Last night I plugged in just the preamp and a cable, no guitar. Holding the cable off the ground, not touching metal, I still had buzz that got better if I touched metal. It was repeatable with 3 different cables. I also tried with extension and went outside, no help. Plugging in just a 1/4” jack with no cable made no noise.
Is that normal?
Often a disconnected cable will pick up noise. They can act like antennas.
The preamp is all tube at 300v but runs off a 2-prong Ac/Dc adapter, sending 1a at 12v to the pre. So it’s not grounded to the house electrical system.
Lots of power adapters cause hum; switched mode adapters are notorious for this. Other power non-switching DC adapters can also cause hum. Whether your power adapter is regulated is another question. Unregulated adapters can be problematic.
F'rinstance, laptops connected via USB can even cause hum problems that go away once the power adapter is unplugged and the computer runs on batteries. Happens often enough to be a known issue in studio-land.
Buzzes can be location dependent, with pickups responding to EMI and RFI, but 60 cycle hum is a different story, unless the pickups are very close to your guitar amp's transformers. And don't forget about adapters radiating hum into pedals and cables.
By the way, you didn't describe what your preamp is going into, other than a cab sim. Where does it wind up? A guitar amp? A computer interface and computer? Powered speakers?
Still and all, when you describe a hum problem that is solved by taking the preamp out of the chain...something's going on that isn't pickups. Perhaps I'm missing something.
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