Elliott Sound Products

Valves (Vacuum Tubes) -
Myths

Copyright © 2009 - Rod Elliott (ESP)
Page Created 09 Nov
2009

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Valves Index
Main
Index

Contents

Introduction

2 - Myths

2.1 -
Distortion

2.2 -
Linearity

2.3 -
Sinewaves are Simple

2.4 - Matched
Valves

2.5 - Valve
Sound

2.6 - Blue
Glow

3 - You Can
Replace The Valves Yourself

Conclusion

References

Introduction

We are all used to myths, and they appear in all aspects of life. Ranging
from emails that tell you your house will catch on fire if you open an email
from <insert name here> to claims that anyone can fix their own
valve guitar amp, they are sometimes merely annoying, but for the latter topic
at least, are extremely dangerous.

You can fix your own guitar amp, provided you are an experienced technician
who knows what to look for and can diagnose faults without resorting to applying
power to see where the smoke comes from. While a valve change may seem simple
enough, it's not, and valve suppliers make matters much, much worse by claiming
it can be done - indeed should be done by the owner.

This article covers most of the more popular myths, but I'm sure I will have
left something out. If you fail to find your favourite myth here, please let me
know.

Every so often, one comes across claims that simply defy belief. The
following (in context but with specific references removed) caught my eye ...

The sonic quality of valve amplifiers can be both superior and possessed
of a unique character

Valves offer full and faithful sound reproduction, rich detail, brilliant
clarity, and accurate tracking of complex waveforms

Valves are also better at reproducing deep bass and extended, sweet,
natural highs

A valve input stage provides magical midrange, soundstage size, air and
overall musicality

Without exception, the above claims are - quite bluntly - bullshit! These
comments imply that transistors (including FETs) and opamps can't provide
the so-called benefits listed, and that's absolutely false in all respects. If
any of these claims were even a tiny bit true, modern test equipment
would be able to detect the difference between a valve and transistor stage. In
fact, the test gear can detect the difference - valves are noisier and
have higher distortion than a competent 'solid-state' equivalent circuit.

It is possible to engineer a valve preamp or power amp to (almost) equal a
transistor equivalent, but it will be disproportionately expensive, heavy and
will liberate (comparatively) vast amounts of heat. Valves have a very finite
life ranging from minutes to many years, depending on how they are used.
Replacements are expensive and often of dubious quality.

2 - Myths

The myths surrounding valves are many and varied, but they fall into a number
of major categories as shown below. The major myths involve the 'quality' of the
distortion produced, and while there most certainly are differences between
valves and transistors (or ICs), most of the stuff you will see is drivel - it's
not true at all. The points listed above are examples of the deluded thinking
that plagues the audio world. Let's go through the list ...

2.1 - Distortion

The most common claim about valve amps is that they have 'nice' second (or
even) harmonic distortion, whereas transistor amps have 'nasty' third (or odd)
harmonic distortion. This is true in only the most limited number of cases, and
is especially noticeable with single-ended amplifiers. The fact is that it's not
the order of the harmonics (odd or even), but how far they extend beyond the
original frequency. High order harmonics (typically from the seventh and above)
do sound relatively nasty, and that applies for both odd and even harmonics.

For push-pull amplifiers, clipping is usually symmetrical. If properly
designed for low distortion (hi-fi amps), there will be small amounts of both
even and odd harmonics, but they will be limited to perhaps the fourth or fifth,
with diminishing amounts of anything higher. Once the amplifier clips, the
distortion produced is predominantly odd-order, and for a valve hi-fi amp with
significant feedback, it will probably sound remarkably similar to a transistor
amp clipped to the same degree.

For guitar amps, most of the input stages are biased to provide maximum
symmetrical swing, which means that the distortion will be a mixture of even and
odd harmonics. At (and beyond) clipping, the distortion is predominantly odd
order. Strangely, very few guitarists actually like asymmetrical
clipping, which will always be a combination of even and order. Severely
asymmetrical clipping usually sounds dreadful - it develops a tonality that may
be described as 'thin and reedy', or 'farty', depending on the severity and the
way the amp is used. The allegedly smooth sound of even order distortion only
(particularly the second harmonic, which is claimed to be especially nice),
produces a waveform that still tends to look like a sinewave, but just isn't
quite right. At anything above 10% distortion, it really doesn't matter whether
the distortion is odd or even order - for hi-fi this is simply unacceptably
high.

To prove these assertions is easy, as the following waveforms will show.
Unfortunately, there are some people to whom there is no proof, because they
'believe'. If you believe the nonsense that marketing pukes and vested interests
dream up, then no proof will work, but if you are willing to see exactly how odd
and even harmonics create new waveforms, then you need to look at the waveforms.
Due to space and article size considerations, I've only added in the first
harmonic of the fundamental, at 10% distortion. In the case of a typical valve
circuit approaching clipping, the first odd harmonic is the 3rd, and is
20dB lower than the fundamental. 1V at 400Hz + 100mV at 1.2kHz ... the waveform
is shown in Figure 1.

Figure
1 - Symmetrical Waveform at the Onset of Clipping

For even order distortion, the typical waveform seen from a valve preamp
circuit looks somewhat like that shown in Figure 2. This waveform is a mix of 1V
at 400Hz + 100mV at 800Hz. The second harmonic is displaced by 90°, for the
simple reason that this is necessary to recreate the waveform seen. The valve
itself doesn't somehow 'magically' generate a 90° phase shifted sinewave, it
distorts the signal, and the harmonic(s) are generated by the act of
distortion.

Figure
2 - Asymmetrical Waveform From Single Ended Stage

The distortion is actually quite difficult to see if you're not used to
looking at sinewaves, so the second half of the graph shown includes the
fundamental superimposed in green. This makes it easy to see that the top of the
waveform is slightly squashed, and the bottom is stretched. At less than about
5% distortion, it is extremely hard to see any difference in the waveform, but
naturally a distortion meter or FFT
will show that it's there.

Audibility is another matter. With a sinewave, it's usually easy enough to
detect as little as 0.5% distortion reliably. With music, that level is unlikely
to be heard unless the loudspeakers are particularly revealing, and the listener
has a keen ear. Even much higher levels can be difficult to detect by ear, and
this again depends on the speakers and the type of music. Note that these
comments are intended as a guide only, and do not include the effects of
intermodulation. IMD results
from any non-linearity, and is measured with two frequencies.

A common test is to apply signals of (say) 1kHz and 1.1kHz. IMD is measured
by looking at the amount of 100Hz (and 2.1kHz) signal produced, since IMD
produces sum and difference frequencies, as well as the normal progression of
harmonics (and their sum and difference frequencies). As a result,
minimising IMD is essential, or the end result can sound revolting. To
demonstrate this, look at Figure 3, which shows a simple distortion circuit.
When operated with a single 1kHz signal, distortion is about 7% and consists of
both odd and even harmonics. When a second signal of 1,100Hz is provided as
shown, IMD can be plotted as shown (the chart is the result of a FFT in the
simulator). The completely new frequencies at 100, 200, 300, 900 and 1200Hz are
quite visible, and only intermodulation can create that. Look at all the
other frequencies shown too. With a single frequency, we expect a diminishing
sequence of harmonics - even and odd.

Figure
3 - Asymmetrical Distortion and Intermodulation

With IMD, we see new sets of frequencies centred around 2.1kHz, 3.15kHz,
4.2kHz, and extending well past the end of the graph. Only a few of these are
actually harmonically (and musically) related to the original two frequencies,
so the resultant distortion is often (highly) discordant. Note that the sum and
difference frequencies are not generated with perfectly symmetrical
distortion (strange but true).

If you were to build a valve preamp operated at a low voltage so that it will
generate significant distortion at low levels (as I have done), it's very easy
to dispel any myth that second harmonic distortion sounds 'nice'. It doesn't! My
circuit was biased so that it would generate about 3% distortion with an input
level of 300mV (this is the level I get from my FM tuner in my workshop system).
The distortion at that level is almost exclusively the second harmonic, and a
fairly clean sinewave at double the input frequency was observed as the
distortion residual at the output of my distortion meter.

According to those who think that such distortion levels give 'body' or
'something' to the sound, this arrangement should have sounded 'pleasant' with
music - much as one might expect from a SET (single ended triode) amplifier
operating at normal levels into a typical loudspeaker. Well, it certainly gave
the sound 'something', and that is best described as crap - it sounded bloody
awful. There was clearly audible intermodulation distortion that cluttered the
sound, and the overall effect was dreadful. No magic sound here - simply
distortion that was audible even on speech. Reducing the level reduced the
distortion too, but it never sounded as good as the direct signal without
the added distortion.

I suppose one could get used to the sound - after all, in the 1930s that's
what people had, they got great enjoyment from it and there was nothing else
available at the time. A lot has changed since then, and we are now able to
listen to music without audible distortion. Given that it's possible to
make a direct comparison between a distorted and undistorted signal with ease
(any ABX test would almost certainly give a 100% reliable differential), I
simply cannot understand why anyone would ruin the sound from vinyl, CD, SACD,
etc. by feeding it through an amplifier that is guaranteed to produce
significant harmonic and intermodulation distortion. It doesn't sound nice, and
designers have been working for decades to produce amplifiers that have
distortion levels that are below the threshold of audibility.

Engineers didn't spend all that time and effort to make amplifiers sound
worse, that much is certain. There are some simply outstanding amplifiers
available today, with specifications (and sound) that was unheard of in
the (mercifully brief) heyday of the SET amplifier. In reality, the era of SET
amplifiers was rather short, because it was quickly found that a push-pull amp
was vastly more efficient, had significantly lower distortion and more output
power, and could use a smaller transformer for better performance at both high
and low frequencies. No sensible designer of that period would even consider a
single-ended amplifier of any kind, except for use in mantel radios and other
comparatively cheap consumer products. Such amplifiers were common in mantel
radios, 'radiograms' (combined radio and record player) and TV sets up to the
1960s, after which they thankfully faded into the obscurity they so richly
deserved.

It's probably worth noting that some of the most expensive amplifiers of the
valve era were designed for very low distortion (such as the Leak TL/12, with
claimed distortion of 0.1%), and their initial (or primary) purpose was to drive
the cutting head in disc cutting lathes. No-one would ever think it was a good
idea to use an amplifier with high distortion to cut vinyl masters, and low
distortion amps would naturally be preferred. Obtaining distortion levels below
0.1% means that good design and plenty of feedback is needed, because no amp
(valve or transistor) can achieve the required performance without generous
negative feedback.

2.2 - Linearity

Many is the claim that "valves are linear, but transistors are not". This is
bollocks! Valves are not linear, and the simple proof of this is a glance
at any plate voltage vs. current characteristic chart. If the valve were in fact
linear, there would be a series of straight lines on the chart, not the curved
lines we always see.

Part of the reason for this myth is easy to explain. When valves and
transistors competed in parallel for the mass market, we had very basic
circuitry in both types of amp/ preamp. The difference was that the valve
equipment operated from 300-400 volt supplies, while the transistorised
equivalents used a supply of 20-30 volts. A signal of 1V RMS (2.8V peak-to-peak)
occupied only 0.93% of the total available 300V supply, while the same signal in
a transistor used 10 times this (9.3%) of a 30V supply. Higher signal voltages
were obviously worse in this respect. Added to this was many years of experience
with valves, and very little experience with transistors, so most transistor
circuits were probably sub-optimal. The early transistors were also a far cry
from those we can get today, with lower gain, higher noise, etc.

Added to this were some fundamental (glaring) errors made when people started
using transistor circuits. One that particularly amused me was the first
(non-adjustable) gain stage in some early transistorised microphone preamps. It
was traditional that a high gain valve would be the front end, and these
typically had a voltage gain of about 50. With high impedance microphones being
the normal ones used at the time, almost anyone could get a 1V or more quite
easily from the mic. These mics used a normal low impedance cartridge with a
step up transformer.

However, even up to 1V can be obtained from a low impedance mic right against
a loud singer's mouth quite readily - I've tested this, and managed it with
ease. The valve would have some difficulty (trying to get 50V RMS, or about 140V
p-p), but distortion was not horrendous. When transistor circuits started to be
used, the gain (also non-adjustable) of the input preamp was ... 48 (nominal -
based on a 47k and 1k resistor). Since this operated from a 30V supply, anything
over 200mV input caused gross distortion. Silly decisions like that were
not uncommon in the early days, and each such mistake provided ammunition for
the anti-transistor terrorists.

Despite that, an early (basic) single transistor stage with emitter
degeneration (local feedback) could still more than match most valves for
linearity, and as transistor circuitry improved it became no-contest. High
quality opamps today can achieve distortion levels (a direct indicator of
linearity) that are almost unmeasurable. No valve circuit can even approach the
distortion - both simple harmonic and intermodulation - that can be achieved
easily with a $2 opamp.

If the collector current in a transistor is plotted for varying collector
voltage at a fixed base current, the 'curve' is almost a straight line - far
more linear than any valve. Several other tests give a very linear result too -
but ... gain varies with emitter current, and that relationship is decidedly
non-linear, especially at low current. A valve is no different in this respect,
and the reasons even have a tenuous relationship between the two devices.

Figure
4 - Linearity Test Circuits ... Transistor vs. Valve

Now, my claim that a transistor in its simplest configuration will beat a
valve is bound to have a few people shaking their heads sadly. I can almost hear
the laments as I write ... "Poor silly bugger, he's really lost it this time."
Sorry to disappoint, but I tested this with real parts, and was surprised at the
results. Very surprised, actually. The transistor circuit is much, much better
than the valve, despite the huge supply voltage difference.

I added the cathode follower to the valve circuit because without it, I
couldn't drive my distortion meters properly because of the high output
impedance, and I do admit that has a small effect. However, I re-ran the test
with two different distortion meters ... one (the 'preferred' unit) has an input
impedance of 100k, and with that connected directly to the plate circuit, output
voltage was reduced and distortion increased to 0.8%. The other meter has an
input impedance of around 1M, and that showed almost identical distortion from
the plate of the amplifier and the output of the cathode follower.

The valve distortion at a very modest output voltage (1.21V RMS) was a
reasonably respectable 0.4%. Not wonderful, but pretty much as I expected. I
know that this can be improved, but this is a simple test of linearity, and the
valve stage is set up for symmetrical clipping for maximum dynamic range. The
distortion climbs steadily as output voltage is increased though.

So what of the transistor, also biased for symmetrical clipping with the
voltages shown? With exactly the same gain (hence the odd value emitter
resistor) and voltage, I measured 0.13%, which like the valve, was predominantly
second harmonic. Biasing the transistor for a collector voltage of 4V reduced
the distortion to less than 0.06%, and although interesting, it's not useful
because there is no dynamic range before distortion goes through the roof.

For both circuits, some of the meter's output residual (distortion + noise)
signal was noise. The valve stage has a reasonable earth plane, but the
transistor circuit was literally just hanging in mid air, supported by test
leads running all over my workbench. Looking at the residual waveform on my
oscilloscope, most of the signal from the transistor stage was noise, and
it was necessary to use averaging to see the 2nd harmonic signal. This indicates
that the distortion is actually lower than measured on the meter! Some of the
noise was picked from a fluorescent lamp that's no more than 300mm from where I
was working, but there was little evidence of this noise from the shielded valve
circuit. This has good supply decoupling, rather than 1m leads to a couple of
power supplies as used for the transistor test. Overall, a very unfair test on
the transistor, yet it still won easily.

Naturally, these are tests that anyone can do. You do need a low distortion
oscillator and a distortion meter (or use a PC oscilloscope with FFT
capabilities), and you can expect similar results. I used a BC546 because I have
lots of them here and I used the first one out of the bag. No selection or
tweaking was done, apart from adjusting the emitter resistor with a decade box
until the gain was the same as the valve stage. I also ran a simulation, using
exactly the same values as the real transistor circuit. The simulator claimed a
distortion of 0.1%, so has obviously under-estimated reality (as I expected).

The test described here isn't designed to do anything more than show that
both valves and transistors are non-linear, and in particular to dispel the myth
that valves are linear. It's not meant to prove that transistors are 'better'
than valves, but more to show that both will show non-linearities under similar
conditions of operation. Both circuits have local feedback, but the transistor
has more simply because it has higher gain.

2.3 - Sinewaves are Simple

This myth applies to both valves, transistors and ICs, but it's still worth
including here because it's a common claim - especially by subjectivists who
cheerfully refute any measurement data. Many people keep complaining that
sinewave testing is 'too simple', and that it's somehow easy for an amp to
reproduce because of this inherent simplicity. A sinewave is simple, only in
that it is a mathematically pure signal, and contains exactly (and only) one
frequency - the fundamental. Sinewaves have been used for testing for decades,
and any amplifier that can reproduce a sinewave perfectly therefore has
zero distortion. If harmonic distortion is zero, it follows that there is
no non-linearity whatsoever, so IMD will also be zero.

No amplifier ever made can reproduce a sinewave perfectly, regardless of its
topology or technology. Some do get very close, but once distortions are below
0.01% it will usually be impossible to pick one from another, provided frequency
response and gain are also identical. As I've pointed out in several articles,
amplifiers have no idea of the signal waveshapes they are reproducing. All that
matters is that at any instant in time, there will be one specific voltage that
must be amplified, and if that is amplified by the same amount regardless of the
instantaneous input voltage, the amp has no distortion. Should the signal cause
the amp to attempt to provide an output signal that's greater than the supply
voltage, the amp will clip the output - this is clipping distortion.

Signals that change too fast for the amplifier to keep up will create
distortion. This effect was dubbed TID (Transient Intermodulation Distortion),
and it can be induced in any amplifier ever made, including those that don't
have any TID according to their designers. All you need to do is apply a signal
that changes so fast that the amp is incapable of the required rate-of-change at
the output (slew rate limiting). My test oscillator can produce a squarewave
that's faster than any amp I've tested or read about can possibly handle. Big
deal - so can hundreds of other squarewave generators.

What was missed (or the authors chose to ignore) in the early TID papers and
claims is that while oscillators can generate these signals, music can't!
There is simply no traditional musical instrument that can generate transients
anywhere near as fast as an electronic oscillator. Even synthesisers won't do
it, because they have filters (and opamps) within the circuitry that limit their
ability to create extremely fast transients. This is necessary, not only to make
them compatible with amplifiers, but because such fast transients can't be heard
(or reproduced) anyway.

CD music is brick-wall filtered at 21kHz, vinyl is utterly incapable of
transients that will stress amplifiers (such a transient would likely tear the
stylus off its cantilever, and can't be cut into the master). Higher sampling
rates don't change anything either. Microphones can't react to an instantaneous
step change in pressure, and even if something could create it, the air carrying
the signal won't support it either. There are scientific devices that can
do it - they use a disk designed to rupture past a certain pressure to generate
massive pressure gradient waveforms. I don't know of any musical piece that uses
them, and no, the 1812 Overture's cannons don't count. The cannon risetime is
actually quite leisurely, and won't stress anything apart from loudspeakers that
can't handle the peak power at low frequencies.

In a sense, sinewaves are simple, and it is their very simplicity that makes
people think that they must therefore be an 'easy' signal to reproduce. I can
easily hear 0.5% distortion on a sinewave with my workshop system (horn loaded),
and a more revealing system may do better. Trying to hear the same level of
distortion on music is much harder - especially if it's only on transients. In
that case, most people wouldn't hear it at all in isolation, and even in a
double-blind test may have difficulty.

I've also seen it claimed that the speed of electrons in a vacuum is much
greater than through silicon, so valve amps are somehow 'faster' or perhaps more
'immediate'. What a load of unmitigated horse feathers! If you see such a claim,
it should be immediately obvious that the author is either mad (i.e. clinically
insane, typically delusional), lying, or also believes that there are fairies at
the bottom of his garden (or between his ears) that make his spinach¹
taste better than anyone else's.

1 - If you don't like spinach, please insert vegetable of
choice ... as a replacement for the word spinach, not there

2.4 - Matched Valves

You can buy matched pairs of output valves. That's what the advertisements
say, and they wouldn't lie to you, would they? Perhaps, or perhaps not, but they
just might lie by omission - the omission being that few will tell you the
matching conditions, explain the test processes, or disclose what is matched and
what is not. To be properly matched, the valves have to be tested with
the plate and screen voltages you have in your amplifier, and operated in a
similar manner to that in the amplifier.

While never stated, there is an implication that since matching involves
testing, matched valves are somehow better than 'ordinary' unmatched valves. In
reality, it makes no difference if crappy valves are matched or not - they're
still crappy valves. No method of matching, regardless of how rigorous the tests
might be, will make a badly made valve work any better or last any longer than
it does with no testing. The occasional completely dead valve might get thrown
away, but that's cold comfort. If there is a significant number of failures in a
batch from a manufacturer, then the whole lot should be binned, not just those
that fail immediately. The ones that pass through the matching process are not
better built than those that failed, they simply haven't failed ... yet.

Any form of matching test that involves a traditional valve tester is
useless, because at best, these testers give nothing more than an indication
that the valve is functional. Scales that simply have red and green sections
marked 'Replace' and 'Good' are worse than useless. Does your supplier match
valves using one of these testers? You'll probably never know, because they
don't (or won't) tell you. Unless there is full disclosure of the voltages used
for plate, screen and control grid, and that the valve is matched for a
specified idle current and gain, then the matching is as good as arbitrary.
There are so many differences between valves that obtaining a genuinely matched
pair, where they track each other over the full operating range, is extremely
difficult.

Ideally, you need to be able to match the valves yourself, in your amplifier,
and under the conditions as those where they will actually be operated. Even
after this is done, there can be changes in the early part of the valve's life
that can cause a significant variation between valves that were originally
carefully matched. As with 'professionally' matched valves, just because you go
through the process doesn't mean that the selected valves will work properly
over their lifetime or last as long as they should.

Some valves with which I had personal experience were 'premium' KT88s. After
the first 100 hours or so of use, every time the amplifiers were turned on the
idle current changed. After adjustment (the amplifiers had LEDs and accessible
trimpots to set the bias) all was well until the amps were turned off and back
on. Sure enough, the bias current was different again. Initially, I was only
able to do a thought experiment on this problem because the system was still in
operation and no replacement valves were available at the time (20 of them in
all). I came to the conclusion that the heater was probably loose in the cathode
tube, because nothing else explained the fault satisfactorily. When later I
broke one apart, I tested my theory, and the heater wire literally fell
out of the cathode. For comparison, I checked an old (dead) GE version for
comparison. The GE valve's heater had to be coaxed out of the cathode, and was
difficult to reinstall because it was so tight.

No matter what anyone may claim, valves such as this cannot be matched. Every
time power is applied, the valve will have different characteristics because
different parts of the cathode are slightly hotter than the rest. In reality,
these valves were fit for nothing - certainly they were of no use in an
amplifier. They were faulty from manufacture, but seemed to be alright when
first used. It took some time before the problem really showed up, so any
matching process could never have picked up that anything was amiss. I doubt
that this was an isolated incident either. Quite obviously, we didn't get the
world's total supply of these faulty valves, so others would have had the same
problem. Sloppy workmanship, poor equipment setup, material substitutions and
any number of other manufacturing faults can play havoc with the performance of
any valve, but unless it completely fails your chances of a warranty replacement
are very slim indeed.

Treat all claims of matched valves with suspicion. Before handing over any
money, find out the exact conditions used for matching, and if these conditions
are significantly different from your amplifier, then don't bother. Anyone who
cannot (or will not) tell you exactly how the matching tests are done should be
avoided, because without knowing the details of the process used, the matching
is likely to be no more useful than deciding that they are matched because they
look the same.

It's less than inspiring to read through a detailed article about how one
vendor matches valves, only to discover through the text that there are some
fairly obvious errors that show that the writer doesn't understand as much about
valves as he thinks he does. This being the case, can you trust the company
involved to know the proper way to conduct the tests? Perhaps they do, but to
include (IMO) serious errors in an article intended to demonstrate that their
matching process is better than (unnamed) 'others' doesn't engender much trust
from me. I do understand that they are making the article readable to the
'average' enthusiast, but this being the case it would have been better to leave
out the wrong stuff altogether.

One thing is for certain though. There is absolutely no point whatsoever
buying matched valves if you don't know which parameters are matched, or at what
voltages. To make things harder, there is considerable variance of output stage
topologies, and some are far more tolerant of a mismatch than others. Valves
that are matched at idle - i.e. for a specific bias current measured under
(usually) unknown conditions - may have widely differing mutual conductance (gm)
and/or plate resistance (rP) at electrode voltages other than
those used for the test. While it has been suggested that µ (Mu, or
amplification factor) should be matched, this generally has to be calculated
from gm and rP because it's difficult to measure directly. It can
be done, but for the most part is not actually very useful at all.

Many resellers claim to have 'computerised' valve matching machines. What
they don't say is what the computer does - for all we know the programme simply
activates a few relays to connect the appropriate pins to their respective
supplies, but nothing else. It might not even do that - there are plenty of
'computerised' systems that use nothing more than an 8-pin microcontroller to
perform very basic logic functions that might be irksome to do by other means.
All of the suppliers who match valves do so for one reason only - to try to
convince you to purchase their product. They generally don't do it because they
are really nice people and want you to like them as human beings, so you should
treat everything they tell you with suspicion.

2.5 - Valve Sound

Often you will hear claims that a certain brand (or type) of valve is
'sweeter' than another, or has greater 'bass authority'. These are weasel words
that have no defined meaning whatsoever, so the claims are equally undefined and
can most likely be attributed to imagination (or vested interest if you like
conspiracy theories). There is no doubt that different valves can sound
different, but unless the writer is willing to take measurements that explain
the difference, treat all claims as apocryphal.

Unfortunately, there are masses (or so it seems) of people to whom a
measurement is worse than an anathema, it amounts to sacrilege or even heresy.
Audio measurements were developed because subjective testing is unreliable, and
can be influenced by a great many things that have absolutely nothing to do with
the device under test. To be really useful, measurements have to be conducted
properly, and the full details of how the measurement was made explained. There
are a few measurements that don't need much explanation, because the methods are
considered well known to the extent that anyone 'skilled in the art' will
immediately understand how it was done.

If one pair of valves sounds different from another, then there's a reason,
and that reason can be measured. Despite claims that there are things in audio
that simply cannot me measured, these are just more weasel words - usually used
by purveyors of magic components and/or snake oil. Measurement techniques are
well advanced, and engineers and scientists know exactly what they can get away
with and not have people bitching and moaning - MP3 is a perfect example. It's
rubbish and sounds dreadful compared to CD, but thousands upon thousands of
people are perfectly happy to listen to it.

Be that as it may, there is nothing about an amplifier that makes an audible
difference (as determined by a proper double-blind test) that cannot be
measured, and test instruments are more than capable of resolving differences
that are completely inaudible. Therefore, any claim that one valves sounds
significantly better than another (for whatever reason) is meaningless without a
series of measurements that show what the differences are. There is no doubt
that there are real differences, and these will show up as lower (or higher)
output impedance, distortion (harmonic and intermodulation), etc.

When people refer to 'valve sound' vs. 'transistor sound' in amps, then there
are some significant differences, although not all are necessarily audible
during normal listening. Some of these have already been covered in another article and will not be regurgitated here, other
than to point out that valve amps have a relatively high output impedance (low
damping factor), so allow the speaker impedance to influence the frequency
response. When demonstrated, higher than normal output impedance is almost
always described as sounding 'better', but the listener is often unaware that it
is no longer accurate. This is of no consequence if you really do like the sound
of an 8 ohm speaker driven from an amplifier with a 2 ohm (or more) output
impedance, and exactly the same thing can be done with transistor amps - I've
been doing it for decades.

There are some other differences too, some subtle, others not. Distortion is
an obvious one - no valve amp ever made can equal the distortion figures (or
distortion inaudibility) of even typical quality transistor amps.
Crossover distortion is very audible with transistor amps, and less so with
valves, but there is no reason or excuse for any amp to have audible crossover
distortion, regardless of whether it uses valves, transistors or a combination
of the two.

Provided we restrict ourselves to hi-fi amplifiers, valves really offer few
benefits and many drawbacks. Duplicating the output impedance and frequency
response are easy with transistor amps, but this is rarely done for the simple
reason that the wider bandwidth and lower output impedance of transistor amps is
now expected, and the low impedance is used as the typical source for
loudspeaker designs. Changing the source impedance for a loudspeaker system
changes the design, so low impedance is assumed. Always.

Many factors are harder with valve guitar amps. Much of the 'sound' is the
direct result of either marginal (or poor) design of the output transformer and
valve output stage, and/or the omission of any means to prevent high voltage
spikes in heavy overdrive. While it is theoretically possible to regenerate (at
least to some extent) the results of these (IMO) common mistakes in a transistor
amp, no-one has ever done so. It's probably safe to assume that it's not been
done because the sound can be bloody awful in extreme cases.

There can be no doubt that different valves can have different overload
characteristics, and these differences may be audible in some cases. Very few
opinions on the Net have backup by way of blind testing, so they are just that -
opinions. Unless there is validation by way of double-blind comparisons or
detailed measurements, pay no heed to the claims that EL34s and 6L6 valves are
'chalk and cheese' (for example).

2.6 - Blue Glow

Rumours persist that any blue glow inside a valve is bad, and indicates a
fault. This is not necessarily the case, and there is some very good information
on the Net that describes the different types of glow one might see and what
each means. In particular, I strongly recommend BLUE GLOW IN ELECTRON TUBES, since it has
many excellent pictures and some very good historical information.

In short, the blue glow depends on many factors, and for power valves it is
common. Much of the cause is fluorescence of the glass when bombarded by high
speed electrons, and the glow is typically a very deep blue, and is seen on the
inside of the class in places where electrons manage to escape capture by the
anode. Many valves have holes or cutouts in the plate to facilitate grid
alignment during manufacture, and these make ideal escape routes for some of the
cathode current. There are other reasons for a valve to glow, and only one type
of glow indicates a fault.

Some valves rely on low pressure gas (neon, krypton, etc.) and were used as
voltage regulators - this was well before zener diodes were invented. These are
meant to glow fairly brightly, and the colour depends on the gas mixture used.
Far less common, except for high power applications, were mercury arc
rectifiers. These have a characteristic blue hue, and also emit significant
ultraviolet light. While vintage enthusiasts may come across them occasionally,
most will never see one. The only one I've seen was a very large 3-phase version
(I seem to recall it was about a 400mm diameter spherical shape), and was
probably used to provide DC power for electric trains.

Should you see an output valve with a faint light blue glow inside the plate,
then it is probably gassy, and is approaching failure. The glow is caused by
electrons striking gas (air) molecules, causing them to fluoresce. All valves
have a tiny residual amount of gas, but there should never be enough to create
the characteristic glow of a gassy valve.

A good indicator that a valve is gassy (rather than just causing
fluorescence) is to look at the vaporised 'getter' material - the (usually)
silver coating on the inside of the glass. In most cases, the first indication
that there is gas involved is that the edges of the silver getter coating turn
brown. A valve with no vacuum at all (or at least lots of gas) may have a
getter that's all brown or even light grey or white (the latter two mean that
there's oxygen in the valve - a sure sign that the glass is cracked somewhere).
As a general rule, don't use any valve where the getter deposit is not as it
should be. Its purpose is to absorb stray gas molecules that are liberated from
the metal and glass from which the valve is made, and if it looks a different
colour from that of another valve of the same type, then assume the valve is
faulty.

You Can Replace Valves Yourself

Note that this refers primarily to output valves, and it is usually
quite ok to change preamp or other low-power valves without much risk. Otherwise
...

You can replace valves yourself. Absolutely. No question at all. They
are in sockets, and anyone can remove and replace them at will. Doing so will
always give you a result, but it may not be the one you hoped for. There are so
many reasons not to do so that listing them all would be tedious.
Instead, I'll note the things that must be part of a valve change, as
well as those things that should be included. In some cases, the tests
will require equipment that the valve amp owner simply doesn't have or even have
access to.

Apart from equipment, there is also a level of knowledge (and experience)
that is necessary to ensure a satisfactory valve replacement. Without these two,
both amplifier and owner are at risk, and that risk is especially great for the
inexperienced owner. Delving around inside an amp that runs from a 500V (or
more) supply is not just not recommended, it's positively dangerous. One tiny
slip of a finger, and your loved ones could be lining up to place flowers on
your coffin - and no, I'm not exaggerating - not even a little
bit!

Assuming that you survive the ordeal, how will you know that the end result
is better, worse or the same as before? If all you have to go on is your ears,
unless the original fault was gross (one output valve not working for example),
you have to rely on audio memory - something that has been proven over and over
again to be extremely unreliable. Without test equipment and (experience based)
knowledge, you're working in the dark, and have no idea if the valve change has
done anything at all. In many cases, it's actually necessary to modify the
amplifier to allow you to measure the current in the valves, and if you don't
know what you're doing you can easily end up with an amp that doesn't work, or
worse, requires very expensive repairs. All fixed bias amplifiers should have 10
ohm 5W resistors between the cathode and earth (ground/ chassis), and this makes
measurements easy. The resistors should be matched first, since most are 10%
tolerance, and you need better than that. The resistors should be matched to 1%
- the absolute value is not especially important, only that the two (or four, or
...) resistors are within 1% of each other.

Output valves do need to be matched, and this usually requires that you have
many more to hand than you'll actually use. Even if matched sets were purchased
(see matching,
above), you will still need to verify that they really are matched, and
you'll regularly find that supposedly matched valves are quite different. The
first process is to match the valves for quiescent (bias) current, and this
usually requires that one valve is deemed the 'reference', and others are graded
against it. Many insertions and removals later, you find a few valves that allow
you to select pairs with bias currents that are within 5% (or better) of each
other. Whether you continue the process to the next stage or not depends on the
application.

The next stage is to run the previously matched pairs to verify that they
have similar gain (gm, or mutual conductance). Before doing so, you must verify
that the grid drive signal is identical for each output valve. Some hi-fi amps
provide a means for adjusting this, but it's rare in a guitar amp. If the
voltages are different, you need to make whatever changes are needed to ensure
that they are not only equal voltages, but have the same waveshape. This
typically requires the use of a low distortion audio signal generator set for a
frequency of around 400Hz, and a dual trace oscilloscope with the ability to
add/subtract one waveform from another. It's nice to have 100:1 probes because
of the high voltages, but 10:1 probes will usually be sufficient. If the
waveforms are identical but inverted, the sum of the two gives a flat line -
zero output. Once the grid drive voltages are correct, you can move on. Note
that if you can't match the output valves perfectly it may be necessary to
offset the phase splitter outputs to compensate, but in general it's unwise to
do so.

Matching at different power levels is difficult to do in a push-pull power
amplifier, because it's very hard to see the possible waveform differences
caused by a mismatch. The 10 ohm cathode resistors come to your aid again. You
can then drive the amplifier to half and full power (using the same oscillator
as before), and verify that the voltage measured across the resistors is the
same for each valve. If it's not, the valves aren't matched for gm, so you can
now go through the remove/insert (repeat as needed) process again, until you
find a pair of valves that is matched for both bias current and equal
current at different power levels. Finally, verify that the amplifier clips
symmetrically with the nominal (resistive) load attached. If it doesn't, that
means that either rP (internal plate resistance) is different,
which affects the maximum current that each valve can draw, or the valves have
screen grids that may not be in perfect alignment, so the valve simply cannot
turn on as hard as another. Well made valves will generally be close enough for
all but the most critical applications, but the test is easy to do and may
improve your level of confidence. You'll also get to see the process in
action.

On no account should your oscilloscope probes be
connected to the plate connections for the output valves. The voltages
developed are lethal, and can easily damage your probes (even 100:1 types
unless rated for at least 5kV) or your oscilloscope. Accidental finger
contact may cause instantaneous cardiac arrest, after which your interest
in valve amplifiers will be terminally diminished to
nil.

Never underestimate the secretly malevolent intentions
of high voltages - they have infinite patience, and are silently waiting
with tooth and claw poised to see to it that you don't do it again. Yes,
I'm being frivolous, but 500V DC is not !

Asymmetrical clipping introduces a net DC component into the output
transformer (as does imbalance at any power level), which can cause premature
core saturation and distortion at low frequencies. While we all understand that
hi-fi amps should not clip, it is pragmatic to expect that it will happen during
high level transients. Whether this is a major issue for you is up to you to
decide. For guitar amps, symmetrical clipping is important, because this is how
the amp is operated much of the time.

For a stereo hi-fi amp, the valves in each channel should be matched against
those in the other channel, or you may have a channel imbalance that disturbs
the stereo image. Check that normal full power is available - a transformer
fault will cause power output to fall dramatically (for example).

You may then decide to run a frequency response test to verify that both
channels are the same from 20Hz to 20kHz. While it's uncommon for valves to have
a significant effect on response, it's easy to test while you have everything
set up. If the results of these tests are all as close to identical as you can
get, the job is done ... until next time.

See, simple isn't it? Provided you have the equipment and the knowledge, as
well as a stock of valves (which must also be the same brand, type and age of
course), the process is pretty straightforward, As described, very, very few
enthusiasts will have the necessary equipment. Before the amp is returned to
service, there are a few other tests that should be run. It's worthwhile to
check the ESR (equivalent series resistance) of the filter capacitors. As these
age, ESR increases much faster than capacitance decreases, and high ESR means
the caps should be replaced because they are on the way out.

Test that all valve sockets have a good grip on the pins. Many sockets use
rather flimsy contact forks, and these must be retensioned before the amp is
returned to service. Also, check for dust or other gunge around the sockets -
especially carbon tracks which indicate that there's been a flashover at some
stage. Replace the socket, unless you are confident that you can make a
serviceable repair to a carbonised/damaged socket (it's not easy). A visual
check of the amp's interior is needed to spot any possible problem areas that
may cause failure at some later date - this requires experience. If you don't
know what to look for, then you don't have enough experience with valve amps.
Any suspect capacitors or darkened resistors need to be replaced, wiring
checked, and if any circuitry is mounted on 'elephantide' with rivets as
connection points, it should be checked for hygroscopic performance. Some of
this material becomes quite conductive (several Megohms) under humid conditions,
and can cause major problems during periods of high humidity.

Finally, the bias adjustment must be rechecked for your newly matched valves
in what is now (hopefully) a fully serviceable amplifier. A final check before
closing everything up and you're done.

So yes, you can change your own valves, as long as you can do everything
above using your own knowledge and experience. If anything above makes no sense
to you, then I suggest that you find an experienced service technician to do the
job for you. If everything looks straightforward, you have the equipment and the
background knowledge, and you are confident (but circumspect) around high
voltage supplies, then you can replace your own valves, otherwise,
DON'T DO IT!

Conclusion

While the above covers the major myths surrounding valves, there are
obviously others. Many of these fall into the 'magic component' category, and I
refuse to give these claims any credibility (or web space) by attempting to
answer them. What I've attempted to do here is state the facts, and as readers
of my site will be aware, I don't accept the premise that magic exists, so my
approach is based on verifiable test methods, and the results obtained from such
tests.

If I happen to have omitted your favourite myth, please let me know. It needs
to be able to be verified though - anything based on subjective tests (without
the benefit of a double-blind test regime) can neither be verified or refuted,
since it is almost certainly imaginary. The words used by the subjectivists are
meaningless, because they don't describe any physical property or measurable
phenomenon. Without these, it's anyone's guess as to what is actually meant, and
in any dispute the subjectivist can argue that one simply 'misunderstood' what
was said.

There are also several minor myths, including the speed of valves (hint -
triodes in particular are s-l-o-w) and silly nonsense such as the points
referred to in the intro. Especially interesting is the inane claim that valves
are "better at reproducing deep bass and extended (...) highs". Sometimes I
wonder if the authors of such drivel actually believe the crap they write or if
they are having a laugh.

References

The primary references used for this article were various valve distributors'
websites, to see what criteria were used for matching. Most tell you nothing
that's even remotely useful, a few provide some worthwhile info that can be used
to verify that the matched valves are likely to perform well, and others say
nothing at all. As in absolutely nothing - not a word about the process, but you
may get to see a glowing review (meaningless twaddle) from some magazine or
another that waxes lyrical about 'authority', 'immediacy' or perhaps
'intimacy'.

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article, including but not limited to all text and diagrams, is the
intellectual property of Rod Elliott, and is Copyright © 2009.
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
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is prohibited without express written authorisation from Rod
Elliott.

Page published and copyright © 09 Nov 2009./ Updated Sep 2014
- minor changes only.