[Amps] TL-922 500W only

[Alex Malyava]

Hi guys,
I think I have a problem with my TL-922 amp.
It gives only 500W out with approximately 50W in.
Grid current, or whatever that meter measures (like voltage between
cathode and ground) shows about 200mA - max "current" permitted by
Kenwood.
Output power slightly goes up to 550W when I increase the drive all
the way from 50W up to a 100W and grid current goes up to 300-350mA.

What could be a problem?
The only thing I suspect - is that the amp gives half of the power, so
it might be "one tube is dead" case. I will try those tubes one by one
tonight to check how much output each individual tube can produce.

A few more questions:
Why their manual says that I shall not operate CW in SSB mode? What
stand behind that prohibition?