Maybe a little more information is needed? How much grid bias is there when you excite the tubes? The tube manual says it should be -200 volts. Perhaps it biased way beyond cut off? The specs also say it take 17 watts per tube for drive. Given circuit losses, it may take at least 50 watts from your exciter. With no drive, what is your grid bias voltage and plate current?
Looking at the parts list, it looks as if the bias resistor R-116 is 2000 ohm. Is it 2000 ohms or did it deteriorate and go to a higher value? Is it hard wired or a clip in type? If it is a clip in type, pull it and make sure there is no corrosion on it.
I found the book
http://collinsradio.org/archives/manuals/300gManual.pdf