Occasional tube failure is inherent in vacuum tube components.
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Consider yourself lucky as a few times they blew out my Tweeters.A tube failure won't blow out a tweeter! Tweeters fail because the amp being used is underpowered and is making too much distortion when driven too hard.
My question is how can this be prevented in future? Autobias means no way of reviewing the tube settings and performance.Autobias assumes that the tubes are OK- it usually can't do much about a tube that is arcing or shorted. If you are concerned about this, I would consider having the amp modified with fuses in series with the cathodes of the tubes to prevent them from damaging the amp.
For Tube Amplifiers Auto bias No Good Ever , for it adds extra circuitry and resistors. It average each channel .
Manual bias far better you adjust each tube individually
And when a tube does go, just review the new tube, recheck others and done .it takes may be s minute per tube some compsnies still want you to use a meter way out dated., I had my own Simpson meters added and switch per channel ,seperste volt meter is primitive and s pain in the ass.
To say that autobias is " no good ever" is hyperbole. I own 3 tube amplifiers, 2 are manual bias and one is autobias. All 3 are very fine amplifiers but the autobias amplifier is every bit as reliable (if not more so, 8.5 years of nearly daily use ) and the best sounding. No component or part is perfect and things occasionally need to be replaced or repaired, stuff happens. To universally denigrate autobias is silly in my opinion. Implementation as with all things involving audio is the overriding factor.