And the wolves howl! Help with a jtm45 build

Discussion in 'The Workbench' started by laycern8, Mar 1, 2021.

  1. laycern8

    laycern8 Member

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    Hey everyone, its been a while since I've popped onto this board. Some people around these parts may remember that I built a jtm-45 clone a while back with a lot of really generous help from members on this board. I am now back with new (old) problems.

    At low volumes, this amp produces some of the most beautiful clean tones I've ever heard. Its all the things that you all know make a jtm a great amp. But when I turn it up the problems arrive! Wolf tones! Always lower than the signal pitch, nice and loud... the least musical thing you've ever heard. Actually I took it outside to play/test it once (don't ask me why) and a neighbor invited himself into my backyard to yell about it lol.

    I did the easy part and isolated the issue to the amp; the home-built speaker cab is not to blame, nor is it the input signal. Without an oscilloscope, is there any way to isolate the issue further? Any ideas on what it could be? Power supply? Bad rectifier tube? Bad power tube? I've even seen people suggest the choke?

    thanks yall
     
  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    can you post a sound clip?
     
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  3. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Does the problem occur with multiple guitars?
     
    Last edited: Mar 3, 2021
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  4. neikeel

    neikeel Well-Known Member

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    If the amp is ok at low volumes then reversed primaries is unlikely.
    Is your head box lined on the bottom with aluminium sheet? That may lead to oscillation as you turn up if it is not.
    However if your lead dress is not ideal (like the purple presence pot wire too close to the other controls) then another source of oscillation
    Sometimes you need 5k6 grid stoppers on the output sockets (pin 5) but that is a sticking plaster vs a proper fix.
     
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  5. laycern8

    laycern8 Member

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    Thanks for the replies. I will definitely post some audio clips soon.

    I haven't tried multiple guitars, but I'd be very surprised if the guitar is to blame, since it sounds fine through other amps and on its own, and with this amp at low volumes. The issue starts when this amp begins to distort. I will make some clips with different guitars to be sure.

    It sounds like 60 cycle fundamental under whatever note I play. Its hard to distinguish on lower notes but very obvious on higher ones

    Neikeel, actually, when I first built the amp I did have the primaries reversed. The thing sounded like a fire alarm, I think I lost some cochlea hairs that day. Anyway the primaries should be in their rightful places now. I also reflowed the entire board (but not the filter caps, so I will do that)

    should I just buy a cheap o-scope?

    thanks again
     
  6. neikeel

    neikeel Well-Known Member

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    If it is a 60Hz ghost not you have to consider that your filter caps may have failed.
    New caps can have short lives - what make are they?
     
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  7. laycern8

    laycern8 Member

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    Alright I have some clips to post. Don't mind the mindless noodling on guitar lol.

    The clean clip is just for reference, the problem begins when the amp is pushed. I mik'd this with an sm57 at the grille cloth, and it didn't seem to pick up the messed up low frequencies very well. If you listen closely, you can hear them on the dimed clip around 1:30 in, where I play some octaves and then some chromatic notes. Its a lot more obvious in person, maybe with better micing I could get it clearer.

    Since it happens in such a distinct range, I would be tempted to say that I am hitting the resonant frequency of the speaker cab, causing it to produce those sounds somehow. But I did play this head through another cab and it did the same thing, so it seems to be happening inside the amp.

    The filter capacitors are supposed to be nice ones, F&T's from valvestorm! Maybe i damaged them at some point in the testing process? Also the head box is lined with aluminum flashing on the bottom where the chassis is exposed. The chassis itself is aluminum as well.

    Other than that, the tone of the amp seems pretty good? But the wolf tones make it unusable for tube saturation stuff.

    I should add that at one point this amp was biased incorrectly, and the kt66's red plated briefly.

    Whats u guys think?





     
    Last edited: Mar 7, 2021
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  8. neikeel

    neikeel Well-Known Member

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    Hmm
    Not clear ghosting, that is worst noodling on neck pup around 12th-14th fret box and moderately cranked amp.
    Are your output valves biased a bit cold?
     
  9. Matthews Guitars

    Matthews Guitars Well-Known Member

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    That's crossover distortion. Check the bias, it may be way too cold.
     
  10. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Is that truly dimed? As it’s on 10? None of my amps sound good on 10, max 7 or 8 on my Jtm45 builds.
     
  11. laycern8

    laycern8 Member

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    Yeah, it was dimed. Even the volume for the channel I wasn't using. I was trying to exacerbate the problem as much as possible. Haven't checked the bias yet but wanted to update that the amp is putting current on my guitar strings - touched a mic and found that I get nice tingles. Also learned that some of these three pronged outlets are farces!

    Found a real grounded outlet and that resolved the issue. But this means something is shorting to the chassis, am I right? I'm new to electronics so all I can think of is electrolytic filter caps leaking AC to ground. Does that sound right?

    Will get some bias voltages but I thought this could be pertinent to the issue.
     
  12. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Sounds like you’re on the right track.
     
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  13. laycern8

    laycern8 Member

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    Could someone please confirm that reading continuity between one of the positive terminals of my filter caps (with caps drained) and chassis/ground is a problem?

    edit: this seems intermittent, it is no longer showing closed circuit.
     
  14. laycern8

    laycern8 Member

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    Update: I can't replicate the closed circuit through any of the filter caps, nor can I get any current through the guitar to ground. I'm thinking this may have been a ground loop caused by the outlet it was plugged into when it happened.

    Still have low 120 cycle hum in the background, and the crossover distortion. Bias looks good @ 44.6mA and 46.4mA
     

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