I am trying all the suggestions with no success so far. I am pretty close to giving up completely on this very nice looking Eimac 4-400A but I have one more thing I have to do before completely giving up.
Further reading from Eimac Application Bulletin #18 [ Ссылка ]
"The key to extending the life of a thoriated tungsten filament
emitter is to control operating temperature. Emitter temperature is a function of the total RMS power applied to the filament. Thus, filament voltage control is temperature control,
because temperature varies directly with voltage. Figure 8
shows that useful tube life can vary significantly with only a 5%
change in filament voltage.
NOTE: If the filament voltage cannot be regulated to within
± 3%, the filament should always be operated at the rated
nominal voltage specified on the data sheet.
It should be noted that there is a danger to operating the emitter too much on the “cold” temperature side. It may become
“poisoned.” A cold filament acts as a getter; that is, it attracts
contaminants. When a contaminant becomes attached to the
surface of the emitter, the affected area of the emitter is rendered inactive, causing loss of emission. Should this happen,
recovery is possible by operating the filament at full voltage
for a period of time. Closely monitored operation of the filament at slightly below the rated nominal voltage, however,
can extend tube life, if done properly.
Note that these filament management techniques should not be
applied to oxide cathode tubes, such as the 3CX1500A7/8877.
Running oxide cathodes too cold will result in internal arcs; and
once that happens, an oxide cathode tube is not recoverable."
From Application Bulletin # 21
[ Ссылка ]
"Large power tubes are subjected to very rigorous processing
during exhaust pumping at the time of manufacture. Active
elements are processed at temperatures several hundred degrees Celsius higher than that to be expected in actual use.
This is done to drive off surface and subsurface gas from the
metals to minimize the possibility of these being released during service life of the tube."

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