As a tangent to this i.e. devices being troubleshot outside of their cabinets, one can also be fooled by amps pulled from their cabinet and the resulting open, unshielded, high-gain electronics hanging out for all the EMI in the world to fall upon, plus testing with a guitar from two feet away. Oscillation, hum, you name it.
In one instance, I had a Music Man amp that misbehaved only when IN the cabinet. As it turned out, the power tubes were sitting almost on top of the (retrofitted) E-V speaker magnet and caused fairly severe waveform distortion as a result of beam deflection. THIS one drove me nuts for a little while BTW.
Oh yes. They put that shielding inside the cab for a reason. Some are sneaky, Peavey uses a conductive black paint inside their cabs instead of gluing foil up. Looks like no shield, but it is shielded nonetheless.
John, that is the first time I ever heard of speaker magnet to tube interaction in real life. Up til now it has always been one of those philosophical debates in tube chats.
Education is what you're left with after you have forgotten what you have learned.
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