NAD old Apeg J-12D; needs some love...help with hum

Discussion in 'Shock Brother's DIY Amps' started by ecoast, Feb 25, 2018.

  1. ecoast

    ecoast Tele-Holic

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    Good deal; should clean up nice and had orig. 6BK11 and 7951 glass.

    This one is diode rectified.

    Has new 40/20/20/20 can, so someone's been in here--- but it still hums

    Powers up; will play at volume, but hum is extreme and raises with amp vol control.
    Exists with nothing plugged in input.
    Have to pull tubes and see also, correct?


    I would also like to rewire with 3 prong and remove death cap.
    Should I change switch/fuse/lamp wiring to more conventional?

    schematic at end


    IMG_20180225_094733.jpg IMG_20180225_094741.jpg IMG_20180225_095659.jpg IMG_20180225_150942.jpg IMG_20180225_151002.jpg IMG_20180225_151033.jpg
     

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  2. hackworth1

    hackworth1 Friend of Leo's Vendor Member

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    Someone replaced some of the coupling caps, but not all. Some of the wiring is bad. Could have an iffy ground connection. Could be some of the old coupling caps are leaking DC. The old fuse on the board is ok and so is the pilot light if you want to leave them stock. Though I would fuse the hot and direct connect the common with the power transformer.
     
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  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Just because work has been done doesn't mean it was done right. There's also a ton of electrical tape splicing connections, which seems odd to me. The cathode bypass caps also haven't been replaced, and since those are old electrolytics they should go too. The lack of 3-prong cord and eath cap removal also indicate to me that it wasn't serviced well.

    I've found ampegs more sensitive to ground issues than other amps, for no reason I can specify. Just what I've run into.

    If you're comfortable doing the work, it's time for some troubleshooting. Pull the preamp tubes one a time from the input/right side and see if that helps the hum. If so, you know the problem is at that point or before in the circuit. From there you can use a scope or signal tracer to track down the hum.

    If you're not comfortable, it's tech time.
     
  4. ecoast

    ecoast Tele-Holic

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    I'm comfortable; just need a little hand holding.
    I have built a 5E3+, modded my J20, and have built a few pedals, so my technique is good, but my theory, well-not so good.


    @hackworth1 , the two pieces of remaining tape are the power black and 3 wires that go to one point on cap can that I *think* was done for convenience rather than in situ.

    I pulled the tubes; dead quiet.
    Installed power tubes; loud hum...so power section issue?

    No noise with just V1 & V2 in place.

    ...if cap can was wired wrong would it pass sound, play, and also hum?
    I have not pulled it to check wiring yet.
    Which other caps should I change out?
     
  5. kennl

    kennl Tele-Afflicted

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    The compaction 6BK11's were microphonic in mine, so
    I had it converted to J-12B specs
    6SL7 preamp tubes, 6v6 power tubes and a 5Y3 rectifier

    my favorite amp
     
  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    In the 5th picture the 2 blue guys toward the center and one silver guy should get changed out for sure. Those aren't going to cause your hum issues, but still should go.

    There are levels of wrong, so it's hard to say what symptom. I would recommend checking all the connections, visually and with a multimeter, to make sure the wires are going where they are supposed to for the repairs.

    See if any of the caps are passing DC - that could cause hum in places.

    Do you have a signal tracer or scope you can use? I love using a signal tracer to send out the exact point I'm probing to another amp. If the hum is there, you'll hear it in the remote amp through the tracer. You can start at the input and work your way through the circuit with the tracer to find the point of the issue.

    If it happens with only the power tubes in you've determined it's likely not in the preamp section. Most people don't have spare 7591 tubes around, so we're just going to hope it's not that :D
     
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  7. ecoast

    ecoast Tele-Holic

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    All the resistors check ok

    I pulled the cap can; caps show 'infinite'; have to check the wiring--- the schematic only shows the 40/20/20 being used; and not the last 20uf?

    it's 120 cycle hum that increase/decrease with volume...

    no scope, but I can make an audio probe; just not sure how I'd use it here as the amp plays but is accompanied by the loud 120?
    Start at beginning of signal path with tubes pulled? And then?

    yes; I will probably order a set of jj7591, but I would like to learn to troubleshoot.

    Other than filter caps, (or a bad tube), what is the most likely places the hum would be introduced?


    Thanks as usual for any and all advice....
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    You're not running any audio through it with the scope. You're listening for the 120 Hz hum at various points in the circuit. For instance, you shouldn't here it at the input with nothing plugged in, or the first grid of the tube, since there isn't any signal.

    You can leave the tubes in while doing this.
     
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  9. ecoast

    ecoast Tele-Holic

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    so I should send a signal into amp input, and then use probe to follow from pre to power section?
    Amp on or off? ...hum is loud with amp on and signal in input; trying to understand how to use probe in my situation?

    Again, regarding cap can, if the grounds were miswired, wouldn't that also cause hum?

    I will go ahead and order 2 new power tubes; can't hurt...
     
  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Nope - you don't need to send in any signal to the amp. You're just listening to the hum from the amp itself, so we don't care about how it's amplifying external signals at this point.

    If the grounds were miswired it could cause hum for sure.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Does that bare connection for the white neutral concern anyone? IMHO, this amp might need to be in the hands of an experienced tech. Everything is suspect, imho, when I see something like that.
    If the hum is present with the 6BK11’s out of the circuit, then the problem seems to be in the circuit downstream from the phase inverter, doesn’t it? Are bothpower tubes conducting current? A touch test on the glass of the power tubes can give a person an idea of what is going on. One should be able to touch a tube for a part of a second without hurtingone’s skin. If a tube is cold, it is not conducting....and there might very well be a hum. If one tube is much cooler than the other, there may be an imbalance of current draw that is problematic.
     
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  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    That's kinda what I was hinting at earlier. The previous work looks off to me, so I would have trust issues with the whole thing.
     
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  13. ecoast

    ecoast Tele-Holic

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    agreed.

    I pulled the tape off that one connection; of course first step is to replace with 3 prong and remove death cap.
    I had literally just got it apart...

    If I cannot figure it out, it will be sent out. But I would prefer to keep learning and resolve it for myself.

    @Wally, pwr tubes are warm...

    Looking at schematic and cap can, I can see "A" "B" and "C"; are only 40/20/20 used in circuit?

    because:
    Currently pin 5 from V3 & V4 is connected to 4th 20uf @cap can, but I'm not seeing that from schematic?
     
  14. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    They could be using that other cap from the can as the cathode bypass cap,but that looks like it would be pin 6 on the tubes and not pin 5. I've seen that done before when there is an extra spot on the cap can available.
     
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  15. Wally

    Wally Telefied Ad Free Member

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    pin 5 on the 7591 is the cathode which has a pin internal connection with the suppressor grid. So, it seems as if that 20mfd section is being used as the bias cap. What is the voltage on those pins 5 on the power tubes?

    http://tdsl.duncanamps.com/show.php?des=7591


    Do you have a picture of the connections on the multisection can?
     
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  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    bah - I was looking at the V2 section - not the v3/v4 section. But that confirmed my theory taking another look at the right spot :D
     
  17. ecoast

    ecoast Tele-Holic

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    IMG_20180227_074038.jpg IMG_20180225_151002.jpg IMG_20180225_151033.jpg
    Pin 5= 295vdc

    Looking at diagram on can and pic, let's call terminals "1 2 3 4" with #1 being bottom right 40uf

    3 4
    2 1

    ________

    1 40uf connects to RED from transformer and one side of diode rectifier, and 1000ohm

    2 20uf connects to 22k and position "C" on schematic

    3 20uf connects to pin 4&8, and opposite side1000ohm from #1

    4 20uf connects to pin5 , and 140ohm


    the 4 outermost terminals are common ground along with the case, one of which is going to transformer.


    Will have tubes tomorrow.
    Please ignore bare white/black; going to rewire fuse when I get a 3 prong and IEC
     
  18. Wally

    Wally Telefied Ad Free Member

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    Take a look at the schematic for the desired voltage to be applied to the cathodes of the power tubes.....
     
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  19. D'tar

    D'tar Friend of Leo's

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    :(
     
  20. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    So it's fine they are using the cap can for that bypass capacitor you see on the schematic. It's NOT fine that you have 295V there. You need to figure out where that voltage is coming from. Something else is either connected to pin 5 or to what you are labeling "section 4" of the cap can.

    You could confirm this by disconnecting the wire from the 140ohm resistor to the cap can. After doing that, what voltage do you have on pin 5? At node 4 of the can?

    You also should pull the power tubes at this point until this issue is sorted.
     
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