If a 12AX7 has gone microphonic, replace it. You don't have to replace them as a set, and you don't have to bias them. (You can't.) Since a noise in the first tube in the signal chain is amplified by every amp section following, simply swapping tubes around to a different position often makes a huge difference. You should keep spare tubes handy, in any case.
Microphonic tubes have loose internal parts that can vibrate with sound. As the parts are electrically charged, they then act as a condenser microphone. Bumping or dropping your amp is a great way to loosen tube parts. They're rather fragile. ALL tubes are microphonic to some extent. What you're looking for when you tap on tubes is one that makes more noise than usual. Don't bang on them hard.
"A cheerful heart is good medicine."
Oh, that's like the Japanese made Chevrolets I laughed at years ago with the stickers under the hood that said "Keep your GM car all GM. Buy only genuine GM parts." They want to sell color coded matched sets of tubes.
A Pro Jr. doesn't have adjustable bias, anyway. Check out the schematic: http://schematicheaven.com/fenderamps/pro_jr.pdf
"A cheerful heart is good medicine."
How do you like that Vintage 10 speaker? They advertise that as the 10" model of the famous Vintage 30. The V30 is a great sounding speaker IMHO, very smooth tone, never harsh. Does the V10 have this super smooth tone??
If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis
