I often repair Valvestates (I have a VS100 on my bench right now) and Fenders, but to be precise, the problems are more mechanical than electronic. PCBs flex and develop invisible cracks just where the thin track (around 30 mils) joins the pot/jack/switch solder pad. Laneys, with a similar construction, are more reliable because they have fatter tracks (40/50 mils). Old Peaveys and most 70´s / 80´s amps, with their hand drawn PCBs, are most reliable in that respect.
Today I also have an Acoustic 118 with beautiful hand-drawn boards (and blown output transistors). Those cracked track problems are often mistaken as "cold solder joints", specially because they are repaired by resoldering, but truth is that resoldering just bridges both sides of the crack. I use a jeweler´s loupe to detect them. Sometimes I also find blown output transistors and very infrequently some blown TDA1514, very hard to find. I have made a replacement mini-board with an LM3886, which is cheap, available, and far stronger, and wire it to the corresponding pins (+-B, ground, etc.) on the original board.