you'll recognize redplating when you see it . . . when you turn the amp on - you watch the tubes. There should be a wire in the center glowing red (this is the filament/heater). That's normal. But if you see the plates around that wire begin to glow red (a lot of red) - that's bad. Turn the amp of NOW. When this happens the amp will usually begin to distort badly, so when it distorts - look at your tubes. If red turn it off NOW.