Perhaps your BIAS?
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Power tubes do short out on occasions - some brands are more prone to this than others. You are fortunate if all you got was a blown tube....in some amps you will take out a bias resistor and/or a fuse. The only way to avoid tubes shorting out is buying tubes generally noted for being rugged in the first place.
As suggested above you could be running your tubes too hot. Are you using the manufacturers recommendations?
Just noted that you said your amp is self biasing --FWIW I've a self biasing amp - put in a new set of tubes and one tube's plates started to glow red. Changed it out thinking it might just be the tube, the next tube also glowed red. Returned the amp to the manufacturer and a minor adjustment to the bias circuit solved the problem.
Thats a common thing in rogue amps. they are hard on tubes, factory Highly reccomendeds FACTORY matched sets only on outputs. those amps are great much better then most people might think. Ive spebt 2-3 times as much on 5 sets of mono blocks only to sell and buy new 120's.
hurricanes, vtl mb300, rogue zeus,ect