I have searched and have not found an answer yet.
The AL-80B is only about 3 years old. I drive it with Ft-897 going through Ameritron ARB-704. A few months ago it occasionally would not kick on and my swr is high, but on standby swr is ok. Also now on the meter, And while just idling the plate IP will jump up to about 100 and the Grid will go negative. Still sometimes when the meters settle down and it will transmit at normal power but sometimes the next key down no power. I have never over driven it. I run between 50-60 watts into the amp.