Update: JCM 800 hum... time for new tubes and caps?

Discussion in 'The Workbench' started by GuitarIV, Jan 23, 2020.

  1. GuitarIV

    GuitarIV Well-Known Member

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    Was playing my 1982 2203 yesterday, suddenly the notes startet to fade, they were still audible but lost some volume and low frequency hum start swelling up. I instantly walked to the amp and turned it off.


    I bought it from a tech who told me he had reformed the original filter caps, but the amp is completely stock including the tubes which are Yugoslawian EIs.

    Now I know that original filter caps are a disaster waiting to happen and I knew one day I'd have to change them. I am afraid to turn the amp back on, I'd rather have a tech swap caps and maybe even pop in some new tubes.

    Just curious what it could be? As said the notes suddenly became weaker and a low frequency hum started appearing, first quiet and then it gradually got louder, it felt like it was overtaking the notes volume. Hard to describe. It all happened maybe in the matter of 10 seconds before I walked over and flicked the standby and power off.

    @ampmadscientist

    Any clues?

    Thanks guys!
     
  2. MickeyJ

    MickeyJ Well-Known Member

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    could be your neg bias failing, could be a preamp tube, could be a cold solder joint, could be a filter cap.
    you did the right thing shutting it off.
     
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  3. MickeyJ

    MickeyJ Well-Known Member

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    If it was here, I'd pull all the tubes out and get a meter onto it.
    I'd measure pins 5 output tubes for bias voltage, id make a voltage chart for all the tube sockets.
    ( I wouldn't do any of that first, first, i'd take the back panel off and switch the damn thing on and watch for redplating tubes)
    Yes, where was I? ah yes I'd make a voltage chart, and if that all checked out then i'd put all the tubes back in where they came from and start swapping pre-amp tubes, then I'd swap a known good set of output tubes in it.
    Then, I'd start sniffing around filter caps.
     
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  4. Seanxk

    Seanxk Active Member

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    This might be worth a try before you or a tech switches it on again, so no power connected, check continuity of standby switch by switching it to on position, check both sides of it, it should be under 1 ohm, if it's not it's full of gunk and needs replacing or cleaning.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Did you change the preamp tubes? (buy 1 new one and swap them out)
    are the output tubes turning red? (failure of bias voltage)
     
    Last edited: Jan 23, 2020
  6. Seanxk

    Seanxk Active Member

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    A big time red plated EL34 as used by a '' monster from the Id '' and another good one. IMG_3407.jpg
     
  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    It’s “completely stock including the tubes”? Try new tubes first.

    Did you recently buy it from that tech? If so, take it back and have him fix it.
     
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  8. South Park

    South Park Well-Known Member

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    I would say it is power related . All filter caps replace . If you have some spare tubes try em . You get the caps on eBay not a hard fix
     
  9. johan.b

    johan.b Well-Known Member

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    I'm with MickeyJ... I'd say Bias supply filter caps...
    J
     
  10. GuitarIV

    GuitarIV Well-Known Member

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    I don't wanna cause any damage to the amp, I love the damn thing too much so I'm afraid if I turn it on something might blow...

    I don't feel confident enough to poke around in a tube amp as I have no experience whatsoever. I sure would like to learn one day but on something less valuable and possibly smaller. Thanks for the pointer though!

    Haven't done anything so far. Just contacted a tech I know who unfortunately had surgery on his eyes recently so he's not available for now.

    I just played the amp as I always do (as quiet as these go, I have neighbors), didn't see what the tubes were doing. The only odd thing was the swelling hum and the notes decaying, losing volume.

    Got the amp 2 or 3 years ago, bought it used. It looks good on the inside, he serviced it before he sold it.

    In that case completely recaning the thing should be the fix I guess?


    This was the amp when I got it:

    IMG-20171224-WA0019.jpg IMG-20171224-WA0022.jpg IMG-20171224-WA0023.jpg IMG-20171224-WA0024.jpg IMG-20171224-WA0026.jpg IMG-20171224-WA0027.jpg IMG-20171224-WA0028.jpg IMG-20171224-WA0029.jpg IMG-20171224-WA0030.jpg


    I will take it to a tech as soon as money isn't as tight as it is now towards the end of the month. Just trying to get some insight by you knowledgeable and helpful folks as to what may cause these symptoms :)
     
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  11. MickeyJ

    MickeyJ Well-Known Member

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    tip- make sure those filter caps stay in the amp if they're ok. tell the tech you wanna keep them even if he takes them out.
     
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  12. Matt_Krush

    Matt_Krush Well-Known Member

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    All and any parts...even if the tubes are shot...
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Oh I remember now.
    Plug into the Low input, does the problem go away?
     
  14. Exotic

    Exotic Active Member

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    what do u keep em for?
    i got my jcm 800 pretty much dead ,so the teck changed a resistor or something ,and swapped out a pair of tubes ,cleaned the pots .
    my jcm 800 lost distortion so the tech retubed it ,cleaned the pots.
    ,y jcm 800 developed a howl ,so the tech changed the caps ,cleaned the pots .

    ps he's lucky? it has flying leads? is that how u say it? i got a solder on the pcb board pots : (
     
  15. MickeyJ

    MickeyJ Well-Known Member

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    1. stops the tech from keeping them
    2. one day, if a person keeps playing, they wake up and wonder" why the fuck did i throw them away". New caps take away the old sound.
    If you can't hear the difference now, doesn't mean you always won't.
     
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  16. neikeel

    neikeel Well-Known Member

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    Make a diagnosis, then fix.
    Don't shotgun parts randomly.
    That hum oscillating in and out sounds what an output tube does when it is redplating......maybe!
     
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  17. GuitarIV

    GuitarIV Well-Known Member

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    That was my plan. Even if I only have them stashed away in a drawer, I wanna keep all the original parts

    Do you think it's safe to fire up the amp again? :/

    So basically if I change the caps... the amp will sound different? But they are ticking timebombs, what other options do I have?

    I'll message a different tech I know and see what he thinks. If it's ok to turn the amp on again...
     
  18. MickeyJ

    MickeyJ Well-Known Member

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    don't believe everything you read.
    What people do unfortunately is tell people to do what they've done, even though they don't understand correctly why they did it themselves, or because they have OCD.
    They're not ticking time bombs, they're ticking tone bombs.

    I saw this ebay ad recently where this idiot advertised his output transformer for sale from his vintage sixties amp, and he stated in his ad that he'd replaced it with a mercury magnetics transformer, He'd ' upgraded'.
    I actually have a stash of vintage sixties transformers from people who have 'upgraded' from that company that I've bought off ebay when those ads first bit people years ago.
     
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  19. Nik Henville

    Nik Henville Well-Known Member

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    I wish more folk thought like this...
     
  20. GuitarIV

    GuitarIV Well-Known Member

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    Interesting man... I mean I would never "upgrade" the trannies, these things are heavy as hell in the 2203 and I'm pretty sure the old iron make a difference... but the filter caps?

    I mean don't they break down as they age? I'd love to hear more :)
     

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