the rating you want is the plate dissipation rating. this figure, expressed in watts, is generally 1/2 of the tube's maximum safe power output level.
the 8950 can dissipate 35 watts and can therefore (generally) produce up to 70 watts. Of the 70 watts, some power is lost due to the output circuit, the coils and capacitors. figure 10% loss from that 70 watts. so your costly 35-watt 8950 can, as a general rule, safely do 63 watts. It is recommended that this be taken as the PEP level. In AM service, the carrier level in watts should be limited to half the plate dissipation rating. (i.e. the 8950 should be run at no more than 17.5 watts carrier level on AM)
safe operating output power for 3X8950 would be 189WPEP. with a typical efficiency of 55 - 60% 300W is the INPUT POWER, not the OUTPUT POWER!
the tubes will not last for their rated MTBF when run in excess of 200WPEP.
while increased plate voltage will allow slightly higher efficiency and reduced harmonic content the key is to maintain minimum input drive to take advantage of the aforementioned. before tank component losses are figured into the equation the maximum carrier level per tube is approximately 17.5W.
maximum MTBF is achieved in AM operation by holding total carrier power to 50W and limiting maximum peak envelope power to just under 200WPEP.
continue operating them as mentioned in the previous posts and you had better plan on having another set of tubes handy.
