One of the most common issue I see with solid state amps and re-issue valve amps is dry solder joints. Solder dry joints are made during the manufacturing process when a component is not correctly soldered and the flux in the solder is possibly holding the board and the component in contact or too low a temperature used on the joint. Good quality inspection processes during manufacture can eliminate this kind of problem. However, these faults are not often found. The result is that the amp works but you have a ticking time bomb with this kind of fault. The heat in the amp during use causes expansion and contraction of the board. Eventually, the solder joint fails and the amp stops working.
The failure can be seen even years after you bought the amp and is dependent on how much use the amp gets and how much vibration it receives.
I recently repaired a modern solid state all bells and whistles with amp simulation. It was about 2 years old. This amp had several dry joints in the power supply and even one joint that had never been soldered !
Having re-soldered all the suspect joints and using a good quality magnifier to inspect all the joints, I put the amp back together and it worked perfectly.
Hand wired turret board amps do not seem to suffer these issues near as much as modern solid state and to a slightly lesser extent, the printed circuit board constructed re-issue valve amps.
Valve amps have high voltages and the amp designer has to take account of the spacings on the board and the choice of the printed circuit board material. There are some amp board designs that have not adquately taken care of these challenges. The amps then end up with current leaking in the board. Symptoms include output valves red plating, excessive noise, and mains hum.
