All active devices make noise. By "active" I mean parts like transistors, FETs, MOSFETs, tubes and such things that do amplification. Heck, even resistors, which are purely passive parts make noise as a function of the random electron movement within generating a voltage across the resistance.
The quietest active devices are bipolar-junction transistors, BJT's, commonly referred to as "transistors." But if you're on this forum, you might not like the sound of them when used as voltage amplifiers. Junction field-effect transistors (FETs) are a bit noisier. They also have a distinctive sound, but at least it's not obnoxious. MOSFETs, a different kind of field-effect transistor are pretty noisy and don't sound particularly swell as voltage amps. Noisiest of all are thermionic devices -- electron tubes, or "tubes." As voltage amps they sound the best (to my ears, I'm biased since I'm a musician) when set up in a good circuit and with good associated parts.
To hear tube hiss when your ear is 1'' from the tweeter is not a cause for concern, nor an indication that the tube is failing. A tube can get quite noisy and continue to amplify just fine, so it hasn't failed as an amplifier, though for audio applications it might not be acceptable. The presence of noise, or an increase in noise, does not indicate incipient failure, it just means that the tube is or is becoming noisier. Some tubes come "out of the box" noisy, some are born quiet and stay that way, and some start quiet and go noisy. Seldom have I seen a noisy tube go quiet. When that happens I am deeply suspicious that the next that that will happen is that my car will fix itself. A very worrisome situation.
Tube microphonics -- pinging, bell or clunking noises when you tap the tube, are purely mechanical issues: the metal bits inside are rattling about, but again, microphonics do not indicate failure per se, though in a practical sense the tube may not be useful for audio. Cryopgenic tempering of the tube may lower the "Q" (resonance) of the metal bits and reduce microphonics.
Some folks do get obsessive about this, others take it in stride, knowing that sometimes you have to put up with a bit of hassle in order to enjoy what tubes have to offer. It's a matter of temperament and experience, I guess.
I hope this helps!
Mike Elliott
www.ariaaudio.com
