Bitar Amplifiers Complete tube tone

"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.

Prove it to yourself. 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.


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.

Rectifier Tube: 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.

Resistors: 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 resistance!

Transformers: 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.

Speakers: 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.

Duty Cycle: 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.
Greensboro, NC
(336) 317-2710