[David]

Most of the failures I have seen have not been from the heaters open. The 12DT8 has a internal shield grounded to pin 9 from some reason the shield fails and will short the tube out. The 14GT8 and 19T8 just seem to fail without any warning. The 12DT8 can be replaced with a 17EW8 without  changes to the tube socket. A 12AT7 can be used as a sub for the 12DT8 IF YOU MAKE A MOD TO PIN 9 of the tube socket or remove pin 9 from the tube. 12AT7 does not have a internal shield to ground on pin 9 but a tap to the heater so you have a dead short to ground your nice new tube will go poof!!! I have done the mod to the tube socket on 2 sets been happy with the results