[Keith]

Do you have a tube shield on the 77 first audio tube? It is right next to one of the output tubes and the grid cap lead is very sensitive to stray signal pickup. The grid cap lead should be inside the tube shield. Had the same type of squealing problem with a model 610 where the 75 audio stage is right next to the 42 output tube. Putting a shield on the tube with the grid lead inside cured the problem.

That was the issue. i put all the tube shields on and that fixed the tone control issue along with the "birdies" while tuning. many thanks!