Some 12AX7, are sensitive, to the electronic fields, that are present in gear. 12AX7's were regularly shielded, in most vintage gear, for this very reason. If the tube, is not defective, then maybe you could try a tube shield, that you can ground(connect to the ground plane, of the pre)and see if that solves the problem. If interested, I have a source, for that very thing.
changing tubes in my phono causes hum ?
i have a tube phono that uses "2"-12ax7's one for each channel. with the mullards i get "0" hum. when i change them out to any other tube like sovtek "low noise" , or electro harmonix i get hum. i like to tube roll and dont understand esp with the low noise tubes. suggestions would be appreciated.
thx,tim
thx,tim