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Help! Hum problem with amp

I have got a problem that I haven't come across before and it has me stumped. I have a H-K Citation 5 amplifier that I have lightly modded. The amp circuit has not been changed. All resistors have been replaced with metal film types. All coupling caps have been replaced with modern ones. The power supply has been replaced with a circuit board from Sheldon Stokes. I have added a regulator for the screen grids of the output tubes. The original amp had a 2 wire power cord. I replaced it with a 3 wire cord, grounded the chassis, and added a diode ground break between chassis ground and circuit/PS ground. It plays music just fine, but has a hum issue that I don't understand. With no input connection, but connected to speakers, the amp is dead quiet. With either grounding plugs on the inputs, or just open inputs, there is no noise. With ONE interconnect connected to a source, either left or right channel, there is no noise. When I connect the second interconnect, either left or right, I get hum. Low in level, but annoying. Both inputs are grounded to the same place. I have tried: 47 pf caps across the RCA jacks hot to ground, rerouting the ground wires, using shielded cable between the RCA jacks and the input tubes, connecting the input jack grounds at the jacks with a clip lead, bypassing the ground break between chassis and power supply ground with a clip lead, and using a cheater plug to lift the chassis ground. None of these make any difference in the quality or quantity of the hum. I have run out of ideas. Anybody got a suggestion as to what I should try next?

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Topic - Help! Hum problem with amp - floydandrews 16:59:29 06/16/21 (39)
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