This is in contrast to a BJT (Bipolar Junction
Transistor), which is capable of less than 1V saturation voltage, regardless of
current (within reason, of course). The closest to a valve in this respect is
the lateral MOSFET - it too has a saturation voltage that depends on the
gate-source voltage (equivalent to the grid-cathode voltage in a valve), and
will reach a limit that reduces the output power in exactly the same way that
the valve amplifier does. The reduction is much less pronounced, and the MOSFET
amp will normally have other characteristics that do not make it an equivalent
to a valve amplifier in any real sense.

2.5 - Overall Differences

To make this whole article a little clearer for those who cannot quite grasp
the technical details above, we can summarise the effects and provide most of
the answers as to the difference between valve and transistor amps. These
include the following ...

Output Impedance - Valve amplifiers have a higher output impedance
than the vast majority of transistor (BJT or MOSFET) designs, allowing the
loudspeaker a greater degree of freedom to exercise its character on the
reproduced sound. This may (or may not) be a good thing, depending on the
drivers used, enclosure type, internal damping, etc.

Soft Clipping - Because the onset of clipping is not as sharply
defined with a valve amp, the harmonics generated are of lower order and are
not as objectionable. Intermodulation products are still almost the same, and
there is relatively high distortion overall at above half power.

Distortion - Valve amplifiers have higher overall distortion
levels, but these increase gradually with increasing power levels. The overall
result is that the amplifier sounds louder (because of the additional
distortion), but usually does not sound anywhere near as bad as the figures
might suggest.

Internal Impedance - This provides a valve amplifier with the
ability to produce a much larger voltage swing into the load when the
impedance is higher than normal. At resonance, many valve amps will produce
as much peak-to-peak swing as a transistor amp of double the power rating.
Coupled with the relatively high output impedance, this gives the music
'life' compared to the (allegedly) 'flat' sound of a solid state amplifier. It
may not be accurate, but to a great many people it sounds 'better'.

As always, there is more to this than the rather simplified points above, but
they constitute the main differences. These effects are all measurable, but one
has to know what to look for. Of all the points made above, a transistor
amplifier can be designed to reproduce all but one of the effects - soft
clipping. Many have claimed to achieve this, but it is actually a technical
nightmare to do it properly - especially coupled with the other factors. Indeed,
it is not easy to deliberately increase the distortion of an amplifier either,
but who would want to do so? Low distortion is a good thing, and increasing it
deliberately is (IMO) non-sensible.

Figure 1 shows the clipping performance of the tested amplifier, measured
directly at the output valve anode. As you can see, the no-load case allows the
valve to conduct fully, reducing the voltage across the valve when fully on to
close to zero volts (it was actually measured at 1.6V).

Figure 2.5.1 - Valve Clipping at No Load and Full Load

Not shown is the measured performance with a 16 Ohm load - the clipping
(saturation) voltage at 16Ω was 70V, versus 132V with an 8Ω Ohm load. This shows
quite clearly that a valve is not normally capable of turning on as hard as one
might have expected, and the saturation voltage is dependent on the current.
This is actually a well known effect, but its significance has (IMO) been
overlooked for too long.

The saturation voltage is reasonably linear with current, and from the above
we can even make a rough guess of the intrinsic resistance of the valve itself.
We know that the supply voltage is 564V when loaded, and there is 132V dropped
across the valve. Therefore, the peak voltage swing on the transformer primary
is ...

564 - 132 = 432V

Since we also know the output voltage (35.4V peak) and impedance (8 ohms), we
can calculate the transformer ratio and then valve current ...

Transformer Ratio = Vin / Vout = 432 / 35.4 = 12.2:1

We already determined that peak output current is 4.4A, so the next step is
easy ...

Peak Valve Current = Output Current / Transformer Ratio = 4.4 /
12.2 = 0.36A (360mA)
Valve Plate (Limiting) Resistance = Vsaturation
/ Peak Valve Current = 132 / 0.36 = 366 Ohms

Finally, we can determine the effective valve limiting resistance at the
transformer secondary ...

Transformer Impedance Ratio = (Transformer Ratio)² = 12.2² =
149:1
Output Limiting Resistance = Valve Limiting Resistance / Transformer
Impedance Ratio = 366 / 149 = 2.45 Ohms

Note that we determined earlier that the total output limiting resistance was
4.36Ω, so there is a discrepancy. The difference will be made up of a great many
different impedances, and the effect of the ultra-linear output stage is
difficult to predict ...

Primary winding resistance

Secondary resistance

The effect of using ultra-linear topology (an unknown)

Stray wiring resistances

Etcetera.

Measurement shows that the actual primary winding resistance is 58Ω per side
(116Ω plate-plate). The secondary resistance is difficult to measure accurately,
but is in the order of 0.4Ω. I suspect that there is also some additional effect
resulting from the ultra-linear topology, since the screen grid of the output
valve shares 50% of the winding - this has not been investigated at the time of
writing. In the end, minor errors are of little consequence - any real-world
load will be vastly different from that used for testing, and all we can say
with certainty is that the effects shown are real.

3.0 - Conclusion

So, do valve amps sound different from transistor amps? Of course they do.
The details above show the major points of difference, and these are all
audible to a greater or lesser degree, depending on the loudspeaker load
characteristics and the amplifier design. While it is possible to design a
transistor amp that has high output impedance and a relatively high internal
limiting impedance (the author having done so over 25 years ago), the solid
state and valve amp are immediately identifiable once there is enough clipping
so that the harmonic structure can be heard. The differences below clipping are
also audible, with the transistor amp having less distortion and sounding
generally cleaner.

The fact of the matter is that there is no point trying to make a transistor
amp sound like a valve amp, since those who like valve amplifiers will still
want to see the glass bottles with their little lights inside, and will never be
convinced that the alternative is superior.

Managing the additional dynamic power that a valve amp can produce into
higher than normal loads is easily achieved by using a transistor amp with more
power - this is so cheap these days that dealing with the additional heat
created by adding circuitry to make the output stage less efficient is a
pointless exercise at best.

As noted elsewhere in the valve section, I happen to like valve amps -
there is something very pleasing about the concept, and they give a great deal
of freedom to express one's creativity in the final design as seen by the
public. Unfortunately, there are a great many negatives as well - high voltages,
fragile glass envelopes with fragile metal intestines, and ... the output
transformer. Good ones are very expensive, cheap ones are generally rubbish. The
cost of building a 100W valve amp is many times higher than building an
equivalent using transistors, and there is ongoing maintenance with valve
replacements, occasional tweaking of the bias to get it back where it should be
as the valves age, etc.

Then there is the search for decent valves that can perform as well as those
made during the valve era. This is unacceptably difficult, and I've seen premium
Russian valves that have a nice easy life fail for no reason whatsoever. Valves
from elsewhere are usually worse, and every time you buy a valve you are taking
a risk. The quality is extremely variable, and few can tolerate the voltages
that used to be common. In my experience, buying valves today is much like
deliberately buying counterfeit semiconductors - you have no idea what you've
purchased until it fails.

Ever wondered why The Audio Pages doesn't
have any valve projects? Well, now you know.

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Copyright Notice. This
article, including but not limited to all text and diagrams, is the
intellectual property of Rod Elliott, and is Copyright © 2012.
Reproduction or re-publication by any means whatsoever, whether
electronic, mechanical or electro- mechanical, is strictly prohibited
under International Copyright laws. The author (Rod Elliott) grants the
reader the right to use this information for personal use only, and
further allows that one (1) copy may be made for reference. Commercial use
is prohibited without express written authorisation from Rod
Elliott.

Page created and copyright © 15 Aug 2012