The first few pages of the Cunningham artcle are hard to read but after that it gets easier. As discussed microphonics are caused by (unwanted)changes in the spacings of the internal structure (electrodes) of the tube. Usually by some vibration. The strength of the electric field between the tube's electrodes is closely dependant on the distance between them. If this distance changes (due to movement caused by vibratioin) it is like getting a voltage change. Various parts of the tube, depending on how they are mounted, may vibrate and have resonate frequencies that can cause trouble in the audio range. This is especially a problem with some bass. The bass that shakes your pant leg may be having the same effect on the tube if you put your amp in the wrong place.
Tapping a tube (generally not a good idea) can sometimes sound the same as if you tap a microphone. Ive seen tubes that will pick up speech..that is you can talk into them and hear yourself. Evans says that even matched pairs can vary in this regard and need to be tested separately for M too. Some parts of the tube can move by internal force (the filament of directly heated tubes).
* J. Evans: Valve Microphonics: http://www.thevalvepage.com/valvetek/microph/microph.htm
* T.M. Cunningham: Practical Consideration in the design of low Microphonic Tubes http://www.triodeel.com/micro1.html
A little microphonics can be a good thing. Older vintage tubes are more likely to be a little microphonic. But these are also the ones audiophiles will pay a premium for. Is there a connection?
A little microphonics can add some "air" to the sound, or slightly emphasize the highs, which might be a good thing in your system? Trial and error as usual.
A tube with absolutely zero microphonics will very likely sound kind of dull in many systems. This is true of many new modern tubes.
Generally speaking, I can't agree with anything Sugarbrie said, but hey, that's audio.
Microphonics is not frequency dependant and it is the last way you want to adjust highs in a tube amp.
Tube amps without micrphonic tubes sound very good and generally better than those with them. If your tube amp sounds "dull" the last thing you want to look for is a microphonic tube to add to the mix. Change amps and use a different tube, change or tweak the circuit, make a better match with your speakers, blah, blah, blah... but I would not suggest shopping for microphonic tubes.
Sugarbrie...OK...If fate gives you lemons, make lemonaid! Now please tell me why turntable rumble is a good thing.
I'm with the Clueless one here. Yikes, talk about an unpredictable tone control/ reverb pedal! Microphonics be bad, very bad!
I appreciate the responses and I will give my case in point. I recently bought a quad of 60's Telefunken 6922's for my linestage. They tested (16.0, 16.0), (15.5, 16.0), (16.0, 15.5), (15.0, 15.0). I had original Sovteks that sounded dry and did not have very good extension. I tried some Tesla that were so-so as far as extension but the midrange was like back in the next room.
I installed the Teles and the soundstage is great, midrange super, and highs brilliant and detailed (but not edgy). However, the low end, especially mid to low bass is really bloated. I had basically the same Teles in a linestage previously and the bass was very tight and controlled. So, I am wondering if this is a tube component mismatch or if it is a problem with one or more tube(s).
If any of the tubes were microphonic, would I have audible problems throughout the range or could it be frequency-specific?
If the tube is only slightly microphonic when you tap on it, then what is the problem? I agree 100% that a tube that is microphonic in normal use is a piece of junk.
Folks including Kevin Deal at Upscale Audio, who sells some of the best NOS tubes, has said similar things.
If all distortion is bad, then why do they add white noise on purpose in CD playback?
tapping and sound pressure have extra large differences in values so ain't no need to compare or giving such examples.
microphony of tubes is realy brand and quality depended. it's caused by loose electrodes as a result of tube's aging or becomming unfunctional in vast majority of cases.
folks at upscale audio despite being respected and honest dealers try to create illusion as many of audiophiles do that microphony is good. it does however help the tube business. well, it was never good and it was a big issue upto mid- 40-s.
not precicely sure but probably different vacuum technology and electrode mounting was introduced ever since and surely tubes produced in 50's are not microphonic. i was never shopping for NOS tubes and don't know if any of the tubes are sold manufactured before mid- 40's but i guess they're too few... therefore if the tube is microphonic you can bet 1000 against 1 that it's already bad.
i'll repeat myself about tapping being not a good example and it can blow the speakers. but you can realy tap on component chassis or mounting board next to the tube to check the microphony.
Tube microphonics, generally, degrade soundstage imaging and detail. As to an "identifying" sound, tubes do not have a sonic character - the sound is a product of its environment (circuitry). Bias, gain, plate voltages, feedback loops, etc., will identify the sound a tube gives you in a particular unit while external elements such as vibration degrade the presentation.
As to the "tube sound" (warmth, LF bloat, etc), it's attibuted mostly to distortion that's inherent in its construction and function as a gain device, especially at the clipping point. I also think that the microphonic characteristics of tubes contribute to the effect, so I'll take Sugarbries' side on this one.
Above Motthird>>If any of the tubes were microphonic, would I have audible problems throughout the range or could it be frequency-specific?
I've never seen anything tying M to F.
above Gs>>Tube microphonics, generally, degrade soundstage imaging and detail.
agreed. Why do you agree with Sugarbrie that it is good thing? Or, is your position that it (microphony) is it simply unavoidable in normal audio usage, in which case, I basically disagree.
>>As to an "identifying" sound, tubes do not have a sonic character - the sound is a product of its environment (circuitry).
Agree and disagree. A tube cannot do work without a circuit. You need a load. No one has ever heard a tube alone. However, tubes do have individual characteristics and this leads to a certain results given a particular circuit. Look at a tube's plate curves and constants. It does not come close to telling you all you need to know but it tells you something. The curves and loadlines will of course change with the circuit.
>>As to the "tube sound" (warmth, LF bloat, etc), it's attributed mostly to distortion that's inherent in its construction and function as a gain device, especially at the clipping point.
basically Agree. And distortion at the clipping point has little to do with microphonics. Some distortions, like microphony, can be controlled in the audio range and in normal audio uses (which does not include tapping).
>> I also think that the microphonic characteristics of tubes contribute to the effect
To a very limited degree in most circumstances. All tubes are microphonic. You can't use a tube that isn't. Tap one (yes it's not a good idea) and they all respond. But this is not the normal use in audio so who cares.
Usually when someone says they have a microphonic tube it means they have one where the internal structure of the tube is weakened and lose. It acts similar to a microphone diaphragm as picks of vibrations (either music or tapping) and amplifies it. The technology has come far enough that this can be avoided for most audio uses.
Guitar players love this effect sometime and exploit it. Listen to Jimi Hendrix's use of feedback (or many others) which is, in part, caused by this.
Some folks like the effect in audio too. Kevin Deals statement is as follows:
True microphony will rear it's ugly head and make itself known with the most obnoxious distortion you can imagine. A smidge of microphony can (at least in the hi-fi world) add a sense of "air". That's why some guys buy tube dampers and find it sounds better without them. Sometimes.
True microphony is ugly. Generally it can and is to be avoided.
When you (Sugarbrie) imply it is the reason NOS tubes sound the way they do, you imply microphonics are a dominant reason for sound in NOS tubes (and therefore all old tubes). I think you are going way overboard and making it sound like it is generally a good thing and at the heart of tube sound. This is just wrong. I do agree above that the tube sound is a tube/circuits distortion characteristics. Microphony play a very minor role in most audio because it is a distortion that tube manufacturers have learned to control although QC is an issue. (again, all tubes are microphonic, but if they are used as intended they will perform with out it.)
Deal says a smidge and sometimes. If you had said sometimes a little M can be a good thing and qualified that in a meaningful way I would agree. You did the opposite, IMHO. Deal is is not saying that it is the secret reason behind NOS tube sound or that it is a way to increase "highs", or that "a tube with absolutely zero microphonics will *very likely* sound kind of dull in many systems" all of which I think are far-fetched and misleading.
There are better ways to get air into your system simply because microphonics are by their nature untunable. As Karls said above, unpredictable. Better to remove them ( it ptretty much can be done) and then tweak with something more subject to control if you ask me. You are more likely to get the sound you are looking for.