Help! JTM-45 build (ish) problems!!! - new guy to Marshalls

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hookstratton, Apr 14, 2016.

  1. hookstratton

    hookstratton Member

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    Hey Amp Gurus! Please help, I'm kinda freaked out:

    I bought a brand new JTM style build with 2 6V6's / 20 watt - the guy is a new builder. It's a basic single channel - Gain/Master TMBP, with 2 inputs, and Fx Loop Option. Tone is pretty nice, but….

    *Please let me know if this normal for a JTM45 isn preamp:

    - The Treble Control adds LOTS of bass when turning CCW from 2-0, and from 5-10 it's really thin and seems to suck gain also.

    - Presence Control - not working at all? No real change turning 0-10


    The builder says it is due to the very high amount of negative feedback in the circuit,



    *ALSO on first play, a bunch of red flags went up (have not discussed this part with the builder yet) - ANY HINTS or THEORIES on this would be very much appreciated:

    - Lots of hum (base level - with controls zeroed, guitar plugged in or not),

    - Very loud hum/buzz on idle with all controls at noon - goes away mostly when guitar plugged in the Clean Input, but no noise change at all when guitar plugged in Crunch Input -

    - Also, if you touch the speaker cable's metal sleeve, the noise changes / gets louder bassier! - with or without guitar plugged in… Bad grounding issues? Stray high voltage maybe??


    Many thanks in advance!!!!



     
  2. garret

    garret Member

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    Get your money back. If he sold it to you as described he doesn't know what he's doing.
     
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  3. pdf64

    pdf64 Member

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    May be a 1M treble control has been used, similar to a Vox / Trainwreck type tone stack?

    May be he's used a master volume after the phase splitter? Does the presence control work if the master is turned up full?

    Seems like he may benefit from technical reading http://www.valvewizard.co.uk/Grounding.html

    It may be that some original JTM45 were wired up with the polarity of their speaker sockets reversed; if it's been built like that, it would best not to use metal bodied jack plugs on the speaker output. The voltage available on a metal sleeve (at max crank) may not be lethal to an adult, but there are also children /pets to consider.

    Just to note that I can't see why this amp is being described as 'JTM45 style'? The features (single channel, clean/crunch inputs, fx loop, master volume, 20 watt / 6V6) all differ to the JTM45; there doesn't much left in there that could identify it as such? May be lots of negative feedback and reversed polarity speaker sockets.

    I think that you may need to work with the builder to iron out these issues; if that could be a problem, it may be best to reject it.
    If the builder is inexperienced, can you be sure that the equipment is compliant to modern electrical safety standards?
     
    Last edited: Apr 14, 2016
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  4. hookstratton

    hookstratton Member

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    Thanks for replying garret,

    I am likely going to keep it cause I got and really really great deal on it - if he can't fix it, I'm ok with spending some more if neccesary.

    Here's some more info:

    I think what has happened is that he offered an Fx Loop option to me, but had actually never done one before… and so he did some design changes to get a lower signal levels on the Fx Send / raise on Return. But I don't think it's an Active or Buffered loop.

    - The loop gain is not unity, with straight cable theres tone change and a light maybe only 3db drop?

    - The loop level is too hot, it seriously fuzzes out my Ana Echo mini delay, and he said he addressed this issue with the design changes.
     
  5. hookstratton

    hookstratton Member

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    Hey pdf64, thanks for the detailed reply.

    I'll check out your suggestions in a few hours! T he "Crunch" input is supposed to emulate "Jumpered channels in a cranked JTM46" The "Clean" input is supposed to be more a "Tweed Bassman" tone. I wish I said "no" to the Fx Loop option….
     
  6. zenas

    zenas Member

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    Is the builder just a buddy making amps for fun or is he in the business of selling his boutique hand wired masterpieces?
     
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  7. xtian

    xtian Member

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    "Presence control does not work" and "hum changes when you touch speaker cable shield" might indicate that speaker output jack is not grounded. Super quick way to test would be to attach an alligator clip: one end to speaker cable shield, other end to amp chassis. If hum drops and Presence control suddenly works, there it is! OR, if you get a feedbacky sqeal when you do this, problem also identified, but OT secondaries need to be swapped.
     
  8. hookstratton

    hookstratton Member

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    Thanks for the helpful replies: I've got a free day tomorrow to troubleshoot before I respond to the builder.

    zenas - i thinks he's a serious repair guy transitioning to builder (not a buddy, an internet buy)

    xtian - Thanks! I'll test that tomorrow. If i can't resolve it maybe i can send it up to you...

    BUMP!
     
  9. zenas

    zenas Member

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    He might be moving a little fast in that direction. Not trying to criticize here but we have to know our limitations.

    Definitely try the stuff Xtian posted.

    If that doesn't get it you might post some gut shots so we can see the lead dress. That's an area that can cause all sorts of issues.
    (don't ask me how I know) :)
     
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  10. Avatar Tech

    Avatar Tech Member

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    All of these problems could be due to a couple of mis-wired connections. It's pretty easy to wire a TMB tone stack incorrectly, and if you do that you can create some pretty odd results. The behavior of the treble control really makes me suspect the wiring in the tone stack. And I wouldn't be surprised if most of these symptoms disappear once you locate and correct the problem. I'm assuming the builder has successfully built amps in the past, and that he followed a fairly conventional layout. Without a schematic and/or some pics of the guts, though, it's kind of s guessing game.
    Colin
     
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  11. icr

    icr Member

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    Who is the amp builder, you or the seller? If you bought it because you like building and repairing amps, ok. Otherwise it should go back, unless you didn't understand what you were buying.
     
  12. hookstratton

    hookstratton Member

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    Gentleman - thanks for the generous input: I haven't had time to open it up and test it - and now I think the builder is going to revisit it.

    - zenas: agreed, thanks! I will post gut pix if i don't send it back to the builder.

    - Avatar Tech / Colin: thanks! yeah, if the treble stays at about "2", tone is pretty righteous actually. I think the noise & hum issues probably come from the fx loop mod; he was never clear if was passive or active, and it's way too hot for my analog delay (fuzzed out repeats), and it isn't unity gain either. nothing was ever written out concisely on an order form, all phone conversations / email.

    - icr: I'm not the builder, I bought it because it was an incredible deal on paper. I think it got off course when he offered the fx loop option, which I'm pretty certain now that he likely had never installed one before... I think he's going to stand behind the work at this point, and I told him if he needs more $ I'm open to it (pretty sure the parts alone are far more than what I paid). On one hand it feels like I've been a unwitting "beta tester", but if he's willing to finish the work I could have a great amp. If he can't fix the noise/hums and fx loop stuff, hopefully someone else can and I won't be out too much extra $$.

    Thanks a million, I'll update soon as I can.
     
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  13. hookstratton

    hookstratton Member

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    P.S. - BTW, I forgot this detail in all the surprises: I've seen gut shots of his work and the wiring was beautifully done...

    Now I think I better peek in and take several shots before I send it back, just to confirm the sample pix I saw are similar to what I have inside my amp - if it looks great it will inspire confidence too!

    Thanks Gear Gurus!
     
  14. donnyjaguar

    donnyjaguar Member

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    This is a pretty simple circuit. Have a copy of the schematic handy and use a hi-liter pen to mark all the connections as you check them. As noted above, slight mistake on wiring can have profound effect on performance. The good news is its probably an easy fix.
     
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  15. hookstratton

    hookstratton Member

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    Footnote: I got into this deal because the price was unreal - but lesson learned for future dealings - be clear on all specs/options, warranty, shipping for warranty work, get a bill of sale / list of added options, basically get it all in writing...
     
  16. donnyjaguar

    donnyjaguar Member

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    Or just do as I do and go on the assumption that anything you buy used it broken. This way you avoid any disappointment. :)

    Mind you, the last two old electronic things I've bought recently (speaker cabinets) worked 100%, which I found disappointing. ;)
     
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  17. hookstratton

    hookstratton Member

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    Lol, but this is a new build!

    I finally had a good talk with the builder - I'm sending it back tomorrow, he's being very cool.

    I'll post when I hear from him or get the amp back!
     
  18. sixesandsevens

    sixesandsevens Member

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    Glad to hear it's moving in the right direction.

    And thank you for taking a chance on this guy and being an early adopter. As you can see, not everyone has the patience required for it and it likely means a lot to this guy.

    If at any point it goes sideways, you can gently remind him that happy customers improve future business way more than unhappy ones. ;)
     
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  19. hookstratton

    hookstratton Member

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    Thanks for the post - 6s&7s. Yeah, I'm a armchair tech myself, and willing to meet any fellow asylum member halfway!

    Communication is not easy but he wants to make it right. Overall I'm getting the impression he's he's really not trying to transition to builder as a business, more a serious hobby. I can't really tell if he wants referrals...
     

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