symptoms of bad preamp tubes?

[67mike]

What typically does a bad preamp tube do as it shits the bed? Would it cause momentary spikes in certain frequencies?

[Chops Malone]

If it's a high pitch squeal, it could be a bad preamp tube. Sometimes the noise is dirty pins on the tube or a dirty tube socket, but not that often, usually it's the tube. Do you have any known good tubes that you can swap out with? That would help you troubleshoot the problem.