AC Indicator Glows

If there is no sound at all and the power indicator glows, there is AC power reaching the indicator. This indicates that although there is power in the box, something in the signal path is not letting any sound through.

Once you eliminate the silly stuff, like speakers not plugged in, etc, you can start looking at:

• tubes:
  • do the filaments on the rectifier tube and output tubes light up? replace them if they don't
  • failing rectifier tube
  • failed phase inverter/driver tube (sometimes the output tubes are quiet enough to not make a detectable hiss or hum without drive)
  • power tube bad, although it's rare that they would both fail
• power transformer faulty
• open choke
• output transformer faulty
• blown speaker(s)
• bad wiring or cord to speakers
• Open/bad speaker cord or wires leading to the speakers.
• First dropping resistor after the choke is open (it is rare that you would not hear hiss or hum in this case, but it could happen).
• Parasitic oscillation