GE T265A sound cutting out

[TheShanMan]

When this 60's era radio first warms up it sounds just fine, but then after about a minute it starts cutting out more and more until a short while later you get nothing. But if you turn the volume up fully you can hear the station VERY faintly.

I haven't dug too far in yet but I have tapped components and wiggled tubes while the problem is occurring (not to mention turning the knobs) so I don't get the impression the problem is a cold solder joint or tube connector.

Since I'm pretty new to the world of tubes I'm not sure if this sounds like a failure pattern for a faulty tube or if I ought to be looking elsewhere. Seems like it must be a problem in the amplifier circuit though. I'm curious what people think is the best way to move forward.