Playing with my band i've been having a high squeal come out when I'm not playing. I have an Epiphone SG with Seymour Duncan Blackouts active pickups and a Line 6 Spider valve tube amp. Right now I have to turn the volume knob way down on my guitar to not get it but I lose all the distortion. I heard that a Noise Gate pedal helped this type of situation. I was also referenced to a "Hot Plate" which i've never heard of before. Will either of those help?
Hot plate is an attenuators. Won't help.

A noise gate could help, but first try lowering your gain.
Originally posted by primusfan
When you crank up the gain to 10 and switch to the lead channel, it actually sounds like you are unjustifiably bombing an innocent foreign land.

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Start with a fresh battery.

Does the squealing stop when you mute the strings with your hand? If so a noise gate might help.

Does the squealing go away if you physically push down on the pickup? If so, the pickup might be vibrating in the cavity. You can fix that by bolstering it with the foam (like the packing foam new pickups come in).

If nothing stops it, you might have a loose ground wire somewhere. Open the back cavity and inspect the wiring. look for a loose wire on your pots, switches and jack.