Most often you can read about the stories here and I would say there are a few very important contributing factors:
1. Old worn tubes (most people are negligant when it comes to replacing tubes but learn very quickly why it is important to replace them every year or two if you gig hard with the amp)
2. Improperly biased tubes (this is a bit rarer in the Mesa family since the stock bias is set a bit cold but some tube vendors can get some fairly hot tubes which can cause problems if you constantly crank your amp)
3. Bad power
4. Bad tubes. This can happen anytime and anywhere. This is part of the world of tube amplification. The unknown factor of what will happen because even though you bought new tubes they can be bad out of the box regardless of brand and regardless of how much testing they went through prior to going into your hands.
The problems associated with a blown power tube can be as little as a fuse blowing but could result in a much more expensive fix depending on the severity of the failure. Mesa doesn't include fuses for the B+, heaters, etc which in turn puts all the stress on individual components (screen resistors are the most common failure).
Basically, regardless of the repair you will have to replace your tubes as well. I say it is safe insurance to replace tubes every couple of years but if you disagree and use your tubes for 10 years without fault then so be it. Personally, I just don't feel I should put that risk on my equipment just to save $60 every couple of years. When you have a failure I just hope you can shell out the $60 for new tubes PLUS the cost of repair if something happened to become damaged.
There are varying views on the topic of tube replacement but in the end it's all up to what you can afford.
Greg
