There is also the unfortunate possibility that it's a parasitic oscillation due to layout. This would be virtually unfixable without seriously altering the tone of the amp.
To change the bias on the PI just change R18 (1.5k) up or down a little. Maybe try 1k and 2.2k there and see if there is any change.
To change the bias on the PI just change R18 (1.5k) up or down a little. Maybe try 1k and 2.2k there and see if there is any change.
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