KAT 6550 problems

[gerrygwm]

Hello. I am having trouble with a KAt 6550 power amp. I built it years ago and used it for many years with no problems.
Then it starts to overheat V8 only. That is the 6550 nearest the back of the amp.
The problem is only on the left hand channel. The amp was retired for a few years and now I am trying to resurrect it.
Tried swapping valves but always the same one getting red hot.
I can run the amp for about 3 minutes then have to turn it off because it looks like the valve is about to blow.
The amp runs fine with V8 removed. Checking voltages I can’t find anything wrong.
Anyone got any ideas?
Or better still is there anybody out there who nows more about it than me (not difficult!) that would be willing to have a go at fixing it?
Thanks Gerry