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Ok, tried two different 6V6’s, no change.

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On the plates of the 6V6’s there is 6.8Vrms of 120hz ripple. If I remove the 6v6’s it drops to 4.7Vrms and the hum is no longer audible (as expected).

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With the 6V6’s back in (Russian tubes BTW), switching the bass “IN”, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple.

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These measurements are made with my o’scope.

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I did some poking through my “inventory” of parts and do not have a suitable choke to swap in for CH2 – still thinking there may be some leakage going on there……?

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Fun stuff huh?

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Tom

W3TA

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Hi Jacques, I just received 1.5 cents from you.

Your proposal may well be, but how do you explain the difference ?the switch setting makes?

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My two cents:

If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise.

Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause.

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don??? va3drl