I'm new here and am trying to learn electronic stuff in my spare time. I find it frustrating but fun at the same time. Once I figure stuff out it seems so simple but then another roadblock. I guess thats how you learn. Anyway to my question, I'm trying to build some simple circuits like a signal tracer/injector and small guitar amps and things like that. I have alot of old circuit boards I rob alot of parts from with all sorts of transistors and ic with different numbers and I've searched for data sheets and got alot of info and I was wondering if its alright to put like a 2n904 in place of a bc547 and 2n906 in place of a bc557 I think there both basically the same but the data sheets show slightly differnt numbers I also have many other transistors I could use that have close specifications. I guess what I'm getting at is do things have to match exactly or can you substitute things with similar specs. Or does it depend on the circuit. Thanks for your help. Also can you use mylar capacitors (most of the ones I have are green) in place of ceramics and vice versa.While asking about capasitors alot of the circuit boards have small blue capasitors I think they are called multi layered with the same numbers as the mylar and ceramic can they be used in the place of the mylar and ceramic ones or does this once again have to do with circuit application?