I've got some 6B4G tubes which take a long time (30 minutes? an hour? more?) to fully settle down. I can't comment on the voltages, but visually you can see when they are "ready". They've got a very distinctive pattern of blue glow on the inside of the glass. When you first turn the amp on, nothing - only darkness. As the cathodes begin to acquire the first traces of dull red glow, there is an ever-so-slight pop, and blue covers the inside of the globe. It dances and wiggles as the field stabilizes. Sometimes the blue collapses and reforms, and you can hear a very gentle pop from the speakers in time with the motion of the blue glow. Some nights I fall asleep with the amplifier running. In the morning, the field is perfectly stable. The edges of the glow are soft and blurred, unlike the hard and sharp edges when the tube is cold.
Weird, huh? Wish I could capture a good video of it.
