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
Failing rectifier tube - try swapping in another one
Failing or open series dropping resistor in the bypass networks
leading to preamp stages
Failing bypass capacitor - treat as in power filter caps.
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.