I doubt that this pertains to your problem, but I had a recent issue with some 6922/6DJ8 tubes. This is a very popular tube in audio equipment and I recently had a problem with Tesla brand tubes in a Audio Research power amp. The tubes were breaking down with plate to grid shorts (visible sparking) when balancing the low level section of the amp. Switched out to Electro-Harmonix brand and no more problem.
This was a very specific circumstance because setting up the low level biases is done with no output tubes plugged in. As a result the high voltage (400V) remains for a long time after the power (tube filaments) is switched off. It was during this time when the high voltage was present and the filaments were off that the internal arcing in the tube happened.
These same tubes perform perfectly in less demanding preamp applications.
There are differences in the 6DJ8 tubes from different eastern european manufacturers.
- Both of the 6DJ8s, the recent failure and its replacement,
have the blue "sparks" when the scope is first
powered up. - They're made by the same company and were
bought at the same time so may be from the same batch. It
could be that they were both bad valves.