If it only happens with the Mullards, then it is probably a grid short in the tubes. Once the tubes warm up, the short clears itself. One or more tubes are probably worn. A tube tester will confirm a grid short.
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Could be a short in the tubes, could be a loose connection in the tube socket, or it even could be metal fatigue, in the tube itself, which doesn't stabilize, till the metal is heated thoroughly. Truthfully, there are so many variables, that if it is not presenting a problem, just accepting it, is probably best. The most vulnerable time for a tube, is at startup, when it begins to conduct.