[Amps] HF amp output power dropping

[jordan moser]

I'm not sure how to respond to my first thread. Sorry if this is a duplicate.  I have been trying to find the cause of my amplifier's output power falling off. Last night I checked it by transmitting a 400 watt CW signal into dummy load. After 1 minute output dropped by about 50 watts. At higher wattages the loss is more pronounced, at 600 watts it may drop to 400 over a minute time. Now when I apply drive the filament voltage INCREASES. It always used to decrease under load.  That might be a clue, I don't know.
Thanks for all the help on this.
 
wj2w Jordan 
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