Regarding the likelihood of premature failure due to mismatched tubes in a fixed/self bias arrangement. If you don't have individual bias for each tube you risk running one of the tubes at enough out of spec for idle bias that it's either too cold (harsh, crossover distortion) or too hot (blowing up tubes and maybe other amp components). OTOH it has a juicy complex clean from those 2nd order harmonics (see many of the pedal tests at my youtube channel: Dr. Whether run in fixed or cathode bias, the amp hums more than it did with matched power tubes. I have a London Power Session amp (10w from a pair of octal output tubes) with a 6V6 and an E元4 in the output section (individual bias adjustment). The more unmatched the two sections are, the more hum. Because of the same issue, the unbalanced output is more prone to pick up noise from the environment (the OT can't cancel it out completely) which will come thru as hum. Because the two sides of a sine wave amplified by the push-pull output section don't match if the tubes don't match, there'll be more second order harmonics. Power tubes that don't bias at the same point create 3 changes in your amp:ġ.
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