"cannister" metal covers for tubes... effective shielding?

[bongomania]

I am a fan of anything that will reduce susceptibility to EMF noise in my gear. Are those aluminum "cans" that fit over tubes actually good for shielding, worth installing for that purpose?

[johnk_10]

i definitely experienced some of the preamp tubes having less hum with them on on my fender heads.

[MuthaFunk]

From my experience with audiophile tube hi-fi equipment I've found they do help. Not for shielding from RF or anything like that but I noticed specifically they reduce micro-phonic induced clouding of the sound at higher volumes. I can't say I've noticed any difference at lower volumes. They don't look very nice so audiophiles have a number of other options to combat the micro-phonic issues like rubber grommets placed on a ring that fits around the tubes.

I keep them installed on my HiWatt and my phono pre-amp as the small signals from record cartridges are very delicate and susceptible to any or all kinds of noise.

[Jerrold Tiers]

They are not as effective if they don't have a connection of some sort to the chassis. Some of the "covers" are really a form of heat sink, with no chassis connection, and minimal shielding effect.

Old type shield covers have a grounded metal ring on the socket that the cover connects to with a "press and turn" or "bayonet" connection.