Grid Heater & Alternator Diode Failure

Here is a tidbit I found that sheds a bit of light on the grid heater element.

https://goodsonengineering.com/wp-content/uploads/2017/08/FiresCausedbyFracturedResistanceHeatingElements.pdf

In Photo 1, we note that the coils (loops) of this element are relatively uniformly spaced. Such uniform spacing creates a relatively constant surface temperature for the element. At 240 VAC, with a fan blowing (as in actual use), surface temperatures were approximately 470 deg F when measured with a thermocouple. This temperature, however, will lessen as the heater element is used. A nominal 5000 watt heater will drop in wattage (and thus heat output) as a function of time. As with the lightbulb filament, the heater element becomes smaller in diameter over time. On the heater element, this reduction in cross-sectional area is due to oxidation. The smaller cross-sectional area increases resistance, which thus decreases wattage.