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Four months after receiving the APPJ amp I am noticing a faint but audible high pitched whistle through the tweeters of my speakers. Think crickets.
Researching tube amplifier's possible problems brought up several possible causes:
1) old/defective tubes. I swapped around all the tubes of this unit several times - first the driver tubes, then the output tubes. No difference. I swapped around my matisse clone preamp out for a little bear P5 tube buffer and it was the same. so...
2) possible microphonics. I dont know about this one. Seems unlikely. Microphonics result in feedback not something subtly bothersome, but loud whine like baby screaming not like many crickets.
3) ground loops? really? no I dont think so.
How else to solve this? Unit has no warranty. Massdrop can't / won't help.