^that can explain the volume drops but would not cause a tube socket to work. Now does it light up at all, as in the amber glow? If it does not as was said, the connection to the heater might be cold or came undone and needs a little solder work. All the heater pins are connected together so if one works and the other doesn't either that joint is cold or the actually socket itself is fucked and needs replacing.
Basically pins 2 and 7 when probed should give a voltage if they don't then you have a bad solder joint or the socket is bad, if you get a voltage reading on those pins, the tube will have an amber glow when you hit the power switch, when you hit the standby and it doesn't glow blue, its the screen grid resistor, or a cold solder/bad socket at pins 3,4 or 8, thats the only way that the tube will not have a blue glow. If you get a blue glow and its creating heat, you have to test to make sure the control grid is putting out a voltage when you are playing as having a bad pin 5 connection will cause the tube to dissipate power but not actually get an audio signal to amplify.
