NUAD Marshall Class 5 With a Question

Discussion in 'Glowing Bottle Tube Amp Forum' started by Joe M, Oct 6, 2019.

  1. Joe M

    Joe M Super Moderator Staff Member

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    Just picked up one of these in mint condition. Newest generation with the three piece back. Original tubes, don't think it was ever played, just sat in somebody's music room. As great as it sounds, and as loud as it is (5 watts????) it has an annoying hum after the tubes warm up. Doesn't matter if anything is plugged in or not, it's always there. Changing any of the knobs makes no difference. Tried a new set of tubes with no change either. Probably being anal since you can't hear the hum when you're playing. So, is there any fix or just live with it?

    Thanks
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If swapping the tubes didn't do it, it's probably just how it is. Unlike older amps, it's all one big board so there aren't any wires to chopstick around.

    It helps to know what kind of hum - 60 or 120Hz? That would at least isolate the issue. I suppose it's possible there is a bad filter cap even on a newer amp that would cause this.
     
  3. printer2

    printer2 Poster Extraordinaire

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    Pull the tubes, if the hum is still there it is the power transformer coupling with the output transformer. If no hum plug in the output tube. It is a single ended circuit and they are more hummy than push-pull. If no hum then plug in the next tube in line. Check for hum then plug in the first tube. Not much more that you can do without a little experience under the hood of the amp.
     
  4. Joe M

    Joe M Super Moderator Staff Member

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    Thanks for the replies, guys, think I’ll just live with the hum.
     
  5. Les Paul lover

    Les Paul lover Tele-Holic

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    Is there a hum balance pot somewhere on the board of the Class 5?

    My Orange AD5 hummed like crazy until I tweaked the hum balance pot in the correct zone.
     
  6. MuddyWolf

    MuddyWolf Tele-Meister

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    I think i remember the hum is a common complaint with those.
     
  7. metropolis74

    metropolis74 Tele-Holic

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    All single ended amps have idle hum to some extent. It's the nature of the beast. Do you have another SE amp to compare the hum to?

    For example my Class 5 hum is quieter than my Champ 600 reissue hum. But when playing the hum is not noticeable on either amp.
     
  8. metropolis74

    metropolis74 Tele-Holic

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    No hum balance pot on the Class 5. There is an artificial center tap on the heater circuit of two 100ohm resistors to ground though.
     
  9. noname_dragon

    noname_dragon Tele-Meister

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    A loose or disconnected ground somewhere in the engine room can cause full-time hum I had an amp with that issue and once corrected the hum was gone.
     
  10. gusfinley

    gusfinley Tele-Holic

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    The artificial center tap trick works pretty well, but is not foolproof. If you put 5% tolerance resistors in there you could end up with a 10% imbalance.

    I like to use 100 ohm 1% tolerance resistors. They can make a noisy heater hum dead silent!

    If you reference the heaters to an elevated voltage ( like the cathode of the power tube ) this can help lower the hum also,

    That is IF the heaters are the source of your hum.

    What tubes do you have installed?
     
  11. Joe M

    Joe M Super Moderator Staff Member

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    Thanks again for all the tips, guys, but I've made my original question moot.....couldn't stand the hum, so I returned the amp.....:rolleyes:....told you I was anal....
     
  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Not having the amp will make the hum less for sure!
     
  13. Joe M

    Joe M Super Moderator Staff Member

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    :lol::lol::lol::lol:
     
  14. tubelectron

    tubelectron Tele-Meister

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    Hi @Joe M ,

    The plate voltage on the Class 5 is not filtered enough, using a 220R/5W resistor (R6). Replacing it with a choke (the 194A for Fender Champ is fine) along with boosting the 2nd filter cap from 47 to 100µF/500V would certainly eradicate that hum. It's what I will do on my Class 5 combo ASAP.

    These 2x 100R (R16, R17) resistor hum balance are already existing, plus the 12AX7 tubes are DC heated and filtered (C16 to C19 : 3300µF). Little chances of hum source here...

    -tbln
     
  15. gridlock

    gridlock Friend of Leo's

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    Congrats a very good amp.

    I owned a new-version 2012 Marshall Class-5 (.1 watt switch on the back). I don’t recall any hum and it was a good sounding amp. Sold it because I also owned two Marshall 18 watt EL84 amps at the same time. I do miss the .1 watt setting on the Class-5.

    Please post some photos.
     
  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    @gridlock - someone didn't read the thread :D
     
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  17. Mad Kiwi

    Mad Kiwi Friend of Leo's

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    I know you have returned it....(I did read the thread :) )

    But as another reference point my Class 5 v1. doesn't hum beyond normal cranked gain / volume.

    At sane levels it is pretty quiet.
     
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  18. gridlock

    gridlock Friend of Leo's

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    Oops...skim reading. Glad that I didn’t post a picture of my old Class-5.
     
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