Three years ago, I diagnosed a problem with my Fender Blues Jr amplifier. There was a distinct fizzing buzz from the amp whenever I played a specific note. It turned out to be the reverb tank, and the sound happened even when the reverb level was at zero. A $30 reverb tank replacement was all it took to fix the problem.
Since then, the Blues Jr has gone away, and has been replaced by a Fender Princeton Reverb - seen here:
And, a couple of weeks ago, I noticed the same issue - a slight buzzing on certain notes played anywhere, on any guitar, even with the reverb set to zero.
And, the same fix fixed it: a new $30 reverb tank from Mod. - cheaper than the stock part from Fender!
Come on Fender, get your shit together.