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There are a number of possibilities, but it would be easier to isolate the source by trouble shooting.
Try shorting grids of each stage from the first sound IF through to the last AF to ground 1 at a time. If killing signal at the 1st IF grid leaves the squeal the problem is in the 2nd IF or later, but if the short kills the squeal then it is in the signal feeding the 1st IF grid..and that same logic follows down the signal chain.If you can pin it down to a stage it should be easier to fix.
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