What is this glass structure on the tubes

[atul.mehta6]

Hi All,

I chanced upon this pic of a C2A on the net (see attachment).. and I am intrigued by this glass bead structure on both the tubes. What are they? What do they do for the sound?

Rgds,

 

[Paul Birkeland]

https://herbiesaudiolab.com/collections/tube-dampers