so the amp is a bad cat bc-50. its a 50 watt plexi style amp with a slightly different power supply and a few different values along the way. but essentially a 1987x circuit with the inputs jumpered. ive switched tubes and it happens with multiple tubes its loud enough to be annoying when playing but i still like the tone. when the amp is run into a load i can still hear it, so i know its not in the audio path. i dont know why but i think it's coming from the output transformer. the output transformer has no markings either so i cant tell exactly what it is. here is an example. the fan noise you hear is the two notes captor. no matter what load i try it into (ive tried 3) it always sounds the same. . the tubes are biased at around 32/33mA with the voltage around 472V. i can get the actual numbers written down at home if that helps. where do i start?