Sometimes the removal and re-install of tubes will clean up (deoxidize) the sockets. Maybe that got it working.
Depending on the droning sound, it could be a microphonic tube. Try tapping them gently one at a time, or holding them with a rag or something to damp the vibration but keep from getting burned. Generally, microphonic preamp tubes will be higher pitched whistling or squealing, where power tubes will more likely rattle or make low pitched rumbling sounds.
Depending on the droning sound, it could be a microphonic tube. Try tapping them gently one at a time, or holding them with a rag or something to damp the vibration but keep from getting burned. Generally, microphonic preamp tubes will be higher pitched whistling or squealing, where power tubes will more likely rattle or make low pitched rumbling sounds.