Put the old tube set in and sub in one of the NOS tubes. If no popping, remove that NOS tube and set it aside, it is not the issue. Now sub in the next NOS tube, and repeat, checking one at a time, until you identify the problem tube. Then replace it.
Installed new tubes. Now a loud pop!
I put new old stock tubes in my PS Audio amps & pre & now a loud pop off & on.
Help!