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All,
I always wondered what the influence is of using microphonic tubes in ones system.
Yes, when I tap on my 6C45P, WE437A and Ba tubes I hear it back in the speakers clearly. But I'm not sure how this translates to the performance of my system when I'm playing music.
I use dampers where possible which add a very small bit of focus, but it for me the microphonic tubes do not seem to pose a problem at all.
Am I missing something? Is it worthwhile to undertake large efforts reducing microphonics?
NC
Follow Ups:
To what Jim said, tubes get a bad rap for microphonics. Many times the sockets are the culprit.
An improperly secured coupling cap can not only mimic a microphonic tube but cause one heck of a feedback loop, even at low volume levels.
If one feels compelled to tap the tube, tap the chassis. Don't tap the tubes.
"Am I missing something?"
Yes. ALL tubes are microphonic to some degree. As long as it isn't causing problems then it isn't an issue.
Nearly ALL tubes make noise when tapped. It's only when the noise is excessive or continues for a while after the tap that it could be considered excessive.
Don't tap on tubes unnecessarily, you can cause damage to the tube.
Finally, the amount of apparent microphonics varies greatly with the circuit the tube is used in. Sometimes a tube that is excessively microphonic in one position is just fine in another.
The rigidity of the internal construction of the tube, the posts and the mica washers/wafers. For instance compare a 12ax7a/7025 with a 12bz7. The 12bz7 was designed as a TV oscillator and sync seperator but it has the same gain as a 12ax7a, twice the transconductance and half the plate resistance BUT notice the stack of mica wafers (many more in the 12ax7) and the size of the posts supporting the structures of the tube. The 12ax7 is better damped. Also though we can't see them the heater maybe wound differently inside the cathode. All of this is a tail wagging the dog and it's a matter of DAMPING those tail wags. Acoustic feedback from the speakers, semi tractor trailers and trains near your home, heavy stampers (I have a daughter this is Miss congeniality but can't walk without shaking the house) all contribute. Moe
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
There are tubes where microphony is a malfunction. But there are a whole family of DHT small radio tubes that used to run off batteries and had very low consumption filaments - like 60mA, 120mA and stuff. Those were horribly microphonic, but a lot of them sounded great. Stuff like 1G4 - wonderful sounding little tube. You can tame the microphonics with starved filaments and massive engineering, but these kind of tubes really need about 2v input. Same with the 4P1L which is naturally microphonic but stonkingly good sounding.
That ALL my WE437A and Ba tubes are microphonic...
I'm not in the position to test tens of these tube to find samples which are not microphonic. Moreover I suspect ALL of these tubes will be microphonic to a degree.
If I want to be anal about this, the only solution is to put my gear in a separate, isolated room.
Since I conclude from this thread that microphonics is relatively 'small' issue, I think I'm going to live with it.
NC
Hi!
> > > That ALL my WE437A and Ba tubes are microphonic... < < <
I have worked with the WE437A in phono stages and did not find them very microphonic. Even in the first stage of the phono. The WE437A can oscillate if not used in a careful layout that can appear like being microphonic. Of course when you tap a tube in the amp you will hear something, but I would not consider that as being microphonic. Just don't tap the tubes ;-)
Thomas
Here is an alternative view (which I do not endorse!)
I recall DLM saying that DF finds only 1 in 15 samples of 7B4 acceptable for his SE DC 2A3 mono amps in term of microphonics.
Apparently this is needed for correct "jump factor" in a low current driver stage.
A microphonic tube would normally be dismissed by the average builder of typical mid-fi tube amps.
"A microphonic tube would normally be dismissed by the average builder of typical mid-fi tube amps."
If it takes a microphonic tube to make a design perform, there's something terribly wrong with the design.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"If it takes a microphonic tube to make a design perform, there's something terribly wrong with the design."
I would agree with you, but the thought process involved in designing "alternative" low power SE tube amps is not conventional or grounded in known theory and measurement sciences.
"I use dampers where possible which add a very small bit of focus"
I think you nailed it.
To me a microphonic tube adds a bit of false "reverb".
Removing the added "reverb" reveals the true focus.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
As you may know now there are levels of microphonic tubes. Like Al said depending on the type of tube and where it is in the circuit they can cause feedback (like from a PA system -squeal) you have tubes now that when you tap on them they make audible noise in the speaker; both are types of microphonic behavior.
When you change a tube it almost always sounds different obviously the same brand/type/run of tube will sound most similar. I would remove ANY tube with any type of microphonic behavior. The sound you get with the replacement will have many variables as you probably know.
In the end you'll decide the cost/benefit factor yourself and weigh all the data. I would replace any microphonic tube and the louder you play your system the more important it is IMO. I'd be less likely to replace a microphonic tube in a headphone only setup. The sound won't get back to the microphonc tube in a headphone system.
Like a turntable susceptible to feedback there are other [potential] ways than just the sound from the speakers feeding back to affect a microphonic tube and its system. Good luck.
ET
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If your amp or preamp is located where sonic vibrations are prevalent you should hear the tube pick up the sounds from playback. I found that this can be noticed as an artifact on the original sound like cymbals that seem to shimmer too long. Best noticed on very familiar program material. Try and compare with a different tube to hear any differences. regards, Dak
And somehow more info. The ringing is often high pitched and masked by the music somewhat. For classical music, getting rid of microphonic tubes can really make violins easier to listen to. Great for some of us that really have trouble with "bright" systems.
It really all boils down to how microphonic a tube is when excited by musical vibrations in the air and the surface on which the tube sits.
Tapping the tube tells you that it is sensitive to a very large mechanical vibration, but not necessarily to sound.
I'd be curious to try an experiment - one where you take a speaker on a different amp, run a sweep through it while pointing it at the tube in a circuit, then measuring the output of the amp that tube is in, to see what actual sound gets picked up and amplified. I think that without doing that, what constitutes a microphonic tube is somewhat in doubt.
a light tap with the fingernail should not produce a 60 second ring but, with very microphonic tubes it can. Subs can certainly set them off by shaking the glass and the box of the amp/preamp.
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