More Questions about Tube Noise

If you will recall, perviously I posted a question about some noise form a pair of RT 12AU7 tubes in my CJ PV10A. (Thanks for the great info everyone, by the way.) I sent them back to the dealer and he found that one of the tubes was noisy and replaced both. I installed the replacements today but am still hearing noise of the tubes. Although, it is very faint and can't be heard unless my ear is within an inch of the speaker. The other tube is making no noise. So, my questions are: How much noise from a tube is acceptable? If a tube is noisy does that mean that it is 'bad' or not functioning optimally?

Thanks again!!!

Brad Day
Atlanta, GA
You are being obsessive. With high efficiency speakers and tubes you are lucky to have no noise at the listening position, much less within an inch of the speaker. Within an inch is great even with a solid state system. Relax and enjoy the music.

It's not that I'm obsessive (well, maybe...), it's that I just don't know that much about tubes yet.
However, I promise to enjoy the music on a regular basis...
(BTW, my speakers are Alon 1's)


Brad Day
Atlanta, GA
In my experiences, tubes always give off some noise. I'd be happy with the replacement tubes you received. Sounds like the dealer took good care of you.
There really are several types of noise and they can be amplifiable or constant. A low level non-amplfiable hiss goes with the territory. It is unlikely that this is an issue at the listening position. If this is the case, relax, sit back and enjoy the music.
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
Good informative post, Ariaaudio. As a musician, you should know that quiet tubes are an oxymoron for many guitarists, who PURPOSELY drive their amp output tubes into distortion! Tubes are know and selected for their distortion characteristics. No guitar player worth his salt will use a transistor amp because they won't produces that funky tube distortion sound without elaborate electronic circuitry which still will not give that same wonderful "chime". Thanks to guitar amps, the 6L6 (EL-34 substitute) will NEVER become obsolete and go out of current production! Thanks guys, and Happy Tunes!
Hi Fatparrot,

Yer darn tootin' that guitarists overdrive tubes to get that fat, rich sound and would not tolerate transistor in that application. But there is a difference between the desirable (from the artist's point of view) sound of a tube being overdriven from "tube noise." Guitarists will not put up with a hissy, crackling, microphonic tube any more than a hi-fi enthusiast will. I've helped many a guitarist replace tubes between sets when one goes noisy.

Mike Elliott