some tubes like Mullards do that all the time..My guess is your not having any problems to worry about..
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When I owned tube equipment, I had a preamp with 12AU7 tubes. Certain brands would flash brightly when powered on. I checked with two tube vendors who assured me that the flash was nothing to worry about -- that it was a characteristic of some brands and not others. So perhaps the same is true for the tubes in your Onix.