Every issue I have had with tube preamps has been the result of faulty tubes. Noisey, popping sound like pop corn, microphonic howl. But never what you describe. But I suppose it is possible. Defintely swap the tubes - that is the first step in the process of elimination.
Are you sure it is your preamp? Hate to state the obvious but you always need a spare set for this reason and for when they eventually crap out.
Good luck!
Are you sure it is your preamp? Hate to state the obvious but you always need a spare set for this reason and for when they eventually crap out.
Good luck!