Originally Posted by A9X-308
I generally concur with Arny in post 2. My own personal experience in three decades of working on SS amps is that (especially earlier generation) electrolytic, and some
tantalum and ceramic, caps degrade over time. I generally replace these in restorations of units > 20yo as a matter of course, especially PS caps.
IME the PS caps are among the longest-lived, while its the small coupling caps that have the worst track record for drying up and falling down on the job.
The actual SS devices tend to be very reliable, providing they have not been run too hot too often, or the designer didn't run them too close to their design limits. this will vary on a design by design basis, but some googling and reading of sites like Audiokarma that tend to have a vintage bent, should indicate whether a given design is prone to aging issues.
There is a long term failure mode of SS devices where overloading or overheating them causes progressive loss of current gain. First distortion rises, and then power of any kind starts falling off. Sometimes amps with multiple paralleled output devices are designed to fail safe, and keep on working even when some output devices short out. The first sign of that trouble is significantly reduced power output, particularly into low impedances.
SS device durability has improved greatly since the early 60s. In the early 60s there were actually power amps rated at say 25 wpc that used a pair of output devices with 25 watt maximum power dissipation. I don't know who would spec such a thing, unless totally desperate!
The next generation of equipment jumped to the same number of devices each rated at 4-5 times as much power. Then, amps started showing up with pairs and quads of output devices in parallel.
Between the continuing progress in SS devices, and the fact that some of the old habits were excessive even in the day, I don't see any need for power amps with literally dozens of large devices per channel. I've had one of these on my bench and in my listening situation, and they only make sense as audio sculpture.
OTOH if you have one of these old amps to work over, putting modern devices into the old sockets will increase the safe operating area by 2 or 3. Open up the protection circuits accordingly, and a marginal amp can become a winner with tough loads.