Tubes, to damp or not to damp?

O-rings are too tight and apply pressure on the glass.

Only if your a total (whatever) and purchase them too small.
here there are sizes that vary in just 1mm ID diameters, from 4mm to 61mm ID (inside diameter) with 2mm thickness to suit every tube you have, on this site. Others have them up to 85mm ID
Just measure your tube diameter and go 1mm less for a neat fit without being tight!!!!
https://www.ebay.com/itm/2mm-Cross-Section-OD-8-65mm-FOOD-GRADE-Red-Silicone-O-rings-Washers-50-100pcs/193009581470?var=493208405430&hash=item2cf0447d9e:g:sOcAAOSwKTddNtGK

Cheers George

Tried the AR, no good. Another vote for Herbies.

I'm a rubber formulation chemist and mechanical engineer, working in the field for 40 years.  Silicone can well withstand the heat generated by the vacuum tubes used in our typical audio equipment (can't speak to the massive exotic 160W SET amps!), as can fluoroelastomer/FKM/Viton materials.  I wouldn't use any other "rubbers" on tubes, as they can't take the heat, literally.  Teflon, a plastic, can take the heat, too, but it has almost zero rebound when stretched, so not a great ring choice.

Silicone can have fillers added to it to enhance its thermal conductivity.  I added ~40% aluminum powder to some custom rings I fabricated to damp the 6DJ8's in my Counterpoint 5.1 way back in the latter 80's (boy did those tubes and that preamp need that!!!).  I did it because I could - - not sure it really mattered.  Adding fillers dilutes the polymer which increases the elastomer's damping quality.

Silicone in general is "livelier" than FKM, which is pretty "dead" and dampens well.  Which one is best in an application probably depends on what your ears tell you.

As JEA48 mentions, placement is an important variable to play with.  You definitely can influence the resulting damping and the sound resulting by shifting the rings up or down.  Experiment and see what sounds better.  It's like working with any vibration control device for your system, offering a degree of tuning, as it were.

I am really surprised to hear that people experience glass deformation with the application of an O-Ring - - that must be some hoop stress fit!  I would hate to have to put those on.  My AR Ref 40 uses two FKM rings per tube, and even after the tubes have played out their 4,000 hour life the glass looks undeformed to me (just slapped a straightedge on a recently expired set).  And AR chooses about the tightest hoop stress fit I have seen - - I hate changing them out!  I am sure that AR is simply buying the standard size O-Ring that best works in their judgement, and that it is a garden variety "A" type FKM (still expensive stuff, by rubber standards).  Number used and placement were likely their more important decisions.

Not a fan of any of these devices. Very simple solution if your tubes are microphonic then replace the tubes. If all your tubes are microphonic then replace the component. In all but a few poorly designed components microphonics is rarely an intrinsic problem. If you like the sound of dampened tubes then more power to you.

I think it is situational to the gear and the tubes.  And some manufacturers seem to design their gear around the tubes with dampers, so it is part of the "voice", as it were (ARC for one).  But these damping devices aren't expensive, and it isn't hard to try them out and hear what you hear.  Herbie's will let you try and return if you aren't grooving on it, which have taken them up on more than once.  My experience with their dampers has never been positive with the squishy clear silicone cylinders strung on the teflon parted-ring.  But I have used the FKM O-Rings strung on the metal parted-ring style with success.  Different horses for different courses could well come into play, but that is my experience.

I would estimate that dampers have worked in ~40% of cases when I tried them over the years, but it all gets down to the particular gear, the particular set of tubes, my system's voice at the time and my preferences - - and, of course, the particular dampers.  Tweeks take work to tune right.  Hard to categorically state that dampers are all good or all bad, IME/IMO.