1989 Jcm 2204 Question. Need Some Assistance.

Discussion in 'The Workbench' started by renips, Sep 26, 2017.

  1. renips

    renips Well-Known Member

    Joined:
    Nov 15, 2010
    Messages:
    756
    Likes Received:
    517
    Location:
    Homestead, Florida
    [​IMG]

    My 1989 JCM 800 originally came with EL34.s. In the very beginning I had 6550.s put in by a tech decades ago when I was not informed about the inner workings of the amp. I am more familiar now but by no means an expert, I am just trying to learn as much as I can. I recently biased my amp. The 6550.s in there are reading 470 volts from pin 3. The wattage of the tube is 35w. Full dissipation (100%) is 74 mv. The highest reading I get on the 6550 is approximately 38mv. It will not go above that.

    I have a question. Looking at the picture above I notice two 5w blocks on the power tubes. Those 5w blocks are not on my 1984 JCM 2204. What are those blocks? Are they resistors? I want to use the correct term. Not being able to question the original tech can anyone tell me what the purpose of those are? Were they original. Could they be causing my low reading?

    Another question. When I tried putting in some EL 34.s the reading I was getting on the multimeter was between 2 and 8mv when adjusting the bias. 8mv was the highest reading. Does anyone from looking at the picture able to tell if something was done in the original conversion to 6550's that is causing the strange reading when EL 34.s are in. I appreciate any guidance. The only mod ever done to the amp was the addition of an effects loop seen in the upper left corner of the picture.

    Thanks
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,951
    Likes Received:
    7,773
    Location:
    Start the reactor... Free Mars!
    THE BLOCKS ARE THE SCREEN GRID RESISTORS.
    They MUST be installed.
    The amp needs a resistor changed in the bias circuit to make the amp bias correctly.
     
  3. renips

    renips Well-Known Member

    Joined:
    Nov 15, 2010
    Messages:
    756
    Likes Received:
    517
    Location:
    Homestead, Florida
    AMS,


    Which resistor and value of the resistor needs to be changed?

    Why do the 6550’s only read at 38 mv max? Resistor?

    Why does amp not read el34? Resistor?

    It does not seem to be reading correctly for either tube. Would the resistor needed be exclusive of tube choice? El34 or 6550 or would there be one resistor to rule them all?

    Thanks
     
  4. wakjob

    wakjob Well-Known Member

    Joined:
    Dec 29, 2011
    Messages:
    5,030
    Likes Received:
    3,224
    Location:
    C-137
    R26... but it looks like somebody has already been messing with it...

    See those two small blue (metal film) resistors right behind the bias pot? Those look like they're tacked on to change the amount of -mV. to the bias network.

    Some other stuff doesn't look right to me either,
    but if it works... run with it.

    P.S. I see you shop at Harbor Freight too. :applause:
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,951
    Likes Received:
    7,773
    Location:
    Start the reactor... Free Mars!
    THE resistor that feeds the bias rectifier diode, my schematic says 150K for 6550.
    Looks as if that resistor may have been changed in the photo.
    PM me your email and I will email you the service manual.
    My schematic does not have the numbers that are printed on the board for the parts....

    Need better close up pictures, I can't see the bias very clearly...on pin 5 of the output tubes, schematic says -30 VDC.
    Check that bias voltage and let me know...

    "Why do the 6550’s only read at 38 mv max? Resistor?"

    Because the bias resistor is set too cold. The resistor must be changed so you can adjust the bias hotter than 38 ma.

    "Would the resistor needed be exclusive of tube choice? El34 or 6550"

    6550 and EL34 need 2 different bias resistors.
    EL34 should be 220K resistor, 6550 should be 150K resistor (on my schematic).
    There is no single resistor that allows both type tubes.

    Measure the voltage on pin 5 of the output tubes and post that.

    "The 6550.s in there are reading 470 volts from pin 3."

    When you adjust the bias hotter, the voltage on pin 3 will go down (decrease).
    This is because the power supply is loaded MORE, the voltage decreases.

    If you adjust the bias colder, the voltage will go UP. Less load on the power supply= more voltage.
     
    Last edited: Sep 26, 2017
    SG~GUY likes this.
  6. wakjob

    wakjob Well-Known Member

    Joined:
    Dec 29, 2011
    Messages:
    5,030
    Likes Received:
    3,224
    Location:
    C-137
  7. renips

    renips Well-Known Member

    Joined:
    Nov 15, 2010
    Messages:
    756
    Likes Received:
    517
    Location:
    Homestead, Florida
    AMS,

    The voltage on pin 5 reads at -46. Here is a close up pic of what I think you asked for

    [​IMG]

    Also Which pin is actually pin #1 in this picture. Is it the one with the orange wire or the one to the left. I just want to make sure. Thanks

    [​IMG]
     
    Last edited: Sep 30, 2017
  8. myersbw

    myersbw Well-Known Member

    Joined:
    Oct 22, 2015
    Messages:
    932
    Likes Received:
    407
    Location:
    SW Ohio
    To the left...the tube's socket key splits the position of pin's 1 & 8. (Which are connected together by the short wire.)
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice