[MrFixr55]

Researched this before I saw Terry's comment, +1 on Terry's comment. The radio works without burning out the 6H6 because it is a series string and 6V is not going to make much of a difference. The bigger issue is that a 6H6 heater draws 0.3A. A 12H6 and all the other tubes in your radio draw 0.15A. So even though this is a 6V tube instead of a 12V tube, it is not heating up. Since the 12H6 is the FM Detector and not part of the AM signal path, failure of the tube to heat will definitely cause no sound for FM. AM is working because the AM section detector is the diode section of the 6AV6 2nd Detector / 1st Audio tube. Hope this helps! Good Luck!!