tkrex wrote
If a circuit calls for a particular transistor, but i can only find similar ones, how much room for error is there? Like, will any old NPN transistor do (provided the voltage is high enough) whenever an NPN is called for?
Really quite a lot. Most silicon transistor-based pedal circuits aren't very specific on which device they use. Often different transistors will have different tones or gain structures, but the circuit will work just as well. In this line of work I don't think you'll ever have to worry about subjecting any transistor to voltages near the absolute maximums so just feel free to swap them about until you hear something you like. The only exception is when a circuit calls for a Darlington, then you should only interchange among other NPN or PNP Darlingtons and not a regular transistor (you'll note though that in some circuits it may be to your liking to replace a regular transistor with a Darlington). This is because the Darlington is usually specified because the extra gain is needed for some reason.
More specifically, what is the difference between different component suffixes? For example, the data sheet for the MPS2222 mentions MPS2222A, MPS2222G, MPS2222AG, etc. However, when it lists actual values, it only makes mention of MPS2222 and MPS2222A (no G).
Lastly, if a transistor is labeled XX2222X, are these all in the same "family"? Examples: PN2222, 2N2222, MPS2222, etc.
There are essentially three different sets of rules for naming transistors. I'll refer you to
this instead of explaining it all myself. Often on transistors, you may have a suffix that indicates a specifically sorted gain range for that device, e.g. BC108A vs. BC108B vs. BC108C in order of increasing hFE. Such a thing also exists for op-amps, where the prefix generally refers to the manufacturer in some way and the suffix is something about improvements or packages, etc. So like the UA741 and the LM741 are the same chip made by different people, and the TL072CP is a PDIP-8 package (what we're used to using) and the TL072CD is a SOIC package (little SMD thing). I believe in your case that the MPS2222AG is a Motorola-made 2222A in a lead-free RoHS-compliant package, the PN2222 is a "regular" 2222 in a plastic TO-92 package, etc. - short of the device type, none of that should mean much of anything to you. You will notice though that the 2222 and the 2222A are actually slightly different devices, but to an extent likely not noticeable in a low-voltage low-frequency audio circuit.
Through all the worry and pain we move on
Locked
it would have been so great if all the big component firms had a referendum and settled on ONE standard pinout for each model of similar device (i.e. all silicon BJTs would have the same pinout, all JFETs, etc) because I really don't see any reason why one transistor has to be BCE and another CBE and another BEC (yes that exists 
) when they're all the same shit. Lol...