12AT nos , flash at start ?

hi, is normal, when i start nagra pre, tube 12at7 make flash ?

like this

Agree with the posters above. It is normal with Mullard 12AT7 tubes. I saw the flash when I had a pair installed in my Jolida 502P amp.
Perfectly normal in a Nagra preamp. (Full disclosure; am US service center for Nagra)
I have a couple of sets of NOS Mullards and a large part of why I like them is the flashing on startup…they sound great also, but the flash is simply a wonderful thing. Gold Pin 6201s…el flasho mundo.