Diagnosing noisy Marshall amp that got worse and worse. Fixed, or is it????

[greengriff]

Thanks for the explanation. I'm glad we got to the bottom of the issue!

[Chuck H]

Along the same lines as what drewl and Helmholts are saying (sort of)...

There is a small differential at different ground points throughout the amp. This is the reason for ground loops and why you don't want circuits blocks grounded in two different places or the same independent circuit grounded redundantly in two different places. But as it happens, guitar amps do that "S" all the time. And it's known to be a source of background noise in many amps. Including Marshalls.

When there's a buss wire soldered to the pot cases redundant grounds are unavoidable. In many older amps the pot cases mechanical connection to the chassis IS the ground. And there are typically five to eight of them (did I mention redundant?). Some boutiquey builds ground the buss bar independently as well making for another redundant ground point. Then there's the issue of metal oxides deteriorating the pot case to chassis grounds. This is especially bad with aluminum chassis. This is a common problem causing bad grounds and noise in vintage amps. This can happen to one or more of the pot case/chassis contacts and the resulting noise and issues vary unpredictably. So...

When I build I don't use a buss wire connecting the pot cases. It's completely unnecessary and a bad practice IMHO. I think a good and permanent solution for your problem would be to just remove the buss wire from the back of the presence pot. Or better yet, remove it from the back of all the pots and bolt it to the chassis with a screw and tooth washer like Pete suggested. The connection should be made at the input end of the buss wire. Slather the connection with anti ox grease.

Regarding Helmholtz oscillation theory I have some thoughts. One is that the purple FB lead routing in Marshall amps can be sensitive. So if you think you might have changed that inadvertently in the course of events it should be routed away from the preamp and approach it's board or presence pot connection (it's done both ways depending on amplifier vintage) from the opposite direction as the preamp leads. This is how most Marshall's I've seen were routed from the factory and it seems to work. And...

If you're sure you didn't alter the purple lead routing and you can solve for unwanted noise without changing lead dress then "I" wouldn't. It may be your amps particular phase interactions, the same thing that may be causing border line stability, that are partly responsible for the tone you like. You wouldn't want to undo any mojo. We don't generally respect that word around here, but the invisible circuits in guitar amps are real and layout/lead dress and how different radiant fields in the amp are interacting can affect the tone as much as the visible (and much more easily quantified) components.

JM2C

[greengriff]

Hi Chuck. I haven't changed any of the routing, and now that the amp is quiet and performing beautifully again I don't intend to mess any further with it, just to play it. The slightly odd 'idling' noise with the master up high isn't that intrusive, and you have to have the master above 5 to even, hear it, at which point the amp is too loud for me to use anyway. It's more that I know it wasn't there before, so the awareness of the change is bugging me!

[Helmholtz]

As there was an intermittent ground contact, the reason for the popping noise when hitting the standby switch might have been purely mechanical.

I wouldn't want to change the ground wiring in a vintage Marshall.

[Chuck H]

I wouldn't want to change the ground wiring in a vintage Marshall.

I would But I understand the sentiment. It's the same reasons I gave above for not trying to mess with lead dress or otherwise "correct" a marginal stability issue.

[Chuck H]

Hi Chuck. I haven't changed any of the routing, and now that the amp is quiet and performing beautifully again I don't intend to mess any further with it, just to play it. The slightly odd 'idling' noise with the master up high isn't that intrusive, and you have to have the master above 5 to even, hear it, at which point the amp is too loud for me to use anyway. It's more that I know it wasn't there before, so the awareness of the change is bugging me!

Ok. Understood and it makes sense. If it gives you any solace I'd bet three beer PNW that the different and/or additional noise is the result of having somehow restored one of the redundant ground I mentioned above. I believe that if you disconnect the ground buss wire from the presence pot case that your background noise would go back to where it was before since that was the fault that was recognized. That buss wire is still grounded elsewhere redundantly. And the closer you get to the power end of the chassis the "dirtier" the grounds get for small signal leads. Since the presence pot is the closest to the power supply adding it's case to the redundant grounds made the whole buss bar dirtier. Since it wasn't connected well before and you were happy with your amps lower noise and tone compared to others I think you could take a tip from that and restore THAT aspect to it's previously broken state for the benefits it offers. Hell, you were willing to leave the whole presence circuit disabled before and that's certainly not stock. Just sayin'.

[greengriff]

Ok. Understood and it makes sense. If it gives you any solace I'd bet three beer PNW that the different and/or additional noise is the result of having somehow restored one of the redundant ground I mentioned above. I believe that if you disconnect the ground buss wire from the presence pot case that your background noise would go back to where it was before since that was the fault that was recognized. That buss wire is still grounded elsewhere redundantly. And the closer you get to the power end of the chassis the "dirtier" the grounds get for small signal leads. Since the presence pot is the closest to the power supply adding it's case to the redundant grounds made the whole buss bar dirtier. Since it wasn't connected well before and you were happy with your amps lower noise and tone compared to others I think you could take a tip from that and restore THAT aspect to it's previously broken state for the benefits it offers. Hell, you were willing to leave the whole presence circuit disabled before and that's certainly not stock. Just sayin'.

Yes, that is logical and makes perfect sense. It would be a great experiment for someone with time on their hands to remove the redundant grounds one at a time and measure how the noise floor changes.

[Chuck H]

I don't know of anyone that's done it one connection at a time. That does sound like a good experiment. I know that some who understood how to reduce noise have done the buss bar not connected to the pot cases (along with some other grounding changes and reported much quieter operation. OTOH some have reported a change in the signature tone as a result. And this is the cautionary angle Helmholtz was coming from (I think).

[greengriff]

Yes his post was quite cryptic - I assumed he was talking about negatively affecting the resale value. Maybe he will enlighten us.

[Helmholtz]

As there was an intermittent ground contact, the reason for the popping noise when hitting the standby switch might have been purely mechanical.

I wouldn't want to change the ground wiring in a vintage Marshall.

For clarification:

First sentence means that I no longer suspect a stability issue and think that the pop was caused by shaking the bad contact when hitting the standby switch.

Second sentence means that to me the empirical ground scheme of a vintage Marshall works well enough - provided ALL ground contacts are very low resistance.


Are you sure that the newly encountered noise is not coming from an outside source?
Do you still hear it with the amp mounted in its cabinet and guitar volume at zero?

[greengriff]

The noise seems not to be there today, so maybe the amp was picking up the noise from another source? I never normally play with the master beyond 2, as that's already a loud gig volume. I noticed when I turned the master up above 6 with the guitar plugged in (guitar volume at zero, and I'm standing behind the amp) there's an incredibly high pitched squeal building. I changed guitar and the effect was still there. I changed cable and the noise then didn't appear until after 8 on the master. Any idea what it might be. If I'd been standing in front of the amp I'd have said feedback, but not when standing behind the amp, surely?