[Amps] AL-811H problems

[John E.]

I was on 80m CW the other night with my new to me (used) AL-811H, when
the
amp stopped working and the high SWR light came on the display on the
FT920.

When I shut off the AL-811H the SWR went down to normal using 90w.

Today started to by pass tuner, external SWR meter etc. I ended up with
the
dummy load connected into the output of the amp and started testing
again.

Found the amp would stop transmitting and the SWR would go high, Changed
out
the jumper between the radio and amp.  no difference.

Found after 45 sec of key down in to the dummy load and multiple keying
on and
off sending CW the amp would stop working.---Must be in the amp--
The amp has not blown any fuses and the HV runs at 1700v.

I have no idea the condition of the tubes(811's),  the amp when working
will  put out 400w CW.

Any thoughts?

Thanks de John KØCQW