Suggestion: A low 60 Hz hum can be the result of:
1)The power transformer's filament winding center tap (TR1 green/yellow wire) has come loose.

-Check to make sure this wire is connected properly and not just hanging on by a cold solder joint.

2)You may have a faulty tube in V1. Sometimes a bad preamp tube will amplify the filament's AC frequency.

-Remove the tube in V1 (ECC83) to see if the hum goes away.

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