Many tubes have a small amount of gas or material inside, forget what they call it but its to facilitate electron transfer. This material can condense in different places when the tube is cold. Then as the tube warms up as the little bits turn back into gas or fly off in little pieces they of course go from electrode to anode and upon hitting the anode create a little voltage surge which we hear as a pop or click or static.
Eventually this all settles down. Usually. We hope. Welcome to the wonderful world of valve amplification.
Eventually this all settles down. Usually. We hope. Welcome to the wonderful world of valve amplification.