Tsl 100 Toasted/loose Resistor R76

Discussion in 'The Workbench' started by thunderstruck507, Aug 10, 2018.

  1. thunderstruck507

    thunderstruck507 Well-Known Member

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    I was going to attempt to install a Dr Tube Bias mod kit in my TSL100 head but upon getting it apart I have found another issue.

    R76 looks discolored and was loose in the board. I was able to pull it right off (I could have shook the board and it would have fallen off). I metered it and it and the other like it, the others come in right at the 1000 ohm rating but R76 is coming in at 9500 ohms.

    Before I call Joe Twist who is local to me I wanted to ask here what this resistor does, what this problem means, and if it's worth trying to continue with repairing this PCB versus my original plan of replacing the PCB with a Revision 20 board?

    All 4 power tubes are Mullard EL34s showing no signs of problems and I could get the amp to bias correctly but it would sometimes run away on one side and it did sound worse once that happened.

    I recently bought a JCM 800 2203 Kerry King which stomps this TSL in the dirt HARD. Plan was to repair the TSL and sell it but now I'm curious if the problem might be why it can't hold a candle to the JCM800?

    Any help appreciated.
     
  2. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    R76 looks to be a screen resistor, like R74, R60, R65. If you don't replace that, the amp will eat power tubes and / or also your output transformer. It could have been several conditions that caused that resistor to burn up, including a defective power tube, or wrong speaker cable, or a bad mismatch of speaker imp.
     
  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Those early JCM2000 boards were quite fickle too; everything burning up on them.

    Does your board also have C46 22pF capacitor at V8? Another problematic issue.
     
  4. anitoli

    anitoli Well-Known Member

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    Ugh, thats what they replaced the 6100 with.
     
  5. thunderstruck507

    thunderstruck507 Well-Known Member

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    c46 doesn't look great either, but I thought I have read of people clipping that one entirely ( I could be mistaken)
     
  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    No, it’s because jcm800 amps kick serious ass.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The screen resistor burns up because the bias is bad.
    The purpose of that resistor is to limit current to the output tube.

    The tube that has that resistor probably burned up...
    Go ahead and do the mod, replace the screen resistor, put in new tubes and bias.

    Yes, there is a tube problem. You know this because the screen resistor burned up.

    Don't reuse the same tubes after you get the bias fixed.
    Obviously, at least one is cooked.

    You can replace the board, but some of these boards developed the same problems after they were replaced.
    The Dr Tube mod might be better than a new board.

    I'm curious if the problem might be why it can't hold a candle to the JCM800?

    First fix the amp so the bias is stable.
    Then you can go for preamp tubes, etc to get the sound fixed up.
    A lot of times, it's just weak preamp tubes.
     
    Last edited: Aug 11, 2018
  8. RickyLee

    RickyLee Well-Known Member VIP Member

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    R76 is for that last EL34 that also has that little 22pF C46. I have had my suspicions in the past of problems with the V8 socket/tube only, and the three other tubes are still good, the problem being related to that under rated C46.

    My advice would be to remove that C46 while doing the R76 repair. Or at least, upgrade C46 to a 1000V rated 22pF.
     
    Last edited: Aug 11, 2018
  9. GT08

    GT08 Active Member

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    Just renew the main board. This will fix a lot more than just a bias drift.
    Rock n' roll
     
  10. thunderstruck507

    thunderstruck507 Well-Known Member

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    Thanks for the tips guys.

    I have messed with all the preamp tubes in this amp before with very subtle changes between Sovtec, EH, and JJ 12ax7. It sounded pretty damn good anytime the bias wasn't running away.

    What's stranger is the bias creep was occurring on the opposite pair, not the pair with the bad resistor. Tubes 7&8 held solid, 5&6 were the ones creeping. The highest I saw them get was 109 mV and I would immediately shut the amp down, usually coming back to it and checking/setting bias the next day or 2.

    As recently as last week I was playing the amp and with bias set a little colder at 68mV per pair it sounded better than ever. Then the 800 came in so I switched heads and haven't played this one since.

    The Kerry King has tons more low end and girth, while still sounding tight. It's a little louder. The tone is also a lot smoother with more good mids even without the beast mode engaged. It could be related to the KT88 power section versus the EL34? I am obviously not well versed in mechanics of 100w heads. The TSL was my first and before that I had a DSL401 combo running into this same 4x12 cab. The TSL definitely blew the doors off the DSL401 in both tone and power.

    My roommate has a 6505+ Peavey 120w head with 6l6 tubes. The 800 is much closer to that than the TSL which while still in the same ballpark is a bit thinner sounding.
     
  11. Jeff Hudson

    Jeff Hudson Active Member

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    The issue 20 board solves all problems with this amp. as far as your preference in sound that could be a different story.
     
  12. RickyLee

    RickyLee Well-Known Member VIP Member

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    Something else to add in about the DSL100/TSL100/122 amps. The bias trim pots and test points on the back are opposite to the pairs of EL34's they control. So I am figuring your bias drift is still on that V7/V8 side.

    I should add, that is how it is on my two DSL100's and my two TSL100's . . . .
     
  13. thunderstruck507

    thunderstruck507 Well-Known Member

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    good info, thank you for that
     

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