Troubleshoot my JVM

Discussion in 'The Workbench' started by Hard40sta, Oct 18, 2020 at 2:08 PM.

  1. Hard40sta

    Hard40sta New Member

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    Over the last few gigs, I've noticed I'm having to slowly creep up my JVM410H's volume to get the same output level. Last night's show was the worst. I had my volume set at 8 (out of 10) and I could not get enough out of the Boost side to get over the top. This is a 100-watt amp! Also, I am noticing a reduction in the smoothness of my Gain on all channels. Maybe related?

    • Am I looking at a tube going out/gone out, or what?
      • If so, where should I start to troubleshoot this?
      • Suggestions on tubes to use for replacement--all?
    • If not tubes, what then?
    Thank you in advance. (I am 2hrs from the nearest authorized Marshall repair location.)
     
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  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Do you have a full spare set of tubes? If not, why not? Swap them in. Tubes get weak as they age and are used.
     
  3. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    For how long you Have you Had it?

    when was the last time you replace tubes?
     
  4. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    Its time to retube your amp and do it before one of the PTs blow and possibly cause additional damage. Im guessing its the PTs just wearing out but might as well replace the pre amp tubes also, they are cheap!
     
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  5. Hard40sta

    Hard40sta New Member

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    This is my first tube amp. I've had it for about five years. I had no idea I needed to change tubes regularly. Do you have a tube-set recommendation?
     
  6. Hard40sta

    Hard40sta New Member

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    Does this look right?

    • JJ Electronic 12AX7 / ECC83 (Gold) x 5
    • JJ Electronic E34L (Blue Glass) x 4
     
  7. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    Looks good to me! Not sure what “Blue Glass” means!?
     
    Last edited: Oct 18, 2020 at 4:05 PM
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  8. Hard40sta

    Hard40sta New Member

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    And having never done it, how hard is it? (I am seeing guys talking about re-biasing when they put new tubes in...?
     
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  9. Hard40sta

    Hard40sta New Member

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    And having never done it, how hard is it? (I am seeing guys talking about re-biasing when they put new tubes in...?
     
  10. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    Yes, you will need to rebias the amp. Its not hard with the proper tools. I use the Weber BiasRite Meter. Others use a multi-meter! Just need to be very careful not to touch anything in the amp as there is high voltage in there that can kick your ass!
     
  11. neikeel

    neikeel Well-Known Member

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    yes you should always check the bias when replacing the output tubes.
    Preamp tubes do not.
    Did you get a manual with the amp?
    I believe the plate voltage on these is 480v so bias would normally be no higher than 36mA per tube, I think the factory setting is 30mA.
    However on these the test points are inside - so it is off to a competent tech for you:agreed:
     
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  12. scozz

    scozz Well-Known Member Gold Supporting Member

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    Or worse!
     
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  13. scozz

    scozz Well-Known Member Gold Supporting Member

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    Is there a reason Marshall puts them on the inside instead of the amp,.....instead of exposed, on the back panel.
     
    Last edited: Oct 18, 2020 at 10:40 PM
  14. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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  15. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    I think it took the manufacturers longer to grasp the idea that guitar players are anal fucks and want to dick with everything in search of the tone.
    If the adjustment points are there they realized that it was necessary i just don't think they took into consideration how much or often some will screw with the bias so putting the external points on the majority of the amps is still long in arriving.
     
  16. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    You may find your preamp tubes, the 12AX7, to be fine. They last much longer than power tubes. While it is a good idea to get a spare set, you might not have to change those. Likely not. Try the EL34 first!
     
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  17. avspecialist

    avspecialist Member

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    I highly recommend getting the Preferred Series EL4 ‘s from The Tube Store in Canada. They transformed my JVM completely. Also, using a volt meter at the test points inside the amp, you can adjust the numbers to get the bias from 70 to 80 mv. It helps to know the plate voltage also, because those measurements work in tandem with the bias. You can look up the Weber bias calculator on line and plug in the plate voltage and bias to come up with a safe adjustment. I have a scope to look at the wave form after adjusting the bias. Also, I biased 1 pair of tubes 1 to 2 milivolts hotter for additional even order harmonics. I believe I ended up at 80/81 mv on one side and 79/80 on the other side at 457 plate voltage. After having the amp for just over 10 years now, it really came into its own after putting these tubes in and taking out the JJ’s.
     
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  18. neikeel

    neikeel Well-Known Member

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    Based on your 457vdc voltage 76mA per pair is max (unless the bias points are straight cathode current (ie plates plus screens) then you can allow 4-5mA for the screens.
     
  19. LPMarshall hack

    LPMarshall hack Well-Known Member

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    So you just plug your tubes into this and it does all the work?

    that seems too good to be true...is it?

    edit: just googled the instructions and it looks like you still have to pull the chassis and mess with the test points inside the amp.

    And apparently they don’t make this anymore?
     
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  20. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    You are correct and I tried to find them yesterday and I couldn’t, I was like WTF. Its a great tool, I know many that have them and love them also. I dont know why Weber would stop making them. I saw a few used ones for sale.
     
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