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The pursuit of the perfect guitar tone

15 Jun 2015  | Paul Pickering

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Compared to other standard configurations, the common cathode amplifier features high input impedance, medium-to-low output impedance, relatively high gain, and good frequency response. The grid bias resistor RGB biases the grid to zero volts DC (negative w.r.t the cathode) and provides a high input impedance. The grid resistor RG combines with the input capacitance of the triode to form a low-pass filter; this has a -3dB point of approximately 20kHz and dampens higher frequency oscillations that might cause instability.

The bias point of the common cathode configuration is set by the cathode resistor RK. The inverted output voltage is developed across the anode resistor RA. A load line representing RA is drawn on the iA vs. vA characteristic curves and the bias point is chosen.

The operating point on the load line is not necessarily chosen for symmetrical operation due to the clipping characteristic of the vacuum tube when overdriven and its effect on listeners, as we'll see.

Clipping in tubes & transistors
When overdriven, both transistor and vacuum tubes exhibit clipping, but their response is different, as shown below.

The transistor clips at the power supply voltage, less a small amount that depends on the circuit design and the transistor characteristics. The result is a "hard" clipping characteristic as shown in red. The vacuum tube, on the other hand, clips in a more gradual fashion. This response is known as gain compression or "soft" clipping as shown in green.

Figure 2: Transistor "hard" (red) vs. tube "soft" (green) clipping (source Elliott Sound Products).

Why does a tube clip (distort) in this manner? Here's a qualitative explanation: tubes produce a "cloud" of electrons around their cathodes. This cloud has surplus electrons available, so that for sudden current demands (such as musical peaks), a tube can respond for a few milliseconds before it clips. The result is that the clipping threshold is not rigidly fixed as it is in a transistor amplifier; it varies depending on the dynamics of the music played.

The spectral composition of the two different responses differ. When transistors overload , the dominant distortion product is the third harmonic. The strong third harmonic produces a tone many musicians refer to as "thin" or "hard". On the other hand, with tubes (particularly triodes) the dominant distortion product is the second harmonic. Musically, the second harmonic is an octave above the fundamental and is almost inaudible, yet it adds body to the sound, making it fuller.

Soft clipping has another beneficial effect. When two or more notes are played together, such as in a chord, intermodulation products are formed, consisting of the sum and difference of individual frequencies. These effects are unpleasant to the ear; soft clipping tends to reduce their formation.

Figure 3: Symmetric clipping – odd harmonics predominate (source: Effectrode).

Distortion & Psycho-Acoustics

Why do musicians find a distorted tube sound more pleasing? This is of course a subjective judgement to do with the psycho-acoustics of the ear. Despite the reference to "Mojo, Voodoo & Secret Sauce" in the title, some facts about the ear are well-established:

 • The smallest step-change in amplitude that can be detected is about 0.3dB for a pure tone and 0.5—1dB for a more complex tone.
 • The smallest detectable change in frequency of a tone is about 0.2% between 500Hz and 2kHz.
 • The lowest detectable amount of harmonic distortion depends on the harmonic content of the signal, but is approximately 1% for lower-order harmonics.

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