JCM800 2204 (1986) problem

Discussion in 'The Workbench' started by CROWEMAG, Sep 14, 2020.

  1. CROWEMAG

    CROWEMAG Active Member

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    A few days ago my 800 was sounding perfect. Nice and bitey full of dynamics. All of a sudden it seems to have less grit and sustain, almost gated in a way. I tried some different preamp tubes and nothing really had a lasting effect. Other than pre amp tubes......what could cause this sudden loss?

    This is really bothering me as she was sounding so good!
     
  2. CROWEMAG

    CROWEMAG Active Member

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    I bypassed one of the cathode resistors with a .68 cap. Now looking at what I ordered it appears as though its only 100v. Could this have been cooked? Also, when you bypass a resistor with a cap, how will that effect a reading with a multimeter? Would it still read the value of the resistor? Say your resistor is 2.7k and you have bypassed it with a cap.
     
  3. CROWEMAG

    CROWEMAG Active Member

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    Also, I should note that when I am on a super low volume....say .5 the crunch and dynamics remain.....inching closer to 1 on the master I start to lose a bit of the special sauce and get the symptoms I have described. I have no clue what would cause that.
     
  4. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    When was the last time the power tubes were changed/biased?
     
  5. CROWEMAG

    CROWEMAG Active Member

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    Power tubes were changed around January of 2020. I biased them recently to 37-38mv. the funny thing is when I ran them closer to 40-41 they sounded better but I was afraid to leave them that high. No red plating or anything like that I was just kind of worried I guess.
     
  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    So, take out the cap and see if that fixes things.
     
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  7. shredless

    shredless Well-Known Member

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    You cant read a resistor with a bypass cap you must lift a leg of the cap. I think 100 may be low, l was recommended to get 160v caps if l remember right.
     
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  8. shredless

    shredless Well-Known Member

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    My 800 at .5 volume sounds like its running preamp only going higher brings in the meat of the power amp and it gets real full real fast. Maybe it is a tube problem but l agree lift that cap and see what happens
     
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  9. shredless

    shredless Well-Known Member

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    I think you would have been fine at 40 41 if it sounded best there. Bias range can depend on probes and meters so if you thought it was better...try it for a while. Some amps like it cold, others like it hot. Within reason that is
     
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  10. thetragichero

    thetragichero Well-Known Member

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    cathode bypass cap isn't gonna see much more than a couple volts under normal operating conditions. literally whatever cap you can buy would be fine
     
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  11. myersbw

    myersbw Well-Known Member

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    Actually, you can do this. Think about what's happening. A meter usually inserts a dc voltage across the resistor, so...yes, you'll have a charge time that's directly proportionate to the cap value. But, once it settles, the cap will charge (be it small) to the meter voltage insert and the meter will read the resistor. UNLESS the cap is leaky and presents a parallel resistance to the cathode.

    The caps that cause large time constants are those first stage Marshall 200-300uF values! LOL!
     
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  12. neikeel

    neikeel Well-Known Member

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    Your description fits with cold output tube bias why it might have drifted ‍♂️
    I’d check that first.
    Which cathode resistor did you bypass? The 10k on v1 or the 820/1k on V2a?
     
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  13. neikeel

    neikeel Well-Known Member

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    Unless it was an electrolytic installed back to front?
     
  14. CROWEMAG

    CROWEMAG Active Member

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    v1

    I need to do some more prolonged testing before I can accurately say that the bias is drifting but what all should I check in regards to this? The bias caps are fairly new. Trim pot and resistors are original to the amp. I did not witness a bias drift. I changed that myself.

    Like I said I need to put a meter on her and play/let her run for a while before I can say for sure that is even an issue. More to report later tonight, hopefully. Thanks for weighing in.

    I may have figured it out though. I recently posted about a lifted trace on the same amp in another thread and I do not think I did the best job at repairing it. I feel like the problem was intermittent so I went back to it last night and redid the job and it seems to be as it should now. Fingers crossed! I hope to do some more testing to night at higher volumes to be sure.
     
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  15. NewReligion

    NewReligion Well-Known Member Double Platinum Supporting Member VIP Member

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    Ah the joys of learning to troubleshoot.

    I hope the lifted trace resolved the issue.

    David
     
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  16. CROWEMAG

    CROWEMAG Active Member

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    It appears it has! This amp is coming to you soon any way! Hahahahaha.
     
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  17. CROWEMAG

    CROWEMAG Active Member

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    ran the amp for almost an hour last night to check for bias drift and I am happy to say it stabilized just fine and had no issues in that regard.
     
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  18. NewReligion

    NewReligion Well-Known Member Double Platinum Supporting Member VIP Member

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    I will gladly help you out. Make sure to call me first if you will. I am having carpal tunnel surgery Oct 5 2020. I will likely be down 6-8 weeks after that.

    Nut as I stated I will be happy to help you with it if you are still interested.

    Best Regards, David
     
  19. CROWEMAG

    CROWEMAG Active Member

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    Certainly still interested! It was great talking to you on the phone a few weeks back!
     
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  20. NewReligion

    NewReligion Well-Known Member Double Platinum Supporting Member VIP Member

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    The pleasure was mine. Let me know when you’re ready.

    David
     

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