Agree that it very well might be a tube. Swapping tubes will tell you in very quick manner.
However, if you find out that isn't the problem, another possibility is that it's actually the tube socket. I've seen this many times. Today's tube sockets often do not make the best contact with the tube pins, and in the "cold" state, the contact with the tube pin can be compromised. Within the first few minutes of operation, as the parts heat up, they expand, and then everything is fine. If you deduce this is the problem, use something like a toothpick to gently bring the contacts of the tube sockets together a bit, after that, the problem should be solved.
Another possibility, though more remote is a less than ideal solder joint, but explore the tube and tube socket scenarios first. Beyond that we get into issues that the average owner must seek outside help to resolve.
However, if you find out that isn't the problem, another possibility is that it's actually the tube socket. I've seen this many times. Today's tube sockets often do not make the best contact with the tube pins, and in the "cold" state, the contact with the tube pin can be compromised. Within the first few minutes of operation, as the parts heat up, they expand, and then everything is fine. If you deduce this is the problem, use something like a toothpick to gently bring the contacts of the tube sockets together a bit, after that, the problem should be solved.
Another possibility, though more remote is a less than ideal solder joint, but explore the tube and tube socket scenarios first. Beyond that we get into issues that the average owner must seek outside help to resolve.