Sounds like Lewm and Jmmorford both have the same problem, and in both cases no evidence whatsoever that the preamp is at fault.
If getting a lot of motor noise the culprit likely has something to do with the tone arm wiring. Perhaps a bad ground so the arm tube isn't able to shield the cartridge from the noise of the motor. If one is using a belt drive turntable, the best designs will have the motor opposite of the arm in order to minimize noise pickup from the motor. But if the motor isn't grounded properly and if its not opposite the arm, the noise from the motor might be getting into the preamp through the cartridge itself.
If the preamp is silent when the turntable is disconnected but on, this is a good indication that the preamp is not at fault.
This would also explain the sudden nature of the onset and why its not tubes (if it was tubes, the problem likely would not have manifested in both channels at once...). All it would take is one bad connection in the turntable and you've got your noise problem.
If getting a lot of motor noise the culprit likely has something to do with the tone arm wiring. Perhaps a bad ground so the arm tube isn't able to shield the cartridge from the noise of the motor. If one is using a belt drive turntable, the best designs will have the motor opposite of the arm in order to minimize noise pickup from the motor. But if the motor isn't grounded properly and if its not opposite the arm, the noise from the motor might be getting into the preamp through the cartridge itself.
If the preamp is silent when the turntable is disconnected but on, this is a good indication that the preamp is not at fault.
This would also explain the sudden nature of the onset and why its not tubes (if it was tubes, the problem likely would not have manifested in both channels at once...). All it would take is one bad connection in the turntable and you've got your noise problem.