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Just reporting back for completeness.
It was indeed the TL072 input op amp that drives the noise gate. Replaced chip and all is well.
A little more info.
Something strange in that input jack PCB.
It has two leads a grey one(carrying signal to the main board and the valve preamp)....
A mates amp died on him and Im trying to help out. I have electronics background but not specifically amps.
After much messing around the...