the problem - the solution

The principle is simple.  We all know tubes are microphonic in some way.  Microphonics means acoustic coupling  exists  to the sensitive inside parts of the tube.  There are two mechanical paths for that: 

1) Direct path:  Loudspeakers -> air -> tube glass -> tube mica ->tube plates -> tube grid.    And...tube grids are very microphonic, because here is where the signal amplification takes place.   The idea with tube dampers is, when you make the tube glass heavier,  and also add some damping  to it, you'll reduce the microphonic effects.   The damping comes from the softness of the Teflon.  It seems ideal audio material. 

2) Indirect path:  Loudspeakers -> air -> amplifier deck -> tube sockets -> tube pins -> tube grid.   This type of microphony is best stopped  by using the Yamamoto Telfon sockets. So you avoid   the mechanical waves to enter the glass.  Without Teflon sockets,   some good part is still absorbed by tube dampers also.  (for Yamamoto, see menu of this website). 

A problem may also come from mechanical transformer hum, guided through the chassis.  Both Teflon sockets, and tube dampers will help against this. 

The construction

The tube dampers are CNC made, in Holland by Duende Criatura.  They are made from a solid block of Teflon, with a working temperature of  maximum 260°C. This material has no burning effects at all.  Teflon is a self-cleaning material, and no dust or colorization will stick to it.   They have a spring around them of Titanium, which is a so called "hard metal" and it is surprizingly flexible. 


 I have measured the decay time of the "ring sound"  of the Electro Harmonics EL34, when you gently  touch the glass with something like a small piece of metal.  (see also note3).  From the speakers you here:  "dinnnng...."    It takes around 2 seconds,  for 2/3 of the ring noise to be fully  gone.   The spectrum contains a lot of higher frequencies in the range of 10kHz.  When the tube dampers were added to the EL34 tube,  the improvement consisted of three things: 

  1. The excitation of the tube was smaller.   That means the "ring sound" is 50% smaller to begin with. 
  2. The decay time was reduced by 60%, giving a decay time of less than 1second.  So the "ringing sound" was gone faster
  3. The higher frequencies of the "ring noise" were strongest damped. 
Glass diameter
Glass diameter
Tubes like:
9 ... 10
 0.35 ... 0.39
6021W, 6111WA etc
17,5 ... 18
0.69 ... 0.71
6AU6 etc (7 pin)
19,5 ... 22
0.77 ... 0.87
ECC83, EL84, EF86
26 ... 29
10.2 ...1.14 
27,5 ... 32
1.08 ...1.26
EL34, 5AR4
32 ... 36
1.26 ... 1.42
EL34-big glass, 6L6-small glass
36 ... 39
 1.42 ... 1.53
6080, KT90, 6L6GC
39 ... 42
 1.53 ... 1.65
2A3, 45
44 .. .48
 1.73 ... 1.89
any EML tube
53 ... 58
 2.09 ... 2.28
211, 845
63 ... 68
2.68 ... 2.36
Some types 300B 

Positioning the tube dampers 

There is often the concern that tubes get overheated from tube rings.  In a way that is true, when you think of bad design constructions that fully cover the tubes. The Duende Criatura  is small and relatively heavy, and covers only a small surface at the top of the tube.  Interesting,  this is the part where the getter is,  and it is supposed to be hot, to make the getters work better. (See also note 4

Some other brand  tube dampers are placed on the middle of the tube.  This is the wrong position, since they obstruct the heat radiation, and can cause severe bias offset of the tube when the grid coolers are covered by them. (See note1)   The tubes will be burned out sooner than you expected.  You can  put these in the Voodoo corner together with non-working medicine, and other magic.  I found those covering types are just bad for the tubes.   Take the  Duende Criatura instead! 

Correct Positioning of Duende Tube rings
Example for EL34 with one rings

Correct Positioning of Duende Tube rings Example for 6021W with two rings

What you see here is one of the "pencil tubes" 6021W, a low noise / small signal tube for high vibration environment. It was made during the cold war in the 1980's. Nuclear explosions destroy all semiconductor electronics in a wide area, like 100km or more. This is why Russian MIG air fighter planes worked entirely on electron tubes. When a nuclear explosion occurs, electron tubes stop working too, but they recover function after the electromagnetic impulse (EMP) is gone. All in theory..... thank God it was never tested. The good part is, there are some stocks of these tubes that the military have started to sell somewhere around 1997. Another typical NOS tube story we have here going on right now. These are PEARLS, sold for low prices until supplies dry up.

This is one of those tubes, the 6021W, with extreme low microphonic effects, made for small signals. Tubes like this have the lowest microphonic effect you can get for tubes. For phono pre-amps you can even further improve the performance of those, with one or two Duende Rings.

Interesting to know is, that the Duende company originally build those rings for this tube at first, for their own use.

This tube, and the Teflon tube socket you see here, we also have in our program

Burning in tube dampers?

Everybody talks abut burning-in, even when things don't get hot. I think there is a lot of personal belief in that, with articles like a 900$ mains connector, of the types that you get send in a rosewood box with gold-plated letters on it. However, as soon as you have mechanical components which are meant to be used very hot, there is in many cases the burn in (or run in) effect. So with the DUENDE tube dampers. The maker of the Duende rings, pointed this out to me. The surface of the Teflon, where it is touching the tube glass, will slowly but fully adapt to the shape of the tube after some days or weeks. This will go faster when the tubes are switched "on" of course. So yes, it's a real burn in effect.

Experiences / Testberichte

Augusto Sá Monteiro -Spain
the order arrived this morning in perfect conditions, as always.  I immediately tested the "Duende Criatura" tube rings and just like a miracle the microphony almost disappeared.  Also the sound performance of the TFK RE-084 improved in all aspects. Therefore I feel encouraged to ask you  4  more tube rings for the phono valves (2 ECC83 + 2 ECC82).  I suppose the reference is the number3 , you will confirm it.  Please send me as soon as possible the invoice in order to include it in the pipeline order, this also to the same address.  Once again thank you very much.

Martino Sanchez Ruiz  - Spain
I have received your consingment. All in perfect state.  The tube rings are very good, more transparency, warm and cleaner soundstage. Better and faster microdynamics.  Best Regards, Martino.   

Die Basskontrolle hat zumindest im Oberbassbereich zugenommen (ich betreibe 2 1/2-Wege-LS, also unterschiedlich angesteuerte Bass-Chassis). So weit, so erwartungskonform. Was mich wirklich ueberrascht hat: Die Leisespiel-Qualitaet hat scheinbar deutlich zugenommen.  Insgesamt wuerde ich den Fortschritt durch Upgrade quantitativ aber in etwa auch bzgl. der konkreten 'Einzeldisziplinen' mit dem durch die BlackBox (die auch auch habe) vergleichen. 

Then I tried the rings for the ECC82s in the power amp. This was ASTOUNDING!!The bass immediately became more tuneful and everything became much better integrated. In simple terms, it all became more real.........Now I want to order some more rings, please. Duende Criatura Tube Rings reduce microphony, distortion and feedback from audio tubes.

Duende Criatura De dempende buizenringen verminderenbeduidend microfonie,vervorming en oscillatie van buizen. Ze worden om het bovenste deel van de buis geklemd. De gevolgen hiervan zijn verbetering van ruimtelijk geluidsbeeld, dynamiek en bascontrole.


Technical notes
Note 1.  Grid coolers  are the two fins you see at the top of  power tubes.  So smaller tubes don't have those.  They are mounted on the grid bars,  and their function is to avoid the grid from getting overheated. If the grid gets too hot, grid emission will start,  and the tube will bias wrong, and consequently will burn from it. 
Note2.   Microphony is not the right word, for what is physically happening here,  but  it will come down to the same result: Noise.   The physical explanation is,  the tube glass and the whole inside  must be regarded  a damped  mass-spring system.  This system is  excited by any mechanical force, and the result will be the typcial inner frequencies of this system  appear.   The tubes will generate this noise by themself when they are excited., for instance  by gently ticking on the glass with a metal piece.  So, you do not only hear the "tick"  noise, but additionally some ringing noise.    Also  the sound  waves in the hearing room will excite the tubes, and this  will appear in the output  signal as the same ringing  noise.  With tube dampers, the mass spring system is given more mass,  and more damping.  Both will reduce the movements it will make, by the same excitation. 
Note3.  A good test  to check if your tubes are sensitive to  this problem,  is the following:  Put on a very loud CD, and play it  as loud as your system  can do.   Now choose a source where there is no signal, like "Aux". (unplug the Aux cable if necessairy).  So now you hear no sound.     When you would go back to "CD" the very loud sound appears again.  This is the set up you  need.   Now,   to excite the system you must,  as short as you can do, switch from "Aux" to CD, and immediately to "Aux"  again.  So you just "touch" the  loud CD signal as short as possible.   This will most likely produce the not very famous "ring" noise, caused by the tube amplifiers. 
Note4.  Getter temperature  plays an important role to keep the vacuum clean.  The getter is applied at the hottest position of the tubes.  This makes the getter function best. They need minimum 60°C to function well.  For optical reasons, sometimes it is put in the bottem, but this is not the best place. 

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