[MrFixr55]

Or unlucky depending on your point of view. I believe that I had another 80 or 5Y3 short on me once. I would think that the older 80 type with the tent type heater may be more prone to failure. The problem with Cathode to heater shorts on 6X5s is that since the heater is powered by the same circuit as the other tubes, that allows for an almost dead short to ground of the cathode. Different illness, same baaaaad outcome.

The 5AR4 is an indirectly heated cathode full wave rectifier tube but is used in a lot of paces instead of the 5U4. The cathode is already purposely connected to one leg of the heater, and an isolated heater secondary is expected to be used, just like the 80, 5Y3, 5U4, etc.