Picked up a rev pro jr 20 watt. I'm looking to see which tube does what as far as the 4 - 12ax7's go. Is there a way to figure it out? I've done countless searches online for just that information and nothing. Unless I'm using the wrong search criteria... 1 tube should be for the effects loop, and 1 for the phase inverter and 2 for the preamp section. Any thoughts?