If the tube is an "Amperex" (Bugle Boy)on the label or the date codes, then it is normal.
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If its the filament itself that glows brighter the first second or two.Make sure it is the filament only.The exact same spot should go back to its normal glow.Those tubes that do this kind of flash will always do this at turn on from a several hour could start.If your not sure,by another tube and check it out to be safe,plus you'll have a spare.
Thanks for the reassurance and facts on the tube flashing.
I've been a member just under a year and really
have learned a trmendous amount by just reading the postings.
I am hooked on the site and continue to improve my system and more importantly my appreciation of the music I listen too.
Thanks again, Lawrence