Boogie Started Humming...help?

Discussion in 'Amp Tech Center' started by MrGibbly, Feb 3, 2018.

  1. MrGibbly

    MrGibbly Tele-Afflicted

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    My beloved Mesa Boogie Mark V:25 started humming quite perceptably recently and I'm looking for ideas on what to try before I call Mesa, box it up, and ship it off to them (under warranty, nearest service center is a 3.5+ hour drive away). Totally bummed because it has been my #1 since I got it and it had a very low noise floor. This hum is loud enough to be heard when playing. Symptoms/observations:

    *It is a lower frequency hum, not a buzz, crackle, etc.
    *Hum is present with power on as soon as I flip the Stand By switch.
    *Hum gets louder as I roll the Master Volume up on either channel.
    *Hum is present in all six modes; clean, fat, crunch, IIC+, IV, and Xtreme.
    *Hum is louder in 25 Watt Mode than in 10 Watt Mode but clearly present in both. There is now a noticeable "pop" when switching power modes (this is new...).
    *Hum is present regardless of EQ on/off or settings.
    *Hum is present and unchanged regardless of whether or not an instrument is connected and no matter which cable or instrument is used (all sound fine through my other amps as well).
    *Hum is present and unchanged regardless of Reverb level.
    *Hum is present when connected to other circuits in the house, with and without the conditioner.
    *Hum is present with or without the Footswitch connected and regardless of the position of the associated switches.
    *Hum is present through another 8 Ohm cabinet.
    *With Speaker Off (switch) and headphones plugged the hum IS NOT present! ...dead silent, sounds like it used to through the cabinet. This should probably tell me something but I don't know what it means!

    Nothing is in the loop and no pedals out front. All other amps (Fender SFVR & Marshall Class 5) have a very low noise floor on the same circuit through the same conditioner. Power tubes look normal during startup and operation. Taps on preamp tubes with a pencil don't produce anything noteworthy. I've ordered a new duet of power tubes and a couple of preamp tubes as I probably should have a spare on hand anyway so I can give them a try in a few days...but, because of the headphone thing, I don't think this is the issue.

    What else can I try? What does the fact that it's silent through the headphones but hums through a speaker cabinet suggest?
     
  2. Wally

    Wally Telefied Ad Free Member

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    According to the manual, that headphone feed comes off of the CabClone D.I. Circuit and is derived from the speaker out leads....the OT secondary. It is odd that you are not hearing the hum in that headphone mode. Before reading how the headphone mode works, I would have suggested pulling tubes...starting at V1...to see if pulling any certain tube would stop the hum. That would still be my approach. Perhaps the hum is there in that CabClone circuit but it is reduced by the circuit and you are not noticing the hum in that mode for that reason??
    I suspect a tube problem.
     
  3. dan40

    dan40 Tele-Afflicted

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    As Wally mentioned, tubes would be my first suspect. Failing power tubes drawing excessive current will create a hum as will bad preamp tubes. Since the amp is under warranty I would definitely let them check it but if you have any spare tubes you could check it yourself real quick before packing it up.
     
  4. Dacious

    Dacious Poster Extraordinaire

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    Power tubes. Mesa drives EL84s. If they're fairly new, phone them and they may send a new set. If you've had them a while like a year of condistent use it's worth changing.

    When I had the three 25 Watters I had I changed tubes yearly as I found they started sounding dull.
     
  5. MrGibbly

    MrGibbly Tele-Afflicted

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    Alright, my new spare set of power tubes came in the mail (needed them anyway) along with a couple of fresh 12AX7s. Tomorrow, I'll try replacing the power tubes first. Then, it would seem like V1 and V6 would be the most likely culprits. We'll see how it goes. Thanks for the input.

    EDIT/UPDATE: I went ahead and swapped the EL84s to the brand new pair and it has the same loud hum, pops when switching between channels, and pop on power off (which it NEVER used to do). I'll work the preamp tubes tomorrow...
     
    Last edited: Feb 9, 2018
  6. MrGibbly

    MrGibbly Tele-Afflicted

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    Thanks, guys! Definitely sounds like swapping the EL84s is the thing to try first so I'll give it a shot in the morning. I ordered a spare set and it came in the mail. I'll post with results.

    EDIT/UPDATE: I went ahead and swapped the EL84s to the brand new pair and it has the same loud hum, pops when switching between channels, and pop on power off (which it NEVER used to do). I'll work the preamp tubes tomorrow...
     
    Last edited: Feb 9, 2018
  7. MrGibbly

    MrGibbly Tele-Afflicted

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    Okay, update after further tinkering this morning and comparing sounds of different frequencies.

    *No change in the hum after swapping to a brand new matched EL84 duet
    *No change in the hum if I jump the effects lump...had the problem with dirty contacts on my ancient Peavey Bravo so thought this might be possible
    *Slight (?) change in the hum after switching V1 and V6 to brand new 12AX7s...but, honestly, it could be all in my head since I've been listening to this thing intently so much. I'm a little reluctant to order another $80 in preamp tubes as a science project since I don't think this is where the problem lies, at this point.
    *Compared to 120hz and 60hz sound samples and it's definitely closest to 60hz
    *I perceive that the hum slowly rises and falls in intensity like a wave over perhaps a 10-12 second interval

    I'm now suspecting either (1) a ground problem [in the amp or elsewhere] or (2) some kind of filtering going bad given that it seems to be 60hz. I went outside and confirmed that the ground line is in good shape, the clamp to my ground rod is tight, the rod is buried well, and all the connections to the ground bar are tight at the breaker box. I'll pop over to the hardware store and grab an outlet tester and a ground lift adapter to see what they may tell me...

    Anything else to try with the amp itself?
     
  8. Wally

    Wally Telefied Ad Free Member

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    Simply replacing tubes might solve the problem but only IF the problem is a bad tube. Pulling the 12A-7 tubes one at a time starting with the input gain stage can locate the problem area.
    Questions: is the hum resent with nothing plugged in? If so, does the h7m go away with a guitar plugged in but it’s the volume on the instrument at zero?
     
  9. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    When you replaced the power tubes did you also replace the driver?

    It's the first thing I would have looked at based on the symptoms. A badly-balanced driver is a very common source of hum and unfortunately the last thing many players and part-time techs consider.
     
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  10. MrGibbly

    MrGibbly Tele-Afflicted

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    The hum is present and unchanged regardless of which of my guitars is plugged in, with different cables, different speakers/cabs, different speaker cables, etc. that are "known good" by testing them through my other two amps. I'll try pulling each preamp tube one-by-one and see, though it's a very tedious task on this little head.
     
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  11. MrGibbly

    MrGibbly Tele-Afflicted

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    Yes, according to the manual V6 is the driver. That's why I swapped both V1 and V6 first while I had it all apart since they looked like the only preamp tubes that could affect all channels and all modes (to my untrained eye, anyway).
     
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  12. MrGibbly

    MrGibbly Tele-Afflicted

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    The more I study this, the less I think it has anything to do with filtering. From what I can tell that would most likely manifest as 120hz hum. So, in the realm of things I can probably fix myself, that leaves me with a ground problem. When I get home from traveling this week I'll test all the outlets and try a ground lift adapter. Otherwise, I guess it's off to Mesa she goes...
     
  13. codamedia

    codamedia Friend of Leo's

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    The amp is under warranty... if tubes don't fix it, send it back. Don't go any further or you will void the warranty.
     
    Last edited: Feb 12, 2018
  14. MrGibbly

    MrGibbly Tele-Afflicted

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    I've rotated good tubes through all the preamp positions with no change to the hum. The ground lift adapter came from Amazon and using it produces no change to the hum. In the same shipment I got a socket tester and every blasted one in the house tests good. Gotta be the amp. Guess I'll be calling Mesa and boxing it up. Bummer.
     
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  15. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    FWIW you were only planning to use the ground lift for testing, right? Just asking because some may interpret the comment as meaning it's a "permanent solution" when it's not a safe one. Useful for testing though.
     
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  16. Veltek

    Veltek Tele-Holic

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    Mesa will tell ya to check the tubes first. V1 is usually the noisiest when it goes.

    Also try plugging it into different wall outlets on different grids. Sometimes the power can be bad/dirty. (my amps sound like arse when it rains due to poor grounding.)

    Also there's a LOT going on under the hood of a mark amp between all the switching and what not. These are very receotive to interferance.

    Make sure your phone or other devices or magnets are not close by.

    Good luck!
     
  17. MrGibbly

    MrGibbly Tele-Afflicted

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    Yep, diagnostic purposes only! Even then, only after using the outlet tester first. I visually inspected as much of my home’s ground wiring as I could, tightened connections to the ground bar, the clamp at the rod, etc. but wanted to rule that out as a potential source of noise before I either (1) shipped the amp to MESA, or (2) bought another pricey [but safer!] gizmo like a Hum-X.
     
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  18. MrGibbly

    MrGibbly Tele-Afflicted

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    Well, exhausted all the possibilities that I could without risking bodily harm or voiding warranties. Called Mesa up and they were extremely professional, courteous, and helpful. Alas, my little Boogie departed this afternoon for a trip back to it's maker but I'm sure its in good hands. I'll post again once I get the diagnosis in case anyone else encounters a similar issue down the line.
     
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  19. MrGibbly

    MrGibbly Tele-Afflicted

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    In case anyone is following this thread or needs it down the line I wanted to be sure and keep updating. Mesa received the amp, ran a full bench diagnostic, play test, etc. and found *nothing* wrong with it. So, a fist full of cash wasted on tubes and shipping but now I know it's not the amp. I trust Mesa. That means that it's something in the "environment".
     
  20. Wally

    Wally Telefied Ad Free Member

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    I reviewed the thread. I suppose that this....
    ...
    kept us from suspecting anything except the amp. IF it is something in the environment, one might expect other amps to exhibit the same symptoms???? IF these other amps truly do NOT exhibit the problem, then one wonders why only the Mesa exhibits the problem IF it is environmental?? Have you revisited the other amps and ascertained that neither of those amps amplifies this noise??
     
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