o-ring size for 12AX7 & AT7 dampers?

I want to experiment with silicone O-rings used as tube dampers for 12AX7 and 12AT7 tubes. What's the right size to use? #116?
Don't know how tight is borderline with breaking vs tight enough vs too loose and therefore useless.

How hot do these tubes get? Silicon will only take so much.... Other elastomers can take more heat. EP, for example....
'snug' is good, apparently.
I have DowCorning Silicon StopCock grease, a white/clear grease used to lubricate O-rings.....VERY sparingly. In the industry I worked, the philosophy was to 'put none on and take it all off'....THAT sparingly.
I've been using McMaster-Carr's part number 116 S70, silcone o-rings, as dampers, for the past 12 years. No need to add anything(grease, etc). They handle the heat of the tubes, without any ill effect, at all. I've had the same rings, on and off, numerous times. Matter of fact; I've still got a number left in the original bag. E-mail me, and I'll send you a few.
UPDATE on the McMaster-Carr part number. It's now AS568A-116. Like I said: I got mine 12 years ago. What I posted was the number off the bag they came in, then. (http://www.mcmaster.com)
I thought this over.....and low power / pre tubes probably run cooler than output tubes. Don't have numbers in front of me, but silicon is pretty good....
Do you get the 'red' silicon? It has oil in the binder which CAN come out at elevated temps.
The Silicon stopcock grease would be simply to make to ring slick enough that it wouldn't 'roll', but instead would slide down the tube. AND, used in extreme moderation. NO blobs or globs.
I'd be careful and keep an eye on silicon if I tried it on output tubes.

Rod, have you tried ringing higher output / hotter tubes with the silicon? Any problems? Any residue on the tube when you slide the ring off, at least on output tubes?
Hello Magfan: No- never tried silicone on output tubes. I've been using McMaster-Carr's Viton O-rings(size 135) on my output tubes(EL-34, now 6550), for as long as I have the silicones, on the 9 pin miniatures. I've had the same 16 O-rings(two per 6550), on three octets now. Never had a bit of residue, or hardening of the rings, with Viton. The silicon rings I have, are the red ones. Never had any signs of anything leaching out of them either.
Tubes must run a bunch cooler than I thought. Even output tubes.
Viton, if memory serves, doesn't take to heat as well as silicon.

I have seen, under EXTREME conditions, NEVER to be found in the home, that an oil will leach out of silicon rings. Not much, but messy.

Their are a few 'exotic' elastomers which take heat better than silion but can be $$$. Doesn't appear that level of nuttiness is needed. Stuff like Kalrez.


More than any rational person would want to know about o-rings.
The max temp ratings I'd seen(way back when), for Viton and silicone O-rings, were 392 and 400 degrees Farenheit, respectively. Not far enough apart to make much diff, I thought. I've still got a package of the Vitons, from the original order, that I've never had to use, 'cause the ones in service stayed supple, and elastic. I've never actually used an IR thermometer, to see what kind of temp output tubes generate. I'll have to find now(all this has me curious).
Post back, or PM me, please.
I have looked at several sites and gotten various answers on temp tolerance of the various elastomers.
I don't even care what they will chemically 'put up with'. Some don't like gasoline or acetone or alcohol or whatever. That doesn't matter, either, to a hifi guy.
Since the silicone and/or Viton seem to be working, I'd skip Nitrile, Kalrez and Ethylene Propolene (EP) since they are simply not necessary.

Be careful to read the envelop temp, NOT the filament temp.
I got plenty.How many do you need? Email me .