"Don Pearce" wrote

OK - I'm interested in how they are supposed to
work. You say they should be fitted at the points
where the mica bears against the glass.

No, I said that's where I use them, there is no
specific location. I just find the best results there.
Purely empirical.

What that is telling me is that they are actually
preventing a flexing mode within the glass, trying
to place a node at the mounting points - a very
laudable thing to do. But compare the modulus
of elasticity of glass and silicone rubber, and tell
me - just theoretically - how much reduction in
motion could be achieved this way.

I can't, I'm not a physicist. I can only tell you from
personal (and many customer) experience that they
sometimes work very well.

Hey, maybe they cause a localized hot spot on the
glass and that changes something, I don't know.
I DO know that raising the heater voltage and
heating a badly microphonic tube above "normal"
will virtually always stop the microphonics. Of
course, that approach has many drawbacks...

The other - and rather more likely - way they
can work is simply by adding mass, and reducing
the resonance frequency with the stiffness of the
pins. In that case all the mass should be placed
as high as possible on the valve.

Okay.

Can you point me at the science please, because
I just don't buy your theory.

Come on, get off your high horse. I never said I was
scientist, I never made any claims as to why they
do what they do. I didn't say I invented them. I never
advanced ANY theory at all.

But I can tell you that I have heard a marked
improvement with my own ears in many cases. And
so have a number of people who got them from me.
As I also said before, not all cases, but a good
number. For 50 cents (or less in quantity) it's worth
a try if you ask me.

I can tell you that 3M developed a tube damper
some years back that was a thick flexible ring
around the tube, and it was quite effective. Maybe
they have some info on their site or something, I
don't know. There was also a respected audio
designer named Robert Modjeski (sp?) who
developed a line of o-ring dampers, maybe
there's some info there.

Jim McShane
Need Tubes? Got a H-K Citation (Pre) Amp?
Check http://pages.prodigy.net/jimmcshane
Repro knobs for Citation gear in stock!