Bias jcm 900 2501 and what to ask

Discussion in 'The Workbench' started by mrjones2004x, Dec 6, 2021.

  1. mrjones2004x

    mrjones2004x Active Member

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    My amp is at Marshalls for a service and got a call today telling me what is required.
    All pots and jacks to be replaced which I kinda knew and hoped they’d do and also the power valves need changing and 2 out of the 3 preamp tubes are bad too.
    I keep reading online that the 900 can’t be biased very hot due to the wrong resister value in R28. Looking at a pic of mine before it went away shows a 56k resister.
    Says this can easily be swapped out for a 47k or even a 39k resister.
    Is this something I should ask Marshall to do?
    I mean I should ask them to bias to a certain percentage? I’m just assuming they’ll go as hot as they can if it’s quite low? I don’t wanna miss a trick just because I never asked them
     
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  2. mrjones2004x

    mrjones2004x Active Member

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    Forgot to say mine has 6CA7EH tubes. EL34 I think.
    Sorry not my strong subject here
     
  3. tschrama

    tschrama Well-Known Member

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    Ask for 35mA per tube and a adjustment to the bias circuit if they cannt get there.
     
  4. mrjones2004x

    mrjones2004x Active Member

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    easy and straight to the point answer! Love it. Thank you.
    I wasn’t sure how to word it without trying to tell them how to do there job and looking stupid.
     
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  5. mrjones2004x

    mrjones2004x Active Member

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    Spoke to Marshall this morning and he said C15 was faulty and the bias is set at 35mA. All tubes changed, pots and jacks also.

    mill collect in Friday morning and see if I can hear much difference.

    I said it was definitely cold biased before and wondered if the bias resisters were correct as was common for them being wrong and not being able to reach the correct values.
    He said don’t believe everything you read online :blah::blah::blah:
     
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  6. XTRXTR

    XTRXTR Well-Known Member

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    Some ppl don't like being second guessed, or questioned, they can take it as an insult to their work. Also once they have completed work is not the time to infer you may want them to go back and do some additional work unless you make it clear you will pay additional costs.
    Even if you are not a tech you can still learn what to ask and how things work and then use that knowledge to your benefit. A good businessman would have welcomed the repeat business.
    Of course learning some simple skills would allow you to check your amps out yourself and be in full control over your amps.

    Don't believe techs that say don't believe what you read online. Just be sure of the information, ask questions, double check, cross reference with others here etc. read some amp books.
     
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  7. ricksdisconnected

    ricksdisconnected Well-Known Member

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    thats kinda what he does around here. :cheers:
     
  8. mrjones2004x

    mrjones2004x Active Member

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    definitely willing to learn how to bias my amp myself.
    I’m an engineer by trade and also do loadsa 3 phase wiring and electrical fault finding on works machinery so I understand the safety side of electrics just never made the time to learn about amp circuits and how to bias and what the biasing really means to tube life and performance etc.
    I find it interesting but reading posts can get way technical and over my head so I think books may be a good answer
     
  9. XTRXTR

    XTRXTR Well-Known Member

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  10. JCM-Andy

    JCM-Andy New Member

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    LOL
    As I see it, as long as one can reach the desired bias there is no need to change any resistors. My Mkiii 2500 (dating 1990) has the JMP50B board with a 56k resistor. Had no problem getting into 35mA range with the new tubes (JJ EL34L) or the old Svetlanas (measuring at the cathodes over a 1-ohm resistor). Note – there are different versions of this amp (circuit boards). When you say R28 = 56k I assume you have the JMP50C board.
     
  11. mrjones2004x

    mrjones2004x Active Member

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    Just back home with the amp. I had a bad cap which was messing with my bias apparently. C15 was changed. He managed to get to 35mA he said and no changes were made to resisters. Yes my board says JMP50C.
    I had one power tube really worn out as drawing way to much current so they were mismatched. New power tubes were fitted and v2 was knackered too.

    I assume the different boards have different layouts of components?

    All pots and jacks were changed as well during service
     
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  12. JCM-Andy

    JCM-Andy New Member

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    That C15 is known to fail, as can be found “online” :p

    Yes, there looks to be some (minor) changes between JMP50B and C, when comparing what is in mine to the schematic of JMP50C, and mainly resistor values. The only JMP50B schem I have found is showing the 100W version...

    Anyways, yours seems to be all good now! :)
     
  13. mrjones2004x

    mrjones2004x Active Member

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    do you have a 2501 schematic that isn’t all blurry? If I tell you my board types?
     
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  14. JCM-Andy

    JCM-Andy New Member

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    Unfortunately I don’t, just have same blurry version, with preamp (JMP51B) and Poweramp (JMP50C), from thetubestore.com. Especially the preamp scheme is a bit difficult to follow and around V1b specifically. Someone has clean it up as shown on page 3, looks correct but I am not 100% sure – still need to trace it and check against my amp/board.
     
  15. XTRXTR

    XTRXTR Well-Known Member

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    Last edited: Dec 14, 2021
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  16. XTRXTR

    XTRXTR Well-Known Member

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  17. JCM-Andy

    JCM-Andy New Member

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    @XTRXTR - Many thanks, much better schematic!
    :coffee:
     
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  18. cozmacozmy

    cozmacozmy Active Member

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