We used to diagnose and repair tube amplifiers with only one instrument: a 20Kohms/V analog multimeter. (I used an Hioki AF105 and an Avometer
First, you must be aware of safety with high voltages because it is lethal.
Only work with only one hand, don't secure the chassis nor anything with the other hand.
Be sure what you do, check twice.
I would touch C31 (V3A pin 2 GRID of 12AX7 with a screwdriver , eventualy with your finger), and you must ear a hum in the loudspeaker.
If this is OK, you know your power stage is ok.
If not, you should first check connections of output transformer and loudspeaker and replace the tube 12AX7.
If still not working, you will have to measure voltages of the power stage and compare with schematic.
If power stage is working, you have to look at the preamplifier.
Check for bad contact at J2 and J3 jacks.
Again, try to touch V1B pin 7 GRID of 12AX7...you should ear a hum in the loudspeaker if it is allright.
If not, replace V1 (12AX7)
After doing all this, report here.
NB: you can also use a signal injector (signal tracer) in place of your finger, that's safer ...