The Vox AC120 is not very common. I wouldn't call it rare but I have met few people who own one and my father is one of them ;) The problem with this amp is, that there is very little information about it floating around and the available schematics are of very poor quality. So when one power amp bottle of my amp was red plating, I had no chance but to open it up and do some measurements without an official reference. My father had some hand drawn schematics which I provide in scanned form here which may or may not be altered. I belive his version of the AC 120 was heavily modified but still: better some schematics as none at all (or unreadable stuff).

So, I had the amp opened up and realized some resistors had gone up in value. I replaced power supply filter resistors along with power amp cathode bypass resistors with 2W metal oxide resistors but still no luck. Voltages all seemed to be correct (B+, grid…) so I suspect one of the earlier stages to be faulty. As I have so little time to dive deeper into this, I've turned in the amp for servicing. Didn't feel too good because up to now I've been able to fix almost everything by myself, but when time is not readily available, you need to choose.