FD5: The Tubes, Part 3: Changing the Tubes 

Either. The cathode biased ones tend to be more prone to squeal because the cathode resistor is a feedback (not many folks understand this). The oscillation (squeal) is grounded when the cathode is directly connected to ground. In amps where the 1st stand cathode is grounded and the 2nd stage has a cathode resistor, it is the 2nd stage that squeals. Either way, the next culprit that causes squeal is when the ground of the preamps is sent to a common star ground. Star grounds also include the grounds from the power tubes. The preamps need to be grounded at the chassis at the preamp tubes or grounded at the input jack (best). Preamp grounds need to be ground as far away from the power tube and rectifiers as physically possible. Which is how vintage amps are physically laid out. "Signal to the left, Power to the right, Stuck in the Middle with you..." :)