The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Tube Microphonics

    Share

    quadaptor

    Posts : 59
    Join date : 2009-05-04

    Tube Microphonics

    Post by quadaptor on Sun Feb 27, 2011 1:34 pm

    Can somebody explain tube microphonics? I have a couple older RCA 12AU7 clear top tubes I've tried in the outboard sockets of a VTA board for a ST-70. I noticed that when I gently tap on one of the tubes it transmits the sound through the amp and into the speakers. Is this an example of microphonics and does it mean the tube is bad? Many thanks for your help!

    Tube Nube

    Posts : 603
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: Tube Microphonics

    Post by Tube Nube on Sun Feb 27, 2011 4:06 pm

    This is an excellent example of the phenomenon. I don't know if it's a bad tube design but it certainly sounds like one tube is audibly worse than the other for microphonics.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Tube Microphonics

    Post by Bob Latino on Mon Feb 28, 2011 10:20 am

    quadaptor wrote:Can somebody explain tube microphonics? I have a couple older RCA 12AU7 clear top tubes I've tried in the outboard sockets of a VTA board for a ST-70. I noticed that when I gently tap on one of the tubes it transmits the sound through the amp and into the speakers. Is this an example of microphonics and does it mean the tube is bad? Many thanks for your help!

    Hi,

    This is an example of "microphonics". It doesn't mean that the tube is "bad" but it does mean that if your amp or preamp is tapped or moved, some of the tube elements can be set into vibration. Personally, I would remove the tube and use another tube in its place. The reason is that sometimes a microphonic tube could vibrate at ultrasonic frequencies which may not be audible to you but whose vibration pattern could take out your tweeter.

    As an aside, certain families of tubes tend to be more microphonic than others. IMHO the 6DJ8/6922 family of tubes tend to be one of the worst microphonic offenders.

    Bob

    pro_crip

    Posts : 5
    Join date : 2011-03-08

    Re: Tube Microphonics

    Post by pro_crip on Tue Mar 15, 2011 8:45 am

    There are dampers you can buy out there or you could use the copper tape that stained glass makers use. Just pit a strip on the tube, if you don't mind the aesthetics. The extra mass makes it harder for the tube to vibrate.

    Rich

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Tube Microphonics

    Post by sKiZo on Thu Apr 11, 2013 2:01 pm

    OLD THREAD ALERT - OLD THREAD ALERT - IT LIVES!!

    Found this one on a local CL listing ...



    Kinda looks like something you'd see in the restroom at an alien gas station.

    Running low on probes? Now's the time to stock up!

    (My apologies to anyone whose box looks like this ... it's probably just me, and I'm crazy!) tongue

    Basically though, from what I've read, those cheap silicone rings are a viable option way less expensive to try than the uber expensive damper/cooler things like the Pearls if you do have a microphonics problem. Same as putting a finger on a bell ... that slight touch deadens the vibration. My concern would be that a certain unavoidable amount of interaction between the glass bottle and the guts were designed into the tube, and that too much deadening would change the tone.

    What surprises me on the rig pictured is there's no damping on the 5AR4, and those have a bit of a reputation for microphonics. I'm thinking the dampers in this case are just a cosmetic kinda thang.

    Given a choice though, I think I'd go with something like this ...



    The wire gizmo is for smaller bottles like the drivers - the springy thingy adjusts to fit most power tubes. Same difference - pressure on the envelope reduces or eliminates vibration that could be transmitted internally. I can attest to the wire gizmo anyway - my Maverick TubeMagic DAC came with one and does make noise with test taps if I leave the clip off. Makes it a bit harder to roll tubes, but hey ... maybe that's a good thing.

    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: Tube Microphonics

    Post by Luddite on Thu Apr 11, 2013 4:54 pm

    "The wire gizmo is for smaller bottles like the drivers - the springy thingy adjusts to fit most power tubes. Same difference - pressure on the envelope reduces or eliminates vibration that could be transmitted internally. I can attest to the wire gizmo anyway - my Maverick TubeMagic DAC came with one and does make noise with test taps if I leave the clip off. Makes it a bit harder to roll tubes, but hey ... maybe that's a good thing."

    Of course the original purpose for these wire and spring clips was to keep tubes from coming out of their sockets in portable gear, especially guitar amps where the tubes are generally mounted upside down. I wonder if these clips are actually damping the vibration or are they merely shifting the resonant frequency of the vibration to a another (perhaps inaudible) frequency?

    Best Regards,
    Charlie

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Tube Microphonics

    Post by sKiZo on Thu Apr 11, 2013 5:22 pm

    Maybe one of those, "I meant to do that" moments? The clips are definitely handy for things like guitar amps and stuff you have to move about, but I've heard their effect on vibration too. Don't know if it's a frequency shift, or just the old "thumb on the bell" thing. Then again, I've heard of instances where adding the retainers exacerbates the problem - proof positive that there's no "one size fits all" solution.

    Like multi-element vacuum tubes themselves ... originally used by Edison's telephone company to amplify call volumes between switching centers. Not sure who the bright one was who had the aha! moment when it came to radio and hifi ... but, hey ... thanx!

    I did order a set of the wire retainers from Weber when I went looking for a Copper Cap rectifier. I think they'll look kewl on the ST-120. Also got some bias test jacks, along with some other nifties. I plan to add dual volume controls, so also scored a couple of Marshall amp knobs for that old school look. Quite a bit of useful stuff there if you start digging.


    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: Tube Microphonics

    Post by Luddite on Thu Apr 11, 2013 6:38 pm

    sKiZo wrote:
    I did order a set of the wire retainers from Weber when I went looking for a Copper Cap rectifier. I think they'll look kewl on the ST-120. Also got some bias test jacks, along with some other nifties. I plan to add dual volume controls, so also scored a couple of Marshall amp knobs for that old school look. Quite a bit of useful stuff there if you start digging.


    Yes, I agree. BTW, another great source for "new vintage" knobs, parts, etc. is Antique Electronic Supply www.tubesandmore.com

    Best Regards,
    Charlie

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Tube Microphonics

    Post by Jim McShane on Thu Apr 11, 2013 7:43 pm

    ALL tubes are microphonic to some degree. Microphonic tubes are not a problem, only EXCESSIVELY microphonic tubes.

    If the only time the tube makes any noise is when you tap it, it's not excessively microphonic. Most tubes will make noise when you tap on them, especially if they are used in a high gain circuit. The sudden impulse of a tap will virtually always excite something in the tube and make a noise. The key is that the noise dies out quickly after the tap. Generally for microphonics to be excessive they need to be self-sustaining.

    As well, the same exact tube may not make any noise if it's in a lower gain circuit.

    I shudder to think of how many perfectly good tubes are damaged or discarded by this poorly understood "microphonics" issue.

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Tube Microphonics

    Post by sKiZo on Thu Apr 11, 2013 8:10 pm

    So ... what's the difference between a tap and a crashing crescendo bouncing off the bottle? Not a lot, mechanically speaking, except said crescendo tends to be more sustained. Porcupine Tree puts my room into "hover mode", everything best be strapped down tight.

    A lot of it is the sockets and their condition too. Good sockets that grip the pins tight will also help damp the tube. Cheaper sockets, and a good bit of rolling that loosen them over time can impact the sound long before you'd notice any purely electrical issues.

    Something else to keep in mind is the age of damping rings. Those apparently turn brittle with age and lose their effectiveness - assUming they were effective in the first place. Again, circumstances dictate. That's something you don't need to worry about with the mechanical dampers, or in this case, retainers. Main reason I put mine back on the TubeMagic is I had a tube that was just wunnerful, except it'd ring like the dickens in the left channel every now and then when I was Hittin'It, not to be confused with (thonk thonk) hitting it. Putting the retainer back on solved that.

    Basically, ya does what ya has to do ...

    I got another mod in mind on mine - adding a wire screen around the VTA board. Seems to me that would tend to soak up and divert any vibration, waves, what have you, down into the chassis instead of the driver tubes. Mostly, it'd give me an excuse to cover up that fugly board. pig

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Tube Microphonics

    Post by Jim McShane on Thu Apr 11, 2013 8:38 pm

    sKiZo wrote:So ... what's the difference between a tap and a crashing crescendo bouncing off the bottle? Not a lot, mechanically speaking, except said crescendo tends to be more sustained.
    It's a HUGE difference in force! The impact of the tap is far beyond the tiny amount of vibration passing through the air. Tap the back of your hand with the same force as you tapped the tube - you'll feel it. But you don't feel music on the back of your hand.

    That's why I (and other reputable tube vendors) say over and over - DON'T TAP TUBES! Here's a quote from one of my competitors who is very well respected for his tube knowledge:

    Okay...listen closely class, because I will say this only once: DO NOT TAP ON TUBES! You can permanently damage them! Repeatedly tapping on a tube's glass can cause a perfectly good tube to become too microphonic for use.
    BTW, I've observed oscilloscope patterns of microphonics generated by tubes, and compared them to even very loud music. The impulse generated by a tap is much greater.


    turbotoy

    Posts : 47
    Join date : 2012-04-15

    Re: Tube Microphonics

    Post by turbotoy on Thu Apr 11, 2013 8:48 pm

    The dampening rings will change the mass and therefore resonant frequency of the tube envelope. The lumped stiffness is basically going to stay the same. Effects of dampening are a little more complex, but would only effect the rate of decay.

    However, the issue of microphonics, as I understand it properly defined, would reside in the structure and support of the grid and plates. I fail to see how bands on the outside of the tube are going to change their response to an excitation (at least first order). All of this is straight forward modal analysis, and I should crack some tubes and do some calcs one of these days to better understand it.

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Tube Microphonics

    Post by sKiZo on Thu Apr 11, 2013 11:59 pm

    I gotta start taking better notes, but I do remember seeing a couple mentions of surface vibration of the bottle itself getting transferred to the internal structure via the micas. Makes sense as there is physical contact there.

    Then again, I see where star type micas with more contact points are supposed to be more rigid and less prone to harmonics, microphonics, and other such ics ... and three mica tubes are better reviewed for sq than two mica tubes ... arghhh!

    Six o'one ... etc.

    I do know I can make the room float and my sternum vibrate like a heart attack ack ack cranking it with my sub-synth ... that can't be good for any tube!

    PS ... off topic ... again, but my notes may be bad, but they're a whole lot better since I started using Treepad. Try it free. You'll like it! Here's a sample from my collection of breadcrumbs:



    http://www.treepad.com/

    Sponsored content

    Re: Tube Microphonics

    Post by Sponsored content Today at 9:44 am


      Current date/time is Sat Oct 01, 2016 9:44 am