apa wrote:

The noise is not quite hum and not quite white noise. It almost sounds
like a mic turned way up in a quiet room, picking up air sound and
motor hum from far away.

It's mostly low frequency rumble?

The noise is uneffected by the 10dB pad on the mic body(the noise level
stays the same).

Is this a capacitive pad?

The noise goes away if the mic is not connected to the PSU.
The noise goes away is the tube is removed from the mic.

I don't have another 6DJ8 around, or I would have swapped it out.
Does anyone have a list of other likely culprits?

I'd swap the tube out before doing anything else, but another very common
culprit is contamination and dirt causing leakage paths. A fingerprint
on the board can be a big problem sometimes with high-Z circuits like
this. Flux remover spray can help clean gunk off, but make sure it's
not the tube first.

It could also be gunk on the capsule, but if the pad switch does nothing,
my assumption is that the capsule is clean and the pad is a capacitive
one that shunts across the capsule.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."