Low Power or Loss of Volume

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Faulty Preamp Tube
Faulty Power Tube(s)
Bad preamp cathode resistor
An unbypassed cathode resistor has drifted upwards (to 5K-10K or over)
Faulty phase inverter
If for some reason the phase inverter input side is good but the inverted side is bad, the power amp will still work, but power will be very low. This can be a bad 1/2 tube, a faulty socket contact, a broken or open plate resistor or coupling capacitor to the output tube, or a bad solder joint on any of these.
Open cathode bypass capacitors in preamp
if they go open, the stage they're in loses gain, but does not otherwise fail. If they short, it dramatically shifts the bias point, and may cause distortion as well as low volume.
Faulty vibrato circuit on neon/LDR vibrato Fenders
If there is a dummy plug in the footswitch hole, or a bad footswitch so the vibrato is always active, sometimes the vibrato tube turns on and stays on, not oscillating. This keeps the neon bulb on all the time, shunting lots of signal away. Same thing can happen if there is a shorted vibrato tube (rare) or a bit of wire or solder shorting the vibrato tube. Check the plate voltage on the vibrato tube to be sure it's oscillating.
High voltage isn't high enough for some reason
Open screen resistors on power tubes
Amp cuts out or "goes dead" when the volume control is turned up higher than "X" or when you hit a specific note
You have a parasitic oscillation above hearing range. This can overheat an output transformer, and really needs to get fixed fast. It can often be fixed by tube swapping, but you often need an oscilloscope to see what's happening in the electronics.