More than likely a bad solder joint. Where is the next question. How confident are you with voltage measurments? The rails in tube amps are high and can kill you so if you are not inclined to do so please don't. The reason I suspect a solder joint is that it is a failure over time, cold solder joints expand and contract with heat. When they heat up they break connection. Not saying 100% that is your problem but it is very likley.