Taken from
https://www.jacmusic.com/techcorner/...-C3m-info.html
Cap Removal
It is really a matter of taste of you want to remove the cap or not. The optical advantage is beautiful. For myself, knowing what is inside, for me it is a tube as well also with the cap in place. Make good note, what the cap is intended for: It is an electrical shield for the tube, and the cap is electrically connected to the metal guide pin, in the center. So via the tube socket you can ground the cap. This is definitely a great advantage in case of very low signal applications. For a driver or headphone, removing the cap for optical reasons is sure nice. You can lift off the edges of the cap relatively easy, it is aluminum, and not aggressively attached or kitted. Also the tube glass will not break easily, because the location is the tube base, and this is pretty thick material. Yet, it is possible to break the glass still when you do not patiently peel the cap away.
After removing the cap, the part with the guide pin falls of, and I would recommend to glue this back on. Put two components glue on the sides. Important: Do not fill glue in the center adjustment hole. That will eventually break the glass pipe in the middle. Note that one pin hole is square, and is used for positioning. Also you can now still use the guide pin to ground this part, and one way or another that is simply good, because the ground plane comes now closest possible to the hum sensitive grid.
Note: Hum is greatly reduced by using a low driver impedance. Also, when working at very low signal level, beware that with all tubes, the grid noise, generated by the tube itself, is actually attenuated (better call it loaded), when using a very low impedance driver circuit. So the noise free signal from the driver and the noise from the tube, will not simply add up, but only one of the two will win. And that will be the driver signal, when it is low impedance. This has nothing to do with the C3g or C3m itself, but since C3g, C3m can be used in pre amplifiers, or even phono amplifiers, I think it is meaningful to mention this here still.