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Some things to check

Gian,

First if all your HUM is 100Hz (assume you are 50Hz) then it is all from the power supply (i.e. doubled from the rectifier).

When the 6922 is shorted on the input what is the AC 100Hz signal at the Grid to the 6A3? I would suspect that it is like 90mv and the 6A3 gain is making it 270mv.

In that case most of your hum is coming from the 6922. I would up the inductor and capacitence there. Also check your grounding. Poor star grounding can have a big inpact in hum.

Thanks
Gordon
J. Gordon Rankin

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