Printed circuit boards comes in different ratings, most common is FR4, but there are others, including some laminated on metal. It would be possible to have a poweramp with a PCB and have chassis mounted tube sockets with wired connections to a PCB. Suitable slots can be routed in the PCB for higher voltage tolerance, as is commonly done in AC line power supplies and high voltage circuits in CFL tubes.
What fails in tube amps?
Overvoltage and overtemp. Electrolytic caps comes with so many hours of load life specified for a max temp. Metal film resistors can degrade over time with too much voltage for their spec. The degrading can take years and results in a noisy amp.
Wirewound resistors don't do this.
Tubes degrade over time but may last a very long time, longer than electrolytic caps. Film caps can break too, causing tubes to loose bias and overheat.
PCBs as stated before, turn brown and ugly, oveheating solderjoints loosening them up making them noisy.
Connectors and pots are common culprits.
Many amps do not have indiviual bias pots for power tubes, but that can be added. Unless of course it is a selfbiased tube, which not so efficient, and not much used in high end amps.
Solid state components can also take a hit from adverse operating conditions.
Don't be afraid of tube amps.
