Bias Problem Changing Valves On Tsl100

Discussion in 'Marshall Amps' started by Fabrizio, Jan 11, 2019.

  1. Fabrizio

    Fabrizio New Member

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    Hallo everyone,
    For the first time i changed valves on my jcm2000 tsl100, and installed new JJ EL34. Testing the bias with a multimeter, from one side ( left pin and center pin) i read 90 mV which is ok, but on the other side (right pin and center pin) i read 300 mV. If i try to adjust it the minimum i can reach is 115mV which is too much as far as I've read.
    What is the problem? Anyone has a possible solution?
    Thanks in advance!
    Fabrizio
     
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  2. Gunner64

    Gunner64 Well-Known Member

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    Oh boy...here we go again.. the dreaded bias drift due to defective circuit board issue amplified by cheap bias pots..cue ampmad in...2..3..1..
     
  3. Fabrizio

    Fabrizio New Member

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    Thanks for your reply. But not sure i understood the answer (my tech english level is low), could you please explain again what needs to to done ?
    What risks do i have using the amp with a bias that high?
    Thanks!
     
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  4. charveldan

    charveldan Well-Known Member

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    [​IMG]
     
  5. Jon C

    Jon C Well-Known Member

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    The guys are trying to tell you that the JCM2000 ( especially early production years ) had thin circuit boards, cheap resistors, and ran hot …… The heat can cause the board to actually become conductive ( NTC ) and then the bias will continue to change on it's own ( drift ) …… it sucks …… but it's such a common problem that the only real solution is to School Yourself on how and why this happens , test before you buy, ask a tech, and be a smart purchaser …… The toughest thing is that Not All of the 2000s do this …… Read This >>> http://www.desertamplifierrepair.com/project/marshall-jcm2000/
    ( it explains the common problem )
    Here is more info >>> http://www.hullerum.de/Marshall/TSL122repair.html
    ( lots of good knowledge about the issue )
     
    Last edited: Jan 11, 2019
  6. Jon C

    Jon C Well-Known Member

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    He will be here shortly !!
     
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  7. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    Welcome to the forum, as Gunner, and Jon C posted, it’s a known issue, and Ampmad is a member that will help you out, sorry I am not a tech :shrug:
     
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  8. GT08

    GT08 Active Member

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    Does the Amp bias OK using the old Valves?
     
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  9. El Gringo

    El Gringo Well-Known Member

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    now you are scaring the poor guy (just kidding )
     
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  10. Jeff Hudson

    Jeff Hudson Active Member

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    About the only solutions for this is.
    1. Replace main board with ISS 20 board.
    2. Use the bias board mod from Dr. Tube(?)
    3. Replace amp.
    Personally I did the first and a few years later I still don't regret it. I have no experience with number 2 so I will not guess about it but that solution is mentioned a lot. As far as number 3 only you can decide if that is worthwhile.
     
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  11. Jeff Hudson

    Jeff Hudson Active Member

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    Strange, he's usually more punctual than this.
     
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  12. Jon C

    Jon C Well-Known Member

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    I hate it when my Coefficient de la Temperature goes Negative
     
  13. thunderstruck507

    thunderstruck507 Active Member

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    I did the Dr Tube Bias board mod on mine and the amp sounds better than before, sadly I had already offered to sell my repaired TSL to a bandmate for $540 and bought my JCM 800 otherwise I might have kept the TSL and my credit card balance would be much happier.

    I do love the JCM 800 though, it has a ton more balls than the TSL and gets me that Marshall growl I really wanted.

    Do you know if the amp was biased correctly prior to replacing the tubes? Did you buy a matched set of tubes? If you move the tube pairs from side to side does the bias issue remain?
     
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  14. Gunner64

    Gunner64 Well-Known Member

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    What they said, and I would not use it until you figure out that bias.
     
  15. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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  16. Fabrizio

    Fabrizio New Member

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    Thanks for your reply and links!!
     
  17. Fabrizio

    Fabrizio New Member

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    The thing is that I replaced the old ones because one was not working anymore, so I can't figure that out
     
  18. Fabrizio

    Fabrizio New Member

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    Actually I am totally new into this inside amps world, but I will try to learn as much as possible now:
    1 No, I don't know if the amp was biased correctly
    2 yes the tubes they were matched
    3 I checked and the problem is the same.

    I will order a new V20 of the motherboard and replace it.
     
    Last edited: Jan 13, 2019
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