Originally Posted by
RothwellAudio
Personally, I think they're useless. The problem is that the metal parts within the glass envelope can vibrate and ring and that is certainly audible, at least when the volume is cranked. The vibration comes up through the valve base, but damping the glass envelope has barely any effect on that. I know because I've conducted experiments to verify it.
The way to cure the problem with PCB-mounted valve bases is to suspend or float the PCB or couple it very lightly to the chassis. That works. I know because I've conducted experiments to verify it.
With chassis-mounted valve bases it's harder to deal with. One solution is to have the valve bases mounted to a floating sub-chassis, but that is really only an option if you're building from scratch or prepared to do some serious modding.
The good news is that the problem is mostly confined to the first stages of an amp, or more realistically the valves in a phonostage where the gain is very high. Yes, I think that vibration isolation is worth doing, but no, I don't think those things that simply damp the glass envelope do any good at all.