Hi Tom and all: 
Well I didn't think that Tom W8JI would go that deep but oh well. 
Personally I'd do a process of elimination with the mics, lower the carrier, even though it won't do much for SSB but it can't hurt. 
As a last resort though don't rule out the possibility of that faulty grid meter diode, if the GLP unit isn't in the amp. 
Its funny, Tom posted about that GLP unit and not recommending it for a 3-500Z tube as this tube is rugged enough to take a beating. 
He actually doesn't think its necessary for the 811as. LOL 
But anyway tom, when you get the time do what us list detectives have suggested and you should trace it down. 
If not, than I guess a call to ameritron is in order. 
73: 
Mike VO1AX