New tubes:
JJ ECC83S (Ruby) & Mullard ECC83 long plate reissues. The OE tubes in the background are just for photographic efffect.
Currently in the MP-1 are Groove tubes (about 20 years old). After about 45 minutes of play, the total output of the preamp appears to drop by about 20%. It just gets more quiet overall, and I have to increase the output volume of the MP-1. Is this a sign of dying tubes? But wouldn't dying tubes be more quiet as soon as I turn it on, not just after a period of time after being warmed up?
Questions about tube rolling. Can I swap out tubes while they're still warm? Or do I wait until they're stone cold? I don't want to damage anything. But at the same time, I don't want to wait unnecessarily long because I have multiple combinations to try out.
edit:
More questions--the new tubes have marker pen numbers written on them, I assume from testing. Is it okay to leave these on, or should I clean them off? Also, the JJs have an "HG" sticker on them (high gain). Should this be removed?