@wharfy the amps are custom made; the only two on earth. The original designer and person who initiated their build was Gary Dodd. He died before finishing them. 6 years later in 2021, his friend and colleague Charlie Cocci completed them and sold them to me. These amps are optimized to run KT77s in 43% tap ultralinear mode; so much so that they literally cannot run any other tube type. So yes, all previous failures where of (Gold Lion) KT77s. From the get go, I was able to set the individual bias for each output tube to just barely 40 mA...usually with the bias pot turned all the way down. Sometimes even at fully turned down the tube would be at ~45 mA. After about half a dozen spectacular tube failures which always took out the sacrificial resistor on the bias circuit, the fuse and sucked in the glass envelope to the point of breaking, I contacted Charlie to ask what the heck was going on with these amps. Long story short ended with him incredulous that the bias circuit could be the problem but that he ultimately was even more incredulous that the three multimeters that measured >40 mA at full down turn of the bias pots could be wrong. So he advised me to buy two sets of resistors, I forget the values. Then one day, while he was on speaker phone I turned one of the amps on that had one of the sets of resistors installed...still got 40-ish mA at full down turn, so he told me to swap those resistors with the 2nd set and try it again. Perfect! At full turn down the output tubes are at 0 mA and 40 mA is at about 5 turns of the 10 turn Bourns bias pots. There hasn’t been any tube failure since then until a few nights ago when the I guess red-plating failure occurred. Much less spectacular and much less hassle as I didn’t have to unplug everything and lift an turn over the 75 lb. beast in order to replace the sacrificial resistor. Soooo, now that the bias circuits are correct, I’m finding great longevity, which I think can also be considered durability, from the Gold Lion KT77.
I'm convinced that the tube just died of old age.