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JTM45 Disappointment

Discussion in 'Amps and Cabs' started by drbob1, Jul 9, 2005.

  1. drbob1

    drbob1 Silver Supporting Member

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    Well, I got my 65 JTM45 back from Edtronics (caps and bias set) last week and I'm kind of torn. It sounds great clean and with relatively low gain (using Tele and Mullard 12AX7s, GEC KT66 into an orange cab with 73 G12H30s or a Carvin cab with blues) sounds but when I really crank on it (volumes at 2 oclock or over) it starts to sound kind of fizzy (like there's an envelope around the distortion and when the notes die out, there's still a buzz that lasts for a split second). Any ideas or is this just not one of the magic ones? I am using a Variac and a voltmeter to make sure it's being fed the right voltage... It's better with single coils, worse with the 59 burst!
     
  2. davebc

    davebc Member

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    That's a huge bummer.What did the amp sound like before you sent it out?
    I'm sure you already tried rebiasing.What happens when you back off the presence and treble?
    I'd be on the phone with them big time.WTF.
     
  3. Mr.Hanky

    Mr.Hanky Supporting Member

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    FOOK dude, that is a lot of coin in tubes and in the amp. That thing should be sounding awesome!!

    I just re-tubed my route 66 and it sounds fantastik.
    Call them!
    Did they save the original caps?

    Just for ha ha's you can swap out the pre-tubes and see what effect that has on the amp. I LOVE teles in V1 but they are not the best higher gain tube. Try the Mullard in V1, also keep in mind that tubes can go bad, even if they are new. Try swapping them and see what happens.

    What did you put in the P.I.?
     
  4. 908SSP

    908SSP Supporting Member

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    Well I think there is something wrong. :rolleyes:


    I wish I had that amp to worry about but instead I have JTM Bluesbreaker reissue and mine sounds great. I run all Mullard pre-amp tubes, Mullard rectifier tube, GEC KT66 power tubes , and C. Blues and mine sounds thick, creamy, smooth with not a fizz anywhere. I typically run the volume at 3:00 with a Weber Load Dump to control the volume. I would try swapping tubes first and if that doesn't fix it take it back to the tech and complain.
     
  5. riffmeister

    riffmeister Gold Supporting Member

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    ditch it and pick up a Germino Classic 45.

    hard to know for sure about your taste, but the C45 sure puts a smile on my face.
     
  6. garfight

    garfight Member

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    is it possible that the bias is off or that the new caps are too big?
     
  7. KLB

    KLB Member

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    With new components, maybe it needs a while to break in.

    Perhaps with the new caps, the tone controls need to be set differently?

    One of the preamp tubes could be weak.

    Occasionally, loud and distorted tones don't sound good to me, no matter the amp. It could be the AC power, the weather, your ears, etc. Try it another day.

    Best wishes.
    Ken
     
  8. drbob1

    drbob1 Silver Supporting Member

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    I'm sorry, I should have provided a bit more info. It had already had the filter caps replaced, and I sent it to Ed (who's very good) because it sounded fizzy. He says it sounds good at his house. He suggested using 113v with the variac because at that voltage the bias voltage and heater voltage were right on. I've tried a couple of different preamp tubes, two sets of power tubes, different speaker cabs and cables, different guitars, so it has to be the amp. I'll go thru the tubes one more time, but if that doesn't fix it, I guess it'll be back to Eds and see what it sounds like at his place (could do that Thursday, so that's cool).
     
  9. davebc

    davebc Member

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    Whenever I experienced the fizzy distortion(and man,I know how you feel,yuk)it was the result of a really cold bias.
     
  10. larrylover

    larrylover Member

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    You shouldn't have to have a variac to optimize the ac to get a stellar sound with that amp. That amp is (should be) designed to sound great no matter what is coming through the wall. I know that crappy ac matters -- I need no convincing on that score -- but a handwired amp should not sound like a solid state crate no matter what the ac.

    Good luck with this. I hope you have it sorted out soon.
     
  11. Mark Kane

    Mark Kane Silver Supporting Member

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    When that amp was built standard wall voltage would have been 110v. Today we're lucky if it gets under 120v.
     
  12. sundaypunch

    sundaypunch Member

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    Your tech may be very good but I personally wouldn't send the amp back to him. If he thought it sounded good as-is there is something wrong. Those amps are about as simple as can be and should be an easy fix for someone familiar with Marshalls. I don't think it's a matter of breaking in components or playing with the voltage. It sounds like you have a problem.

    Do you have any other tech's you can send it to? I'm sure you could get a recommendation over at the Plexi Palace board.
     
  13. drbob1

    drbob1 Silver Supporting Member

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    Now I feel like a complete idiot. I had changed the first and 3rd preamp tubes but wrongly assumed that the second tube wasn't as important, and left the current tube in there (it tested good). Today I tried a different outlet, a voltage regulator and a Tele in the second slot as well. When it worked well I dropped the voltage regulator part and it still sounds killer. In fact, it sounds great with wall voltage, but the problem there is that the heater voltage goes up to 7+v, so it makes sense to stay with 113, where it's a nice comfy 6.3. Anyway, the amp just came alive-humbuckers, single coils, old guitars, new guitars, TS808 or not, it sounds great (I prefer the cab with the Weber Blues to the pre-Rola G12H-30s). Thanks for all the advice, guys! Bob
     
  14. Chi

    Chi Member

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