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Marshall 8100 head hum query

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  • Roob65
    replied
    Sorry folks one final update...
    Got the amp back into it's cabinet (I had been re-covering it as part of the project - made a few wee mistakes but can live with it), and it's damn near silent! Remaining hum/buzz must have been from ambient. Amazing result!

    One thing also for anyone browsing this thread in the future:
    I decided to treat it to a new tube on the basis it was the original branded Marshall one in it and was presumably 26 years old. I bought the wee guy a nice shiny new Tung Sol 12AX7 and it absolutely hated it! Boost channel completely washed out in terms of volume and tone. High end was miserable. Swapped back to the old one and everything is great. Wish I had more tubes to test, but not being in the 'biz' I don't have the resources. If it does need replaced in the future I'll just stick with the stock (I think?) Shuguang ecc83.

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  • Roob65
    replied
    ok think I'm gonna put this thread to bed. Thanks to everyone who commented and gave advice. I think I've got a pretty decent result.

    The best result I could achieve was by adding one more additional ground 'jumper' between R120 and R23. It's possible I could have gotten further noise reduction by adding more but I have a feeling it would be deminishing returns and no magic bullet! Suppose it's possible the ground traces are fatigued with age which is why we see more posts about hum from these amps.

    Anyway, thanks again to all concerned! Cheers!

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  • Jon Snell
    replied
    Bigger caps will, in my experience, do practically nothing. Ground routing is the issue.
    Your link is mainly for valve amplifiers. Slightly different approach for SS is required.
    It is all down to grounding issues on a poorly designed stage amplifier. Crate and Trace Elliot are the same.
    To make a studio amplifier, requires that different approach and attention to detail with the grounding is required.
    I have repaired a number of similar amplifiers using this circuit and all are very noisy. Budget I suppose.

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  • Roob65
    replied
    Hi Jon, thanks. I'm not knowledgable enough to know how to effectively get rid of the remaining buzz bar throwing bigger caps at it? I was reading this (https://robrobinette.com/Tube_Guitar...m#Hum_and_Buzz), which suggests a 'noisy' rectifier. Do you think it's worth trying to add caps as he suggests?

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  • Jon Snell
    replied
    Originally posted by Roob65 View Post
    Quick update as I can't do anything else today as other stuff on my plate. I removed VR13 and the hum is still there. I attached a temp wire between C40 & C41 and C18 & R25 (with VR13 still removed) and no change. Not tried anything else yet.
    In that case the hum is not coming into the IC. It is either ripple from the +-15volt rails or the main HT rails.

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  • Jon Snell
    replied
    Originally posted by g1 View Post
    It would be very hard to see the hum if it were at the inputs of that IC, which is probably the case. Try working your way back from there.
    As I wrote in post 6.
    Doing that will remove any possibility of hum on the input of the voltage gain IC.

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  • Enzo
    replied
    What is the difference between "correcting" a ground loop and curing one? It would seem to me that if connecting two grounds together stops a hum, then it was what was needed in the first place.


    And 50Hz hum (rather than 100Hz) means a grounding or radiated hum, power supply ripple would be at 100Hz.

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  • Chuck H
    replied
    Originally posted by Roob65 View Post
    OK progress update for anyone following....
    Massive thanks to Glen from Mars Amp Repair!

    I found a rather old thread where Glen had a similar issue with a 8080 (essentially the same amp): https://music-electronics-forum.com/...alvestate+8080
    I reached out to Glen and he gave me some great advice RE signal tracing. Armed with this I managed to obtain similar results to him and reduced the hum/buzz by around 50%. My solution wasn't quite the same whereby the best result I could achieve was jumping the ground trace between the star point (where R119 meets ground) and the ground trace at the line out jack.

    There is still a bit of buzz (def more buzz than hum) which seems to have been being masked by the hum and I'm going to keep at it but this may be the best the amp can achieve. I'll update the thread again if I get anywhere or give up!
    IMHE often when you can decrease hum by jumpering two grounds it's because you've actually grounded a ground loop. Not that it could ever be easy in that amp, but locating any possible ground loops and correcting them would be a more ideal solution. If you have some sense of isolation for the hum you need to look for shared ground traces and circuits that might be grounded redundantly at more than one ground point. Even via components. If, say, a voltage divider is grounded at one point and then before the next amplification stage another voltage divider or mixer stage is grounded at another then any voltage differential between the two ground points can inject itself into the signal chain. So that's the sort of thing you'd be looking for.

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  • Roob65
    replied
    OK progress update for anyone following....
    Massive thanks to Glen from Mars Amp Repair!

    I found a rather old thread where Glen had a similar issue with a 8080 (essentially the same amp): https://music-electronics-forum.com/...alvestate+8080
    I reached out to Glen and he gave me some great advice RE signal tracing. Armed with this I managed to obtain similar results to him and reduced the hum/buzz by around 50%. My solution wasn't quite the same whereby the best result I could achieve was jumping the ground trace between the star point (where R119 meets ground) and the ground trace at the line out jack.

    There is still a bit of buzz (def more buzz than hum) which seems to have been being masked by the hum and I'm going to keep at it but this may be the best the amp can achieve. I'll update the thread again if I get anywhere or give up!

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  • Roob65
    replied
    Measurements from power amp +/-VE rails:
    +VE, 39.14dc, 0.25vac
    -VE, 39.25dc, 0.15vac
    100Hz frequency.

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  • Roob65
    replied
    Thanks for that G1. I've been probing around and get a lot of noise from the outputs of IC9 and IC10 too (goes quiet again after R104/105 etc.) The rest of the op-amps seem quiet. Must admit I'm getting a bit lost. What would be acceptable ripple (if any) in the DC rails?

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  • g1
    replied
    It would be very hard to see the hum if it were at the inputs of that IC, which is probably the case. Try working your way back from there.

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  • Roob65
    replied
    That totally makes sense Doom. I've checked the +/-15v for ripple and looks like a high of about 3mV peak to peak.

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  • doombass
    replied
    Originally posted by Roob65 View Post
    ok I picked up one of those cheap handheld DSO150 scopes as not much happening on the auction front for scopes in the UK at the moment.
    Definitely 50Hz noise on the speaker output. +/-15v rails squeaky clean. Will try and work backwards from the speaker output to see if I can find the source.

    edit: decided to work from vr13 instead. Looks like it's the TL072 (IC8) causing it. From what I can see the signals and power going into it are clean but 50hz signal on pin7.


    If power supply and inputs are clean and you have 50Hz on one output of IC8 it is not likely it would start "producing" that frequency all by itself. Have you checked the power amp rail supply VE+/VE- for 50Hz?

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  • Roob65
    replied
    ok I picked up one of those cheap handheld DSO150 scopes as not much happening on the auction front for scopes in the UK at the moment.
    Definitely 50Hz noise on the speaker output. +/-15v rails squeaky clean. Will try and work backwards from the speaker output to see if I can find the source.

    edit: decided to work from vr13 instead. Looks like it's the TL072 (IC8) causing it. From what I can see the signals and power going into it are clean but 50hz signal on pin7.
    Last edited by Roob65; 05-22-2020, 09:54 AM.

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