Let me add.......
Seems like many people have issues with popping, cracking etc...me too.
Seems like one of the culprits to watch out for are the connections for the leds attached to the 12AU7 tube socket...
Mine was working well for several months and after changing the signal tube several times over weeks...wiggling it in and out of the
socket my leds developed a mechanical "fracture" on the leads coming out if the led...make and break, wiggle the tube in the socket and
get loud popping and when doing this with the chopstick method from the underside I saw the LED actually going off and on...
My solution was to replaced and re-install the leds with some "relief" in how these are soldered to the socket and term strips...leaving some
excess leas slightly bent up to provide a little stress relief when changing out tubes.
The other thought is now after having the crack for awhile I find myself changing out tubes less and less and thus the decrease in creating a
fracture due to tight fitting parts or a cold marginal joint the becomes worse by the wiggling of tube removal and insertion...
FYI..
Alex