it makes them more prone to it versus a short plate version, but none are to always be expected to be microphonic. most people assume it could happen and only use them in a head. short plates are for combos.
you're right, depends on the tube. i have used long plates in combos too. i'm just saying what the "conventional wisdom" is. as with anything, YMMV. :lol:
my V1 is a telefunken long plate NOS and is the sweetest preamp tube i ever heard so far(i use it in heads, no combos thou), and i didnt have problems at all with microphonics.i guess it depends as mentioned above on the tube/brand.