I've had a 6550 and screen resistors die twice in the last 12 months (new amp). It's happened with both the original sovtek 6550's and a replacement set of EH tubes. Am I just unlucky or is it likely there's an underlying problem here that's yet to be diagnosed? Is there a particularly rugged tube (other than mega $$$ nos) that's worth trying? Would changing to KT88s be a good move? What's your advice tube oracles?