Rectifier Mod For Jcm800 2203 Ri

Discussion in 'The Workbench' started by tman, Sep 14, 2018.

  1. tman

    tman Well-Known Member

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    Hey any Marshall techs out there is it possible to change the rectifier in a JCM800 RI 2203x from SS into a valve driven rec?? And if so how? Thank you
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    For a 100 watt power amplifier it will require two rectifier tubes and a transformer to power their filaments.

    But why?
     
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  3. tman

    tman Well-Known Member

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    Well I love the 800.But I like the feel and the sag of a tube rectifier
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Installing fresh F&T caps + a nice set of preamp tubes would make a noticeable difference you could hear.
    Then a PPIMV could be better if the stock one is a normal master volume.
    That's an upgrade.

    It would need a different power transformer with a 5 volt filament winding to run a rectifier tube.
    And even that would not power 2 rectifier tubes.
     
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  5. danfrank

    danfrank Well-Known Member

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    It will actually need a new power transformer if done the traditional way, as the stock PT doesn't have a HV winding with center tap and won't support a 2 diode full wave rectifier like a 5U4 or GZ34.
    BUT, You can get the rectifier sag by placing a 5U4 rectifier after the diode bridge and first filter cap of your 2203. This way, you will need a separate 5 volt 3 amp filament transformer and this will be marginal at best because the 5U4 will be pushed beyond its limits.
    Easiest way to do it is to get an original 2204 and a GZ34 and extra 5 volt filament transformer.
     
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  6. tman

    tman Well-Known Member

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    Yeah I dont think I want to go through the adding two rec tubes etc.. But maybe a way to mimic that "sag" feel. How about a resistor of some kind after the bridge rec in that area or something.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Actually the amp already has a fair amount of sag, that's what makes it sound good.
    If you take the sag out of a Marshall it looses the unique tone, and sounds sterilized.
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    http://www.londonpower.com/sustain-sag-kit-selection
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The should call that the Saget modification.
     
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    By the time you install two 5U4 or especially two 5AR4 rectifiers the so called sag will be reduced dramatically. Of course its still not near solid state rectification but much closer.
     
  11. charveldan

    charveldan Well-Known Member

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    [​IMG]

    Nothing like a little sag ...
     
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  12. ampmadscientist

    ampmadscientist Well-Known Member

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    "Well I love the 800. But I like the feel and the sag of a tube rectifier..."

    Reality is that (sadly)
    it's not the rectifier tube that produces the sag. (But it never occurs to those who read the advertising)

    Although, vendors who sell rectifier tube amps want you to believe it is.
    And many players who read this advertising believe what they are told.
    And they buy "rectifier" amps because they believe the advertising.

    Rectifier tubes can only use smaller size capacitors. Rectifier tubes are limited in surge current capacity....therefore smaller capacitors which have smaller surge current.

    The size storage capacitors for a rectifier tube are much smaller than solid state, and therefore sag quicker / easier compared to the larger caps used in solid state amps.
    The smaller capacitors run out of stored energy quicker....producing more and earlier sag !

    I think the truth is that: Rectifier tubes VS solid state rectifiers - do the same exact thing.
    You are attributing the effect of smaller capacitors to the rectifier tube (because the advertising told you to do so....and you believed it.)

    Of course the tube is more nostalgic and romantic. You would like to think that the tube has some magical property.
    But...not really.
     
    Last edited: Sep 15, 2018
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  13. tman

    tman Well-Known Member

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    Would it be worth trading my 2203x for a JTM45 1989 RI?
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The reality is that the system produces what people consider sag. Yes the capacitor power supply input for a tube rectifier does have limits which requires lower filter capacitance but the tube rectifier also has resistance and reactance. The resistance value at 450VDC for a 5U4 is between 60 and 70 ohms. As the power stage is pushed and plate voltage drops the internal resistance also drops.

    Only bassier frequencies require a reservoir of power for surge currents. People with guitars have not that much to worry about.

    Example: the famous Bassman, one 5AR4, input capacitance is 40uF. I have never had my Bassman loose steam not to mention it was intended for for for BASS.

    And that is why you have dual and triple tube rectifiers, to increase power supply input capacitance and lower power supply plate resistances which in turn handles more current.
     
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  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Oh yes, something else to add is that you will usually see a choke in the B+ rail with use of tube rectifiers. This helps the voltage to hold. The 2203 has one.
     
  16. Chris-in-LA

    Chris-in-LA Active Member

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    You mean with solid state rectifiers?
     
  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    No. While a choke may be used with solid state rectification you will often see a power resistor in its stead. There always seems to be a choke in line when tube rectifiers are used.

    I either case the choke helps to hold voltage more steady.
     
  18. tman

    tman Well-Known Member

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    So if I remove the choke and place a resistor in its place it would help to drop some voltage and give me more "sag" like feel?
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Yeah, maybe, I am not sure how to answer. Everyone seems to have different opinions and descriptions for sag.

    Its different and it depends what you like. The choke tries to hold the voltage up (once the magnetic field is induced) but it still gives. The power resistor drops voltage no matter what.

    Maybe this will help or confuse, :). I see rectifier tubes and chokes providing reverse sag on demanding transients. The resistor will basically drop voltage in a linear fashion.
     
    Last edited: Sep 16, 2018
  20. tman

    tman Well-Known Member

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    I had a amp tech tell me to increase the choke for "sag" from stock to something higher.
     

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