I know this is going to sound like a smartass reply but seriously it isnt....just figure it out!
people have been doing repairs for years on stuff for 50+ years and rarely did the techs need a schematic. unless it's was a uniquely complex circuit or it had been screwed with and needed to be returned to stock, I almost never need a schematic for a repair or maintenance. It's particularly true with tube amps but if you cant find the source of the problem without a schematic, it's doubtful you'll be able to find it even if you've got one in front of you.
Understand why and how certain components fail, basic operation of active components, etc., and you can pretty much skin any cat, solid state included, with a scope, meter, and a function generator. Consider some of the guys here that can diagnose a problem based on a short text message and they dont even have the amp in front of them!