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I have owned the st-150 and mb 450's The st-150 blew resistors once in a while. A pain because it weighs 75 lbs.
I have had no issues with my mb450's. they do not even run that hot. It is beautifully made. I will say the sound is amazing. It rules the speakers with an iron fist, but sounds smooth as silk. It ranks with the finest equiptment made . I think it is almost a bargan in the current used market. I also own tube amps from CJ and ARC and have had no problems ARC never even blows a resister when a tube blows. depending on how much you listen 6550 type tubes last around two years .
It is not a flawed design, it is a protection feature to stop current from entering the transformer and possibly damaging it. Every tube amp of competent design has a 10 ohm resistor, or there about, on the cathode to ground return...in effect when the resistor blows out...it turns off the tube because the tube failed.
If your amp is other than a strictly triode amp, then it also has screen resistors, value between 450 and 2500 ohms depending on the amp and power, as well as number of output tubes. If one should blow out the screen has shorted, and the power will drop so low that you can't bias the tube anymore.
In both cases, it is more a matter of the owner not keeping track of the tube condition with a quality tube tester that can measure LEAKAGE on an element. Most testers including low end Hickok can only measure 250K leakage and buy then you are close to a short. Only better or well conceived testers can register leakage above that 250K. Hickok would be 539 types, KS or RD, or 752(a), Triplett 3444 or 3444a, Heathkit TT1, Eico 666 or 667 ( emission only), and a few others like Better Jackson, AVO, and a few European ones.
The simple answer to expensive repairs is a good quality tester. Keep track of tube hours, and watch out for a red plate on a tube, beware of flashing that has grayish edges.