Bruce J. Richman wrote:

Tube amplifiers have both their proponents and detractors. Comments about lack
of reliability and/or high repair needs from the latter group are, AFAIK, not
supported by any empirical evidence indicating those claims are valid.

Lack of reliability or the converse is relative. Given equal design
attention to reliability, tube amplifiers are simply not as reliable as
solid state amplifiers, especially as regards power amplifiers. The natural
manufacturing variances in tubes themselves, the the increased heat produced by
them, (meaning increased stresses on associated components) and the higher
voltages dictate the empirical facts of the matter.