Help identifying this capacitor in my JCM 800?

Discussion in 'The Workbench' started by 1956tvmodel, Jan 23, 2021.

  1. 1956tvmodel

    1956tvmodel Active Member

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    The other night I changed the filter and bias caps in my 1985 JCM 800 2204. I figured after 36 years it might be time. After alot of research and youtube videos I managed to do it correctly and safely without killing myself. The amp sounds amazingly quiet now and the tone sounds alot richer to my ears. While I was inside the amp I noticed this capacitor soldered onto the PCB board. I know its not part of the stock circuit and I was wondereing if anyone knew why it might have been out there. I dont reminder having it done. Its possible I did because Ive had the amp so long but cant recall. Thanks Mike
     

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    Last edited: Jan 23, 2021
  2. neikeel

    neikeel Well-Known Member

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    Cathode bypass cap over the second stage 10k resistor) - not stock will add some gain at certain frequencies.
     
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  3. thetragichero

    thetragichero Well-Known Member

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    big old cathode bypass cap on the cold clipper if that's r6. pretty dumb, kinda lose the whole 2204 vibe with it in
     
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  4. South Park

    South Park Well-Known Member

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    Not the right cap remove it . That cap is for power filtering it is not hooked to ground .
     
  5. tschrama

    tschrama Well-Known Member

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    Carefull now. It's a 25Volt cap.. so surely NOT a power filtering cap, at least not for tube amps. .. (and it is connected to GND it seems ;) )

    Leave it as is, if you like the sound.
     
  6. 1956tvmodel

    1956tvmodel Active Member

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    Thanks for the info. Kinda curious , could this be the reason the amp is super gainy on the high input channel? Almost harsh sounding? Ive been putting a lower gain 12 AT7 in V1 just to make it more usable. Thanks
     
    Last edited: Jan 23, 2021
  7. AndyD

    AndyD Active Member

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    Totally agree with Neikeel. This would indeed add gain. Agree with the others.......remove it!
     
  8. 1956tvmodel

    1956tvmodel Active Member

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    Well that explains why the bright channel was so gainy all these years. It was almost unusable.
     
  9. tschrama

    tschrama Well-Known Member

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    If it's too gainy for your liking... try a 10k-100nF across that 10K instead of the 47uF. Or a 2k7-220nF, which is what the Custom Audio Electronics 3+ SE and Bogner XTC uses.
     
  10. 1956tvmodel

    1956tvmodel Active Member

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    Thanks guys! So I just clipped out the capacitor and Wow the high channel sound way less harsh and gainy like im assuming the amp should sound like. After all these years of running pedals into the low gain input because the high gain input was so harsh it was basically unusable and all it was was this one capacitor some one must have put in at some point. Not sure why it was put there. I guess it helps to know a little bit about servicing the amp. Even now with the proper AX7 its just a nice amount of crunch. My 2204 is reborn!
     
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  11. cruisemates

    cruisemates Active Member

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    I also recall reading about this mod - that it rendered a lot more gain. I never tried it though. I now have all the gain I want with mostly stock 2204 parts (from the high input). I have enough that I don't even miss not having an overdrive pedal.

    It is kind of redundant to aim for getting high gain straight in if you know you will be using an overdrive at some point, though.
     
  12. 1956tvmodel

    1956tvmodel Active Member

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    For literally 15 years I complained about the high input channel on this amp being to gainy and harsh to the extent that thought the amp was broken and all this time it was one small capacitor. I just blew it off and used to low gain channel with my pedals. I did that for 15 years never thinking twice until I got a JCM 800 2203x 100 watt reissue and became reacquainted with the high input channel and how good it sounded. Way better the my 2204's low gain channel and my overdrive pedals. So I finally revisited the high gain channel on the 2204 and hating it more than ever just like always. At this point I was just gonna sell the amp. To get it ready to sell I decided I would change the 36 year old filter and bias caps and while inside the amp I see this this small non stock cap. Took it out and what do you know. A great sounding stock 1985 JCM 800. Now im loving it and use nothing but the high gain input. What a journey. With a happy ending!
     
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  13. cruisemates

    cruisemates Active Member

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    Glad to hear you found the one thing that was your bane. I read about that mod and actually put a 1uf in my shopping cart - then a few days later I couldn't remember why so I took it out. I later heard it was not a great sounding mod and I'm glad to hear you confirm that - we learn something new every day.

    Good job.

    I am in the same process - getting my 2004 back to stock as much as possible. Another thing I have found that causes more havoc than I expected is the treble bleed cap between the wiper and pin 1 of the volume control. Spec is 1n, but some people just clip it out or go as low as 100p. I tried both of those and it made the amp very unstable; microphonic and the tone controls very susceptible to hum.

    I am now at 470p and it seems pretty stable. I would not go back to removing it completely again.
     

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