Marshall Super Lead 100W Build NFB Issue

MasterEvan07

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Here's a video of the NFB being hummy. If I swapped the OT secondaries and do the same test I get the crazy squeal, so I know where they are now is the right placement but I don't know why the NFB is doing this...

The amp is 98% of the way to roaring, full Marshall Plexi goodness but something's hinky with the NFB. I know once I fix it the whole thing will tame up (in the way you want it to "tame up") and the noise floor will drop.

What's going on?

 

dan40

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You mention a switch. Are you referring to the impedance switch or did you add an NFB switch mod?
 

Phrygian77

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Hum could be a sign of parasitic oscillation. I know the lead dress and grounding around the presence control can be an issue.
 
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Tom Kamphuys

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Section 15.8 of http://valvewizard.co.uk/Grounding.pdf shows how to handle grounding and NFB.

Sounds more like hum/buzz to me (50/60Hz and harmonics) than parasitic oscillation which can be at any frequency in or out of the audio band where the gain and phase are such that oscillation occurs.

Can you provide detailed schematics/layout/pictures?

How does it react to turning of knobs?
 

MasterEvan07

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Screen Shot 2022-06-17 at 4.34.17 PM.png
Section 15.8 of http://valvewizard.co.uk/Grounding.pdf shows how to handle grounding and NFB.

Sounds more like hum/buzz to me (50/60Hz and harmonics) than parasitic oscillation which can be at any frequency in or out of the audio band where the gain and phase are such that oscillation occurs.

Can you provide detailed schematics/layout/pictures?

How does it react to turning of knobs?

 

MasterEvan07

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I assume you also tried it without your fingers on the wire, right?
I did. The NFB seems to have stabilized with a little more lead dress, apparently. It's still got a significantly higher noise floor than I find acceptable, so that still needs taming.
 

Tom Kamphuys

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Grounding seems to be a bit of magic. Some builds get away with breaking almost all rules, others don't. I think I see quite some chassis grounding points in your pictures. Ideally you should have only one at the input. It's described in the document I attached earlier and several posts on this forum showed improvement by only implementing a part. However, I can't find them right now.

Then the noise. I think most resistors are carbon composite (based on colour only...). You could try metal film, especially in (front of) the input stage where the signal is low. You could also try shielded wire where the signal is low.

You could switch some tubes. The input stage is most sensitive to noise.

And chopsticking is very cheap. Maybe you find something that has some effect.
 

MasterEvan07

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So I did sort exactly where the initial problem was coming from that prompted this thread.
What happened was the solder tab wasn't completely filled in which was somehow causing an incomplete connection from the OT to the output jacks to finish the ground - even though the OT ground lead appeared to make a proper connection, and the ground buss itself metered to ground, it wasn't actually completing the circuit. Perfectly explains why the NFB would trip when connected and was fine when it was disconnected. It seemed to "fix" the issue but still left grounding problems and lead dress concerns making more noise even after the perceived fix.
All's well now!
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