Hello, CUH -
Not sure this will illuminate anything for you...perhaps more by way of commiseration. I have a JoLida 502B. It came with stock Sovtek 6550s that ran without problem for almost 6 years. They seemed bullet proof. I went to JJ/Tesla KT88s and had reliability issues similar (I think) to the failure frequency you encountered with the TungSols. I am now using Penta Lab (Shuguang solid plate) KT88s. So far, so good. My B is now almost 7 years old. Haven't replaced any of the circuit components to date but have been thinking lately about a rehab.
Not sure this will illuminate anything for you...perhaps more by way of commiseration. I have a JoLida 502B. It came with stock Sovtek 6550s that ran without problem for almost 6 years. They seemed bullet proof. I went to JJ/Tesla KT88s and had reliability issues similar (I think) to the failure frequency you encountered with the TungSols. I am now using Penta Lab (Shuguang solid plate) KT88s. So far, so good. My B is now almost 7 years old. Haven't replaced any of the circuit components to date but have been thinking lately about a rehab.