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.

[bobloblaws]

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.

Yes, you are referring to post #14 in this thread. I just double checked. There is no connection from V4:2 to the FET and the drain is connected to the .005/33K junction. Myself I'm not able to determine what the FET does or where the gate is supposed to connect or what significance the wiring anomaly has (forgive me if this has already been addressed and I've forgotten).

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.

Sorry, I'm not sure what you mean "behind" it?

[Chuck H]

Sorry, I'm not sure what you mean "behind" it?

Upstream in the signal path.

Since there's basically nothing at the hummy grid node and grounding that grid stops the hum it's implicated that the hum must be either on that triodes cathode circuit or is being fed to that grid from a previous circuit. Of course you already know this and that's why this one is such a corker.

EDIT: I think the FET in the NFB circuit is changing the feedback level and frequency analagous to channel switching. Probably less feedback and high end cut for the clean tones but that's just a guess.

[bobloblaws]

I think the FET in the NFB circuit is changing the feedback level and frequency analagous to channel switching. Probably less feedback and high end cut for the clean tones but that's just a guess.

That rings a bell, x-pro did mention something to that effect in #17.

J175FB changes the frequency response of the output stage when RHY mode is enabled.

[bobloblaws]

There is something that confuses me. When talking about the trace I posted in post # 124 we've been describing the waveform as "ripple". My understanding is that ripple is the AC component left in the rectified DC that doesn't get filtered out by the filtering caps and typically shows up as 120Hz hum rather than the 60 cycle waveform in the photo.

I grabbed this trace from a different amp, a Music Man RD50 (schematic attached), from the +30V power tube bias voltage, more or less equivalent to point "F" on the F-30. It is 120Hz and is roughly the 50mV level that Helmholtz anticipates a typical value for ripple would be in post # 139.

Since I seem to have a 60 cycle hum issue in various places, is it more likely I'm dealing with a grounding problem rather than ripple/filtering?

Musicman-RD50B-Schematic.pdf

[Helmholtz]

60Hz ripple is produced by power supplies using half-wave rectification like the bias supply and low voltage supplies.
Other sources of 60Hz hum include a bad ground.

[bobloblaws]

60Hz ripple is produced by power supplies using half-wave rectification like the bias supply and low voltage supplies.
Other sources of 60Hz hum include a bad ground.

OK, that makes sense, the bias supply on the F-30 is not full wave rectified. Thanks for that explanation.

[Tom Phillips]

.... Since I seem to have a 60 cycle hum issue in various places, is it more likely I'm dealing with a grounding problem rather than ripple/filtering?

That’s a “Maybe.”
It’s good that you have verified that the errant hum is 60Hz rather than 120Hz.

60Hz hum can be caused by:

• Radiation from wiring carrying 60HZ AC that is sufficient to be picked up by nearby sensitive circuitry. (Power line wiring between the AC cord input and the PT primary & the heater wiring)
• Shielded wiring that has lost the shield ground.
• Issues with the heater circuit balance.
• A bad tube which is picking up the 60 Hz from its own heater.
• Half wave rectifier circuitry that has excessive ripple. The bias supply in your amp uses a half wave rectifier.

I scanned back through the whole thread. One thing we do not know is the absolute value of your objectionable hum level and how that relates to a normal F-30.

Looking at the photo in post #1, I can’t make sense of the diode installation. Are you sure that your high voltage supply is indeed full wave rectified?

Edit: I agree with posts that went up while I was composing this.

[bobloblaws]

Looking at the photo in post #1, I can’t make sense of the diode installation. Are you sure that your high voltage supply is indeed full wave rectified?

Yes, that photo shows the cluster of 4 diodes in the heater supply. The 4 diodes for the high voltage supply are partially hidden underneath the 180uF filter cap. (the board is a different rev than what is shown in post #34).

[Tom Phillips]

Yes, that photo shows the cluster of 4 diodes in the heater supply. The 4 diodes for the high voltage supply are partially hidden underneath the 180uF filter cap. (the board is a different rev than what is shown in post #34).

OK. Have you verified that the ripple on the high Voltage B+ supply is 120 Hz. Sorry to ask, but so much has already been checked that I just think it's good to verify.

[bobloblaws]

OK. Have you verified that the ripple on the high Voltage B+ supply is 120 Hz. Sorry to ask, but so much has already been checked that I just think it's good to verify.

Hey Tom, here's a couple of traces for you. The first one is B+ at the reservoir cap. The AC ripple is pretty much what I measure with my multi-meter, 600-700mV. For B+ supply of roughly 400V it is good S/N ratio I think.

The second pic is point F again. The amplitude of the wave is lower than in the previous shot from post # #124. The file name I gave the old one indicates I had V3 pulled when I took that shot. Maybe the power tubes were pulled as well? Anyway, this is what I see currently with all the tubes installed.

Bottom line is there is still significant hum at the output and is detectable at V4:7. Grounding pin 7 kills all signal and noise at the output, including the hum.