THE TUBE SOUND
"Complete Tube Tone" means that we design circuits that preserve the
maximum amount of tube tone coming from an amp, and we don't do trade-offs
just to put out more power. If your amp is giving you the goods
tone-wise, you'll spend just a few seconds dialing in "that" tone.
That should be the objective every time you power up an amplifier.
The tone you dial in with an amplifier carries the effect of
intentional tone shaping as well as distortion. Speakers, signal
tubes, power tubes, rectifier tubes, transistors, transformers
and even resistors can contribute to the output tone.
The sound of a vacuum tube is described as being "smooth", "singing", etc.
but what's happening here is plain and simple, it's compression
Tubes and speakers are the main contributors to that end.
When tubes reach the threshold of distortion, the condition is
referred to as "overdrive". The effect is to flatten out the top
of the waveform, but with no sharp edges. As you push the tube
a little harder, the distortion is mainly due to clipping. Clipping
occurs as the gain stage reaches a hard power limit, and the top of the
waveform is clipped off, sharply. A transistor gain
stage, when fed too much signal is capable only of clipping,
which always includes sharp edges on the waveform, contributing to
a harsh sound.
The waveform defines the tone...
The best way to understand tone is to view the one note you play
as a group of sine waves that add up in combinations to yield
the waveform you hear. This is a composite picture of the tone.
For many instruments, including guitar, a clean tone is embodied by
a pure sinewave at the fundamental frequency, with no
harmonics. This is illustrated by the top-most waveform in the
oscilloscope patterns shown here.
The sweet spot of compression, with no hard limiting, is the point
at which you get the fundamental plus a strong 2nd harmonic. This
tone is referred to as "overdrive", or "soft clipping", and is the
point at which the 2nd harmonic will dominate the odd harmonics. In
these cases, the fourth harmonic is above the noise too, but
typically remains weaker than the third. This is illustrated by the center waveform in the
oscilloscope patterns shown here.
A heavily clipped tone
approaches being a square waveform. This turns out to be a combination of the
fundamental, along with the odd harmonics (the 3rd, 5th, 7th, etc..).
Here, the odd harmonics are much, much stronger than the evens. This is illustrated by the lower waveform in the
oscilloscope patterns shown here.
Some of the major design features of the electronic signal path that contribute
heavily to the tone, include the following:
- single-ended vs. push-pull (aka differential)
- triode vs. pentode mode
- cathode-bias vs. fixed-bias
- open-loop vs. negative feedback loop
These innovations (inventions really) were originally realized for needs far beyond
home audio or music production (let's keep in mind how important signal processing
is for a myriad of other critical needs). But their use did find it's way into the
realm of "mere audio", and one by one were used to get more efficiency, power, and
clean headroom out of public address systems, living-room systems, and even guitar amps.
Well... the hi-fi audio enthusiasts --thinking themselves the vanguard of all things audio--
rejected each of these innovations in turn, citing a decrease in tonal warmth, or just plain
harsh tone, in any design where a circuit departs from triode, single-ended, cathode-bias, open-loop
operation. To most electrical engineers, this is crazy talk, because in theory and
in most applications, the usefulness of these advances was indisputable.
In fact, using tubes vs. solid state devices (transistors) is actually saying that you prefer
a certain type of distortion; that which is dominated by even harmonics. In the relatively
distortion-free realm of music reproduction, it's a hard-sell. But in the world of
music production (guitar amps mainly), where harmonic distortion levels routinely exceed 5%,
it's easy to demo a tube and solid-state amp, side-by-side for non-musicians and find that
they can detect a difference in the distortion signatures. And this is really saying that
the human ear is capable of distinguishing a waveform that includes more power
in the even-harmonic content, than in the odd-harmonic content. So, not necessarily
true for all comparisons of tube vs. solid-state. It depends on the circuit design.
For example, a guitar amplifier like the Bitar Blue Moon, which employs a triode,
single-ended power section, generates the maximum amount of even harmonics.
More importantly, the even harmonics generated in the power section are left intact,
and not cancelled out in the output transformer, as is the case in any
differential power section. The single-ended arrangement allows lower
voltage swings, and thus, less power for a given type of power tube. This
type of design would seem extreme to some, but when done well, the tonal difference
can be very obvious.
At the other end of the design spectrum, take a tube amp that employs, push-pull,
pentode-mode, fixed-bias, with considerable negative feedback. This will increase the
potential for tones that are deemed harsh. We need to be careful here, because it's
still true that an intelligent, robust design can employ these methods and find a
very willing audience of players and listeners.
A good player will stand out on most any amp, but for any player it's more fun to
use an amp that gives you the touch sensitivity to fully express
yourself, and allows you to easily dial in the tones you hear in your head.
Ask yourself, "Why run tubes at all if you're going to cancel out the
best part of the tone before it gets out of the amp?" You do owe it to yourself
to try a wide variety of gear, and gain a broad frame of reference. But the whole idea is to
determine which tones make you think differently as a player, to determine which tones
allow you to get "your sound" without having to compensate for a lack of touch, harmonic
content, or clarity in the output. In the end, it has to be the right choice for you.
CLASSIC TUBE AMP TONAL ARTIFACTS
The following discussion of components and design tradeoffs is
intended to describe briefly the basis for the typical sound we
hear coming from tube amplifiers.
When Tube amplifiers are designed to get the most possible power
out of the tubes, that usually means a pentode, differential power
section. In terms of tone, the price can be high, as the even-harmonic
content can be reduced. In this case, the design trades tone for more power.
Since the 1950's, the differential power section has been the typical
power amp configuration for guitar amps rated above 10 Watts.
Preamp and power amp tubes:
A vacuum tube's native response allows the possibility for considerable
compression and clipping. This is illustrated in the transconductance
curve for the device, and is most pronounced in triodes, and in pentodes,
and beam power tetrodes when configured to run as triodes.
When the input to the power amp stage is cranked, and a player
picks a note, the situation demands peak current from the power
supply. A vacuum tube rectifier, due to it's effective series
resistance, will cause a lag in the voltage rise. This effect is
called "sag". It tends to soften the attack of the note. This is
most pronounced in differential stages (typically the power amp stage)
running in class A-B, since they idle at low, low current and demand peak
current at the instant the note is played.
Carbon composition resistors manufactured several decades ago offer
lower resistance as the voltage across them increases (specified
as the voltage coeffiecent of resistance). This response distorts
the waveform and can actually add some compression. In theory, that effect
can be used to some advantage in one or two locations in an amp, where the
resistor sees high voltage swing. Still, the net gain in compression here
is very small compared to that achieved by other means.
As for using them throughout the amplifier, the dominant effect is that
you get the downside of this technology: hiss, and temperature drift.
And that makes for a considerable tradeoff in performance. In addition,
when using newer carbon comps, you get even less of the effect because
manufacturing has improved and resulted in a much lower coefficient of
Transformers are a magnetic circuit in and of themselves. When pushed
to the limit, they will add distortion effects of their own. Some people
like the effect for the character it adds, while others view it as an
undesirable feature of an underdesigned (cheaper) system.
The speaker voice coil, at the extremes of its travel, can add
a pronounced compression effect. This topic would take a good bit
more space to treat fairly. Let's just say that speakers are one
of the most subjective choices in any audio system, not only because
of their frequency response profile, but also due to non-linear
side effects as you drive them closer to their limits.
All vacuum tubes have a DC bias point, allowing positive and/or
negative signal swings around this "center" voltage level. If the
positive and negative swings have an equal pulse width, then they
are said to have a 50/50 duty cycle. Each pulse width is 50% of
the full cycle of the waveform. If the bias point is set such that
the positive and negative swings produce a slightly different
amplitude for the same input stimulus, then the two halves of
the waveform can take on different shapes and pulse widths. The
duty cycle can pull toward 45/55, etc. When produced by a single-ended
power section, the situation means there is increased even-harmonic
content in the waveform.
A differential amplifier can produce this effect, but the situation
can easily be accompanied by "crossover distortion". This is another type
of distortion found in class A-B operation, where part of the waveform
sits at zero volts for an instant when transitioning between positive
and negative halves of the waveform. That sound is undeniably harsh,
and un-musical, requiring the presence of plenty of odd-harmonics.
Crossover distortion is eradicated by correctly biasing the output
tubes in a differential amplifier stage.
Currently, DSP modelling and some well thought-out solid-state circuits
can produce and emulate some great sounds, but there may be more hope for
research that has recently occured in the electronics industry that would produce
vacuum chambers on silicon (or some other solid-state material). These chambers would then
be used to produce actual thermionic emission, the main principle of vacuum
tube devices. But how far off is this stuff? Dependent on what markets
it might satisfy, this technology will easily take years to develop, and
millions of dollars to commercialize.
Bitar Amplifiers, Inc.