Marshall JCM 800 2204 (1985) gain

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

  1. CROWEMAG

    CROWEMAG Active Member

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    Hey all. I have an 800 that I really like a lot. I have had several over the years that have had more gain (unboosted). I have known good preamp tubes and have tried several of the known 'good ones" since owning this amp. I feel like this one has nowhere near as much stock gain as the others I have had. I want to test the components that effect the gain of the amp and I need help pointing out where I should start. I have safely discharged the caps and have done a full cap job. I just am not 100% familiar with what I should be checking. Hoping you fine folks could help me out!
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    You can start with measuring the voltages and writing down all the information that you find.
    If there is a oddball voltage showing up, it will help zero in on the problems.
    Are you able to measure when the amp is turned on?
    Do you have a volt ohm meter?
    If you are not sure about your ability or safety, you can also take the amp to a qualified tech.
    The PC board is easily damaged by inexperienced soldering and you should understand this before you try to change or repair anything.
     
    Peter McAteer likes this.
  3. CROWEMAG

    CROWEMAG Active Member

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    I am experienced with soldering, just changed the power filter caps and bias caps myself with no issue. I have been doing pedal and guitar repair for almost 10 years. I have a DC volt meter and can test with amp on. I just did not know if there was specific components I could target to ensure they are in spec. thanks for the response.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Make a list of all the tube sockets. Pin 1,2,3, etc.
    V1 is closest to the input jacks.
    Measure DC volts on all the preamp tube pins except 4/5 and 9.
    Measure AC volts on pins 4/5 and 9.

    Measure DC volts on output tube sockets pins all except 2 and 7, and measure AC volts on pins 2 and 7.

    Then report the readings so we can see all tubes, all pins.
     
  5. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Did the amp sound the same before and after the cap job?
     
  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Just asking.
     
  7. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The two components (in any given preamp gain stage) that most affect gain and drive are the plate resistor and the cathode resistor. If the plate resistor value increases, you get more gain. If the cathode resistor value increases, you get more cold clipping. Since the components have a tolerance range of either 5 or 10 percent, variances in actual component values can account for the gain difference, and yet the components may still measure within their tolerance range.
     
  8. RickyLee

    RickyLee Well-Known Member VIP Member

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    Something like that could be relatively simple. It could be one or more resistors that are slightly out of spec. Maybe the gain pot is not measuring up to full 1MEG value. Could be a few things combined together that are giving you that result.

    Are you the original owner from when it was new? Maybe someone changed some parts in its past with slightly different values?
     
  9. johan.b

    johan.b Well-Known Member

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    Some 800's have a cap anode to cathode on the first stage. It tames the high end. Depending on the sound you're after, it could be perceived as less gain... if so, snip one leg and push it gently out of the way..
    J
     
  10. CROWEMAG

    CROWEMAG Active Member

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    It sure did.
     
  11. CROWEMAG

    CROWEMAG Active Member

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    All original. I am not the original owner but I bought it from him and my tech looked her over and says no changes or modifications.
     
  12. CROWEMAG

    CROWEMAG Active Member

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    Where would these components be located? Specifically on an 85 2204? thanks!
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Make your voltage measurements. Don't guess or replace parts randomly.
     
  14. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I would suggest, gently, that if you do not know where the cathode and plate resistors are on the gain stages, then you should not attempt this job.

    This is a job for an amp tech. Knowledge of electronics is mandatory in order to avoid making matters worse. TUBE electronics knowledge.

    You would not want to take your car to a mechanic when you were having transmission problems, tell the mechanic about it, and hear him call his
    helper over and tell him to do the job, only to hear the helper say "Gee, boss, I ain't never fixed no transmission before!", would you?

    The cathode and plate resistors for V1a and V1B will be on the PC board connected directly to the wires leading to the plate and cathode of V1a and V1b.
     
  15. South Park

    South Park Well-Known Member

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    Check the bias on the drivers mite be a little low. Roll some preamp tubes for a start
     
  16. CROWEMAG

    CROWEMAG Active Member

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    I am in the process of learning and do not want to have to rely on a tech and shell out money every time I need something. I am a fully capable person. I know enough not to kill myself or when to bow out if things are past my comfort level. I build pedals and rewire guitars all the time. I know how to treat a PCB as not to damage it. I just need to know where exactly these items are located, which you have provided and I appreciate that! Thanks .
     
  17. Exojam

    Exojam Well-Known Member

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    Have you located the schematic of this amp as of yet?

    Have you made the voltage checks ampscientist asked for?

    @matthewsguitars why not try to assist him instead of pushing him to a tech? Everyone has to start somewhere and I would believe he understand’s that there are deadly voltages inside the amp.
     
  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    This is something you should be able to figure out with information easily available on the 'net. Get the schematic for the amp in question. Identify the plate and cathode resistors on the gain stages. Look for matching value resistors on the PC board that connect to the correct preamp tube.

    That amp should have an Issue 202 version ST1 board. Schematic and layout information for the ST1 board is easy to find. Preamp tube socket wiring diagrams are easy to find. So are the schematics.

    It will be easy to figure it out from this point if you're ready to work inside a 2204.

    V1 is the input gain stages. V2 is a gain stage and a cathode follower. V3 is the phase inverter.

    I'm not opposed at all to helping someone who wants to learn to work on an amp, but he needs to be ready for it. If you can't identify described components from diagrams and the schematic, you're not ready.
     
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  19. Exojam

    Exojam Well-Known Member

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    See now that is what I am getting at.

    We can throw some suggestions out there and also tell him that google can be his friend in a lot of this stuff.

    Than if he puts forth no further effort in finding the information (or at least showing he is trying) and providing the information people requested of him to help try and fix the issue with HIS amp, than we say, take it to someone else.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    That's pretty much it.
    If there are no measurements then the rest of it is guessing.
    Preamp tubes can help a lot and there is a big difference between different tubes.
     
    Exojam likes this.

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