Tube Tech

A lot of audiophiles (myself included) are in love with tubes, due to their superior sonic properties. What makes tubes so special, anyway? People have been trying to figure that out for years. Personally, I don't think it's any one specific thing. Although tube amps figuratively fall of the test bench when it comes to hard measurements, people hardly listen to white noise or square waves in real life (well, unless they're autistic or something). The attributes of tube sound we find most pleasant are clarity or resolution of fine detail, and freedom from "harshness" or "grain".

In terms of resolution I think that there are two contributing factors, one being high voltage, the other being space charge. Tubes operate at much higher voltages, up to 600V for a power amp, compared to transistors, which don't usually go beyond 48V in that application. I believe higher voltages in an amp reduce the resistance relative to the potential in things like sockets and hookup wires, and allow capacitors to charge and discharge more quickly. Speaking of capacitors, those rated for higher voltage are generally more well constructed than their low voltage counterparts. The other contribution is from the "space charge". The hot cathode of a tube is surrounded by a cloud of free electrons. This space charge provides a pool of instantly available electrons, smoothing out the supply and providing extra power for rapid transients in excess of what the power supply by itself would be able to deliver.

There are a few likely candidates for the reduction in grain provided by tubes. One of the most popular ideas is that tubes don't clip the same way as transistors. Clipping is where a signal is overdriven past the point that the power supply would be able to handle. Tubes are usually operated in a way that minimizes clipping, this is mainly to protect the cathode from "stripping", but has the fortunate side effect of minimizing their ability to clip large signal transients. When a tube does clip, it doesn't clip abruptly. When a transistor amp clips a signal it tends to literally cut off the top of the wave, while a tube amp would round it off, which more closely approximates an undistorted signal.

Tube amps typically produce more total harmonic distortion (THD). While I've seen transistor amps with THD as low as 0.001%, there are some highly regarded tube amps (single-ended triode, mostly) that have up to 5% THD. This is counter-intuitive, since low THD is widely considered to be a good thing. Some people think that the (over) application of negative feedback, which is used to reduce THD can lead to a very dry, artificial sounding amplifier.

But what about tube rectifiers? Some maintain that tube rectification makes a difference in the sound of an amplifier. Personally, I think this is only evident in systems with very simple power supplies. Tube rectifiers do have less switching noise than a solid state diode, and may help to filter out some external powerline noise. I haven't really made up my mind about this yet, but I do use tube rectifiers in my projects, mainly because it's cheaper and easier than wiring up timers and relays for a soft start.