[The Lost Bears]

You should check and if needed, adjust the bias often when the tubes are new. The bias on new tubes can drift until they settle in. That is why it is important to purchase well matched tubes from someone like ARC. ARC burns their tubes in a bit before the hand match them. Some tube dealers also do this but most don't If the tubes are only (tube) factory match, they will tend to drift a bit more in the beginning. If you are running matched pairs than they can drift in opposite directions and one tube will run hotter than the other. Running tubes too hot can affect tube life.

I bias my tubes at .63ma. I usually check the bias after listening. I mute the preamp and let the amp settle for at least 5 minutes. The first few days with new tube I check the bias often. After that I check the bias about once a month. I also dust the top of the tubes so the have better heat dispersion.