1. Try substituting the tubes with new ones, to rule out heater to cathode leakage.
2. Verify that the new capacitors are grounded to the same point as the original can, this can be critical.
3. Verify the "across the line capacitor" is good and connected properly
4. Check if the volume control is bad, jumper the middle and negative terminals of the volume control to see if hum goes away.
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