Marshall loud humming ...help!!!!!

Discussion in 'Glowing Bottle Tube Amp Forum' started by Johnnypops, Jun 16, 2019.

  1. Johnnypops

    Johnnypops TDPRI Member

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    Hope someone can help me out here. I have DSL40 Marshall and its power tubes would drop out after about 20min of high volume (7-8) playing, so I replaced the tubes, was biasing the tubes and the amp developed a hum as I was finishing but bumped the multimeter probe against the chassis while still on middle biasing probe. May have earthed something
    Thought I’d blown the tube but fear it’s worse
    So anyway I can play at low volume 2 or 3 ok for at least 10 minutes, but turn up above about 4 and play and very LOUD hum kicks in while playing. It will go if master volume turned down but comes back as soon as turned back up. Putting on standby doesn’t fix this either but tapping either power tube will make it go away for a minute or two
    Thought reverb but turning reverb down or off does nothing
    Put old power tubes back in and they now will hum the same
    Interestingly if in triode mode hum is constant once it occurs, but in pentode it will hum for a second or two then go for a minute or 30 seconds then come back for a second or two and go again. Only seems to happen if playing and will happen with any of my guitars so not guitar or lead related
    Happens with or without foot switch and also on all clean crunch and overdrive channel options

    Any suggestions?

    Would a sound clip help?

    Thx John
     
  2. kbold

    kbold Tele-Meister Silver Supporter

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    Now you have a MMMMMarshall
    Perhaps try replacing the phase inverter - thats possibly the easiest diagnostic step before getting out the est equipment.
     
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  3. VintageSG

    VintageSG Friend of Leo's

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    Dry joints?
    Moving the valve causes the noise to go away for a minute or two, then it comes back. That possibly implies a mechanical issue. That it differs between triode and pentode mode may give a further clue as to the whereabouts of any iffy joints.

    To isolate whereabouts, try running your guitar straight into FX return, this'll bypass the pre-amp stage, leaving just the phase inverter and power stage ( usually ).

    When it's humming, insert a cable into FX send. This'll break the signal path, leaving only the phase inverter and power stage. Any hum now has to be in that region.

    Or just reflow any and all solder joints. Note that power supply capacitors can hold a charge that can spoil your day. Unplug the amp and discharge the caps before doing anything.
     
  4. tubelectron

    tubelectron Tele-Meister

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    I'll complement what has been said :

    1 - open your amp and put in on a table, with access to guts
    2 - connect a speaker
    3 - power it and check the voltages to power amp tubes
    4 - plug a guitar and play for a while until the amps becomes faulty
    5 - just put the guitar and check as is the voltages to power tubes now - that is to say under fault

    Is there a noticeable difference : I mean more than +/- 10% ?

    Chances are that if there's a trouble in the power amp or power supply destined to it, you may trap it...

    That said, if you're not confident to do it, rely on a knoledgeable tech, of course... ;)

    -tbln
     
  5. kbold

    kbold Tele-Meister Silver Supporter

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    Or worse!
     
  6. Dacious

    Dacious Poster Extraordinaire

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    I'd suggest you've got a bad shield or bias resistor. Your tubes are probably redplating. Needs to go to a tech before you melt something.
     
  7. Johnnypops

    Johnnypops TDPRI Member

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    Hi tubelectron
    Thanks
    This is something that makes sense and I’m comfortable doing this without frying myself
    I’ll break it open and see if there is a voltage difference when normal and when humming

    Will also try isolating Fx loop to see if that has an effect as I’ve not tried that either

    Likely got to go to a tech but had a 100w Valveking turning on but no sound, tried all the Fx isolation etc and in the end cracked it open to find an obvious fried fuse, 5 min fix and good as gold

    Assume the hum is some sort of oscillating as a result of change to voltage to the tubes but strange that it comes goes and is resolved by a tap on a tube or thump on top of the cabinet

    Will report back once I’ve checked those couple of suggestions out, but thinking if a cap or resistor was blown or dry joint it wouldn’t function at all and wouldn’t just appear after changing tubes unless it’s on the tube holder itself
     
  8. tubelectron

    tubelectron Tele-Meister

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    Yes : clever suggestion to check.:cool:

    Ah - that could possibly mean :
    1 - microphonic power tubes (or other tubes).
    2 - poor contact in a power tube socket in need of retensioning its pins.
    3 - intermittent contact somewhere.

    for 1 - : try as best to identify the faulty tube by taping (gently, Ok ?) on them with a drumstick (it's what I use) or a pen, or a wooden dish. If it is a microphonic tube or ringing tube, you should locate quite easily. Include the preamp tubes in your search.

    for 2 - with a cloth (to avoid burning your fingers), while your amp on, move the power tubes on their sockets and see if something weird occurs like a loud crack / pop / hum heard in the speaker. Again, if its a poor contact there, you should locate it also - or at least the area where to search.

    Note : sometimes poor contacts occurs in the tubes themselves : poorly welded pin socket or electrode, or short occuring time to time, or when hot... If you suspect the power tubes, try a substitution if you can - even with used tubes - just to see.

    for 3 - On those amps where there is multiple PCB like computers with zillions of traces, components and solders... Good luck, if it is not obvious like wire, connector, etc...o_O

    -tbln
     
  9. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Since you were recently biasing a new set of power tubes, check for redplating. Whenever a power tube begins to redplate, a loud hum begins to develop because of the increased current being drawn by the redplating tube. Watch the power tubes as you play the amp. The grey plate structure inside of the glass envelope should not show any signs of heating up or "redplating". If you do begin to see a red or orange glow on the plate structure when the hum starts, shut the amp down. You either biased them up to hot or you may have a bad tube in the new set that you installed.
     
  10. Johnnypops

    Johnnypops TDPRI Member

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    Hi Dan, these tubes and the last set are red playing then. My fenders have a blue glow but these have been red since day 1
    I did get a new multimeter today as my other one was auto adjusting and would always default to higher setting so all I’d get as a reading was .00038 etc. I should be able to bump up that accuracy with my new one to get to decimal places but they were set at 38 which should have been fine according to the biasing charts

    I will back the biasing right off and see what happens. A tech I spoke to in an amp hire place told me Marshall’s are touchy for biasing in Australia because we run closer to 250 volts than 240 while UK is often more like 230 - 235

    I will check and drop the biasing and do a bit if a check on the tube sockets also
     
  11. dan40

    dan40 Tele-Afflicted Silver Supporter

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    JUst be sure that it is the plate structure that you are seeing a hot spot (redplating) on. Many tube's filaments will glow at different intensities but this glow will come from the center of the tube structure. Redplating would be an actual hot spot on the grey plate element inside of the tube. The fact that it starts occurring at higher volume levels does point to the tube's being over biased slightly so bring the ma's down a bit should stop it. If not, there may be a problem in the bias circuit that is causing the issue.
     
  12. Johnnypops

    Johnnypops TDPRI Member

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    Hi Dan

    Well I hope I have it sorted. Watched too many YouTubers telling me to bias around 38. Turns out my plate voltage is 475 not 440-450 like in the US so have dropped my bias down accordingly and can’t seem to get it to hum... only time will tell and I’ve not had time to really hammer it but it did get it up to 9 volume with about 1/4 preamp and it should have had it humming but didn’t

    May return in a bit once it gets a decent playing but looks like it might have just been too highly biased

    Rule 1. Check the plate voltage on the sockets
     
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  13. Greg_L

    Greg_L Tele-Holic

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    Glad you're on the right track.

    I don't know if your amp has this feature, but some modern Marshalls have a "hum pot" that look a lot like and is very near the actual bias pot. Make sure you're not inadvertently jacking around with the hum pot.

    Also, learn how to calculate bias instead of using charts or internet numbers.
     
  14. Johnnypops

    Johnnypops TDPRI Member

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    Was using Jim Jones bias charts, not sure how to actually calculate but something I’m interested in learning as knowledge is king!!
    Definitely using the right bias pots but getting a manual multimeter instead of auto range made life a lot more accurate also.

    Hoping problem solved and will give update one way or the other once I get to give my baby a decent session
    I had a Peavey Triumph I bought new in the 80s and never changed a valve in 30 years, had a Marshall a few months and it’s chewed one out...sold that Peavey a few months ago after falling in love with the Marshall.... might have set some bad juju in motion
     
  15. Greg_L

    Greg_L Tele-Holic

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    Yes, learn, but at least you're measuring plate voltage. If you have accurate plate voltage measurements then the charts can get you close enough.

    I've read tales online where people say "just set your bias to xxxx" without any consideration of plate voltages.
     
  16. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    Could you clarify this? Were the tubes falling out of the sockets, or was the sound "dropping out"? I can't see how they could physically drop out because of the tube retainers - and if the sound was dropping out, how did you know it was a power tube problem? The whole tube replacement issue is a bit unclear, and it would be good to get a hale on WHY you replaced them first.


    You can't properly bias the tubes without knowing the plate voltage.

    ^ This - but there's more. See below.


    Use Google - the information is easily found!

    But - do not bias your amp strictly on the basis of math. An equation has NO TONE. Not good and not bad. Or terrible. ALL the math does is tell you what the general "safe" range is for your tubes. You don't just calculate 70% and say "OK, I'm done.". 70% dissipation is safe, but may sound like crap with THOSE tubes in THAT amp.

    You HAVE to use your ears, and at the playing volume you use - not low workshop or bedroom volume. - 70% is a good starting point, but adjust it a few ma up and down until you find what sounds best with THOSE tubes. Any setting that sounds good and calculates from 50-80% dissipation o so (with no replating at higher numbers) should be fine. Up to 80% or so won't hurt anything as long as the tubes are not red plating - if they are I'd use different tubes, as any decent ones should be able to handle that.

    Start with the plate voltage , then calculate 70% dissipation and start at that ma setting without playing. then play, listen, adjust up a bit, down a bit, find the sweet spot - and check the dissipation. If it's safe and the tubes are not redplating - you're done. If it's too high or the plates are red, bring it down just enough to stop the redplating and re-check.

    If you can't get the tone, headroom or breakup to work for you at a safe bias setting you have the wrong power or preamp tubes - or the wrong amp. And the latter is far more common than you may think., especially if you can't get the tone you want at a reasonable volume level. That's an entirely different problem - the "too much amp" syndrome; and you may very well need an amp with lower output for the places you play (most players I know use at least 3 different amps with different output for gigging - and select one based on the venue).
     
  17. Johnnypops

    Johnnypops TDPRI Member

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    Hi silverface

    By dropping out i would be playing with the band and volume would go Frm cranking to about 1/2 watt and fuzzface even on clean channel and once volume drops it’s on all channels but came good by going to standby and then flipping back on after a few minutes but would then go again after a few minutes of playing
    I tried swapping out 2 preamp tubes and moving 2 but didn’t fix
    I then did some googling and it sounded pretty consistent with power tubes so replaced them and made the mistake of just going with the accepted advice to bias around 38.5 and that’s when things got a lot worse, fearing I’d completely stuffed my amp.
    As mentioned last night checked plate voltage which is at 474 so considering the charts have backed it off considerably and so far so good but time will tell as it needs a good work out with the band to tell if it’s fixed

    If the ears have it I’d have it running at 80%_90% as it’s awesome at that and super fat and responsive but just falls apart by cooking the tubes. I’ve never had this happen before as it’s not been a problem with my Fenders or my Peaveys and my Vox is too new to know yet

    Thx John
     
  18. Johnnypops

    Johnnypops TDPRI Member

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    Oh BtW old tubes were Marshall and new ones are Ruby’s
    Currently got the Marshalls back in but biased a lot lower than they were to see what happens
    I’m at 31.5 at the moment just to see whether they still die after 1/2 hour or so or hum as soon as I turn up the volume and hit a few chords like they did the other night when I put them back in without rebiasing
     
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