Deepak, congrats on your new amp! As you're learning, tube amps can be so much fun!
Since you were flying blind and may have left the bias set too high, connect the voltmeter before start up so you can watch it rise from zero and prevent it from exceeding the recommended value.
Tubes can tolerate a few seconds of too high bias, but if it overheats then it starts drawing more and more on it's own, aka runaway tube, which will not last long. This might occur after 30 seconds or a minute if the bias was double or more of the recommended value.
I like to turn the bias all the way down before changing power tubes. Use a meter to watch, or make sure you remember if down is counter clockwise.
Have fun!!