I own a 502B. 6550 and KT88s are interchangeable (have done that) but, to the best of my knowledge, without circuit modification, EL34s shouldn't be used in in the 502. On the other hand, the 302B is designed to run EL34s. Maybe things have changed recently but that's been my understanding. Why not contact JoLida directly (I'm sure they're well aware of what Shenda is doing) or Underwood HiFi (very knowledgeable) if your seller can't help?
