Fargen Mini Plex + Mighty Plex gain & tone mods

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by BENJAMIN FARGEN, Oct 2, 2004.


  1. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    Hi All,
    I have had many requests for ways to add gain and brighten up older mini plex models.....here you go:

    adding gain to dual input newer models:

    change the first stage plate resistor from 100k to 220k
    change the slope resistor from 56K to 33K

    adding gain to older single input models:

    change input grid resistor from 68K to 33k-10K
    change the first stage plate resistor from 100k to 220k
    change the slope resistor from 56K to 33K

    adding more highs to older single input models:

    add 500pf across volume pot.
    change 270pf cap that feeds the 250K treble pot to 500pf.

    DISCLAIMER NOTE:TUBE AMPS CARRY LETHAL VOLTAGES AND CAN KILL YOU IF YOU DO NOT KNOW WHAT YOU ARE DOING. IF YOU DO NOT UNDERSTAND THESE TERMS OR HAVE NOT WORKED ON TUBE AMPS BEFORE...TAKE IT TO A QUALIFIED TECH TO HAVE THESE MODS DONE. PLEASE DO NOT E-MAIL OR CALL FARGEN AMPS FOR TECHNICAL ASSISTANCE ON THESE MODS.....WE JUST DON'T HAVE THE MANPOWER OR TIME.

    Enjoy!
    BF
     
  2. Axemeister

    Axemeister Member

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    Hi Ben,

    I am awaiting receipt of a MiniPlex that was built in April of '04.

    Does this have these updated mods included at this time of production?

    Thanks
     
  3. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    Yes... all MP amps from 04' include these mods as stock circuitry now.

    BF
     
  4. scottl

    scottl Member

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    Ben,

    I was wondering, if you raise the plate resistor to 220k without raising the cathode resistor, wouldn't that raise the voltage dramatically? How would the tone change if you raised the cathode resistor to 3.3k? I know raising the plate raises gain, but how about raising plate and cathode together??

    Of course, maybe I am off base since I am unfamiliar with your circuit.

    Thanks!

    Scott
     
  5. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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  6. scottl

    scottl Member

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    Hi Ben,

    Have you seen this page?

    http://www.tone-lizard.com/Mods_and_Odds.htm

    They have a table that explains the effects of changing these resistors. Unless I am reading it wrong, they are contradicting you by saying you get less gain by increasing the cathode resistor.

    My brain hurts now!

    Btw, I had breakfast at Namm with your Fargen Amp guy.... Can't remember his name! But cool dude!

    Someday I'll play that Bordeaux or maybe the new VOS...
     
  7. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    Scott,

    I think you are reading his chart wrong........

    This info is pulled directly from the site you linked which is basically exactly what I listed but in less technical fashion.
    Increase value of R1(plate resistor) (typ 100K) Increases gain, to a point.

    Increase Value R2 cathode resistor (typ 1.5K) Lowers gain/increase bias.

    The ultimate tone by Kevin O Conner is a good reference for understanding this type of info
    Take care,
    BF
     
  8. rick66

    rick66 Supporting Member

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    after sending out an email to ben, i just now realize he didn't want anyone emailing him directly on these mods. so, i'll ask the tech people here:

    my question is does the voltage rating on the cap matter for the 500pF recommendations ben listed above? my cap are rated 100V. thanks for the help.

    (and sorry ben for the email)
     
  9. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    No worries:>)

    Yes & No..... your100V 500PF cap will work fine accross the volume pot...there is no high voltage accross the Volume pot

    As for the swapping the 270PF out for a 500PF you will need at least a 300VDC cap...silver mica 500VDC-1KV can be found at most electronics stores and would be the best

    BF
     
  10. rick66

    rick66 Supporting Member

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    thanks for the reply, ben. i'm having trouble finding a silver mica cap you suggested for the swap of the 270 pF. thanks for the clarification.
     
  11. aroman

    aroman Member

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    If I tell that to an Amp tech will he be able to find the first stage resistor and slope resistor without a circuit diagram ?

    How much should I pay an Amp tech to do something like that ?
     
  12. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    Hi,

    I would say no more than a 1 hr bench fee and the parts cost should be under $5.

    Thanks,
    BF
     
  13. calieng

    calieng Member

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    Can anyone offer some suggestions...

    I did the mods on a single input model and now it is too bright...

    Should I adjust the 500pF bright cap value (maybe 120pf)??? Or the 270pF cap I changed on the board to a 500pF that feeds the treble pot??? Or just remove the bright cap and leave the 500pF on the board alone???

    I want to get about halfway between how the amp was and how it sounds now.

    Thanks for any help.
     
  14. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    Remove the bright cap accross the volume first.

    Regards,
    BF
     
  15. calieng

    calieng Member

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    ...yep...that's right on the money.

    Thanks Ben!
     
  16. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    No problem....glad I could help.

    BF
     

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