Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

Does that mean the ripple seen there probably isn't the problem? I've never had to quantify how much bias ripple is a problem so I don't know. If that's the case I only have a couple more ideas to test for.

What else did you have in mind, Chuck?

[Tom Phillips]

Regarding the bias supply ripple, I can give you some real world examples.
The typical stock Fender supply ripple used in a push/pull 6L6 or 6V6 power amp is 0.3% to 0.5%. That is for the traditional single stage filter. With that setup, the extra hum caused by the bias supply ripple can be heard but it is not irritating.
If a two stage filter section is used in the bias supply, reducing the ripple to 0.04%, then the hum caused by the bias supply ripple becomes inaudible.

[Helmholtz]

The typical stock Fender supply ripple used in a push/pull 6L6 or 6V6 power amp is 0.3% to 0.5%.

That would correspond to around 50mV here?
Are your numbers peak-to-peak or RMS?

[Tom Phillips]

That would correspond to around 50mV here? Are your numbers peak-to-peak or RMS?

RMS. I use an HP34401A bench multi-meter to measure the AC and the DC component and calculate the percent ripple from those values.

[Chuck H]

What else did you have in mind, Chuck?

Well you mentioned that the feedback loop isn't wired as per the schematic. There IS a low voltage supply device included there. So I wondered if that might be part of the still questioned hum on the LV supply. But without a proper representation of the circuit it's hard to say. As per the schematic it shouldn't make a difference. As per your "description" it might. No accurate circuit representation has become a problem.

And I'm still troubled that grounding the actual PI input grid (via the "mute" switch" does stop the hum. That tells me that the hum is going into that grid. Yet none is being measured behind it.?. There has to be some dirt in the testing.