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For audio amplification, the first active device remains the best active device: the triode vacuum tube. The triode is unique among the active devices: vacuum tube pentodes and transistors of all sorts. With these devices, the plate/collector/drain current is largely independent of the voltage across the device. This property makes them excellent approximations of an ideal current source.

With triodes, however, this is not the case. The plate current can vary with the plate-to-cathode voltage. This is the meaning of "amplification factor" --

μ= ΔV_pk / ΔV_gk (at Ip= constant)

For most triodes, the amplification factor will usually be between 10 to 100. For power triodes, this can be smaller (audio power finals, vertical deflection power amps, series pass regulators) or higher ("zero bias" RF finals). Amplification factor is largely meaningless for other active devices since it's so difficult to measure directly.

The triode, like every other active device, has an inherent degeneration when operated as a grounded cathode amplifier. This being the cathode resistance: r_k= ~1 / g_m. This resistance acts in precisely the same manner as if it were an unbypassed resistor soldered into the circuit. Just because you don't see it doesn't mean it's not there.

There is another feedback mechanism at work as well. When the signal pulls the V_gk less negative, the plate current increases, and with it, the voltage drop across the plate load. This results in a decreasing V_pk. As this voltage decreases, it tries to pull the plate current lower. Since V_gk and V_pk are pulling in the opposite direction, this is negative feedback by definition.

It is this additional source of NFB that serves to correct for harmonic distortion to a greater extent than you will see with other active devices. What harmonic distortion remains is mainly the second harmonic, h₂. Sonically, h₂ is the least detrimental. It is this h₂ that lends to the so-called "tube sound", described as "warm", "rich", "full", etc. More h₂ is described as "dark". No distortion would be the best, but the perfect amplifier, like the perfect lens for telescopes, has yet to be invented. The question becomes how to minimize those defects which you will never completely eliminate.

Of course, not all triodes are equally good sounding. As far as the effectiveness of plate current control by plate voltage, this is measured by the term: plate resistance: r_p. A good many high-μ triodes attain that large amplification factor by driving up the plate resistance. For the 12AX7, the spec sheet gives: r_p= 80K (nominal). This is comparable with the r_p's of small signal pentodes. Is it any wonder why the 'AX7 tends to sound like a small signal pentode?

Even if the r_p isn't excessive, some triodes just won't perform all that well for audio. This being caused by excessive variation in g_m with plate current variations. For all active devices, amplification tends to increase with increasing current. If the positive going half cycle of a sine wave receives more amplification than the negative half cycle, then the two half cycles hit different peak voltages. That's not a sine wave any more, and something has been added. Since this is asymmetrical distortion, it is even order, and mainly h₂. Types such as the 12AV7 tend to produce much more of this distortion than you'd like to see. (Although it just might be useful for audio effects.)

Nothing sounds better. It is for this reason that this "obsolete" device is still around over a century after its invention. This has been known at least since the early-1950s when the quest for a solid state device that could match the distortion performance of the VT triode began. So far, no one has found such a device.