Vacuum Tube For Audio and Tesla Coils

The vacuum tube was invented by English physicist John Ambrose Fleming in 1904 as a basic component for electronic devices and used throughout the first half of the twentieth century. It brought about great innovations in television, radio, radar, sound recording and reproduction, telephone networks, industrial automation and, most importantly, the development of analog and digital computers. It was essentially the predecessor of the modern transistor, which brought about a revolution in technology and paved the way for the development of the personal computer.

What does Vacuum Tube mean?
A vacuum tube is a device used to control the flow of electric current using a vacuum in a sealed container, which usually takes the form of a glass tube, hence the name. The vacuum tube is the predecessor of the modern transistor and was used in similar ways as electronically controlled switches, rectifiers, amplifiers, oscillators and in other creative ways that transistors may be used today. The cathode ray tube (CRT) used extensively in early television sets and computer monitors as screens is a kind of vacuum tube.

Why do vacuum tubes sound better in audio devices?

It's not universally agreed that they do!  But I am of the opinion that they do (in certain situations), and I'll state why I think so and also give a few examples where tube-based equipment might not necessarily be the best fit.

I'll also go ahead and state for the record that I'm not one of those silly audiophiles who worships gold-plated connectors and $500 cables and views music as simply a necessary input needed to listen to his sound system.  In the stuff that I build, I don't spend absurd sums of money on "audiophile-grade" capacitors that have supposedly magical properties.  I'm a music enthusiast and electronics hobbyist who has tube and solid state equipment, and listens to music on vinyl records and iTunes alike.

First, tubes are very good at producing 2nd-order harmonics.  These harmonics are at double the fundamental frequency, one octave higher.  They sound musical.  They sound "velvety" because they add some extra stuff.  This is especially true of single-ended amplifiers; unlike push-pull amplifiers, the single-ended topology has no way to cancel out the even harmonics added by the power output stage.  Consider a tube like the 6L6 ... in a single-ended configuration, this tube produces on the order of 10% total harmonic distortion, but that distortion is overwhelmingly 2nd-harmonic.  It is a fat-sounding tube.

Now, a technique used in tube and solid-state amplifiers alike to reduce distortion is negative feedback, where a potion of the signal is fed back out-of-phase into the input of one of the amplifier stages.  This can take the form of local negative feedback (feedback from an amplifier stage to itself) or global negative feedback (feedback from the amplifier's output stage to an earlier state of the amplifier, usually the one after any of the user controls).  Solid state amplifiers use negative feedback to a large extent, partially for some of the reasons it's used in tube amplifiers (reduction of distortion, flattening of the frequency response, lowering of output impedance), but also because some solid state topologies require it to function at all.

One drawback of negative feedback is that not everything in a circuit happens instantly, and phase shift can occur if there are many stages encompassed by the global negative feedback loop.  This especially an issue if the stages are RC-coupled (i.e. capacitors used between the stages to couple them together).  This phase shift is not readily apparent when bench testing an amp with a sine wave (which is how THD measurements are taken).  But this phase shift can come into play in dynamic conditions, such as musical transients (a quick crack of a snare drum, for example), and can be seen on a scope when testing an amplifier with a square wave signal rather than a sine wave signal.  The phase shift creates distortion during these transient events, and tends to be odd-harmonics.  So, like an engine's horsepower rating, an amplifiers THD rating doesn't necessarily tell the whole story.

One thing solid state amps do have going for them, however, is their low output impedance.  The output impedance of a tube amplifier can be improved (reduced) with global negative feedback, but solid state amps have them beat in this regard.  Lower output impedance means a higher damping factor, which roughly translates into e.g. tighter bass response (if your damping factor is too low, bass response can sound "flabby").  If you listen to a lot of music that has tight bass (electronic dance music, for example), solid state amps might be a better fit.  That said, Daft Punk sounds great on all of my tube amps!

List some of the best vacuum tubes for audio and electronic circuit design

0. Complete list of vacuum tubes: The best and cheapest vacuum tubes

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Vacuum Tube - Tesla Coils

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