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JCM2000 to JCM800 2203 conversion !

Discussion in 'Shock Brother's DIY Amps' started by nasdak, Feb 3, 2019.

  1. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    So, here it begins!
    the project : reusing all i can salvage from a broken JCM2000 to make a nice JCM800 2203 :D
    I keep the head, the chassis, the OT and PT, and all the rest goes to junk

    But it's not that simple...

    Bias : my PT ha a bias tap in form of 25-0-25 V ; so i need to make my own bias board - a 2203 has a 90V bias tap
    But that should not be too hard.


    Center tap : my PT has no center tap. So i'll have to wire it like a jcm800 and not a JMP :

    [​IMG]

    wich leads to Filtering : i have to drill the chassis to fit the infamous cap cans. I don't like drilling. It's scary. So the less cap cans, the best it is for me.
    In the original layout 4 cap cans are used in // and series. the 2 first filter caps join at the center tap.. but i have no CT on my PT.
    And today we can find good 500v capacitors. So can i use only 2 good 100uF 500V cap (or a 50uF 500V cap can) instead of the 4 cap cans ? that would be a relief !
     
    Last edited: Feb 3, 2019
  2. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    Ok, looking at the 2203ri schematics, it has no CT and uses the 4 cap cans for filtering.
    Seems that i'll get way more than 500v at startup so i need 2 caps in series for the first filtering node(s).
    The choice is : 4 2x50 cap cans, or 4 x 100uF axial caps!

    Envoyé de mon Redmi 4X en utilisant Tapatalk
     
  3. Sean Mac

    Sean Mac TDPRI Member

    20
    Mar 2, 2016
    Dublin. Ireland.
  4. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    A drill press *with a vise* and a hole saw works. I've also used a large step drill bit in the drill press to make chassis holes as well. Since I can't tell someone's metal working experience by internet, and because other folks may follow along, deburr those holes thoroughly after cutting. The finger you save may be your own!

    Sent from my Pixel 2 using Tapatalk
     
  5. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
  6. Sean Mac

    Sean Mac TDPRI Member

    20
    Mar 2, 2016
    Dublin. Ireland.
    Hi,

    the earliest superleads used axial cans inside the chassis.

    Qmax punches are rated for 16 gauge/1.3mm and cost less than 20 euro for a 31mm.

    After posting yesterday I measured a 70's Marshall chassis at almost 2mm.

    Chaindrilling and a half round file for that one I think:)

    Axial caps inside definitely the easiest way....
     
  7. Lynxtrap

    Lynxtrap Tele-Holic

    641
    Feb 29, 2016
    EU
    I would not bother with cans, but take the opportunity to place individual filter caps locally on the board grounded with the circuit they serve. At least in the preamp.
    Better performance and solves the real estate problem.
     
    Snfoilhat likes this.
  8. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    Well i'll try the can way ; i managed to drill 1 hole, i must be able to drill the 5 others :)


    About biasing : so my PT gives me 25-0-25 AC for biasing.
    2203 schematics use a 90V tap.

    I can rebuild the bias as it is in the JCM2000 shematics but it uses a lot of capacitors (2x 22uF before the 4 rectifynig diodes, then 2 others 50uF smooting caps)
    Is there a simpler way to get the -40 / -50v DC range i need ?
     
    Last edited: Feb 6, 2019
  9. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    Here's the jcm shematics:
    [​IMG]

    Envoyé de mon Redmi 4X en utilisant Tapatalk
     
  10. Sean Mac

    Sean Mac TDPRI Member

    20
    Mar 2, 2016
    Dublin. Ireland.
    A bridge rectifier with 50 volts into a cap/resistor/cap filter should give you 70 volts.

    Leave the centre tap unconnected.

    The schematic you posted is essentially the same idea....
     
  11. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    yeah... but do i really need the 4 caps ?
     
  12. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    i mean, would that simple circuit be okay : center tap to ground, 25VDC taps connected to only 2 diodes
    [​IMG]
     
  13. Sean Mac

    Sean Mac TDPRI Member

    20
    Mar 2, 2016
    Dublin. Ireland.
    The bias is negative or "minus" to the ground.

    The caps on the input isolate the transformer.

    The circuit you drew will not develop a sufficiently negative voltage.

    Marshall are kind of famous for keeping their production cost to a minimum so I think you will find it impossible to do things any cheaper than they did.:)
     
  14. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    Thanks!
    You'e right, the input caps are here to isolate the circuit from the transformer, because there's another output from this same winding to power AOPs in the jcm2000. I don't need that in the 2203 so i don't need thoose input caps !

    As for négative voltage : why would the 2 diode schematics with grounded center tap provide less négative voltage than the 1st schematics with 4 diodes ?

    Envoyé de mon Redmi 4X en utilisant Tapatalk
     
  15. Sean Mac

    Sean Mac TDPRI Member

    20
    Mar 2, 2016
    Dublin. Ireland.
    I'm guessing AOP's are opamps au Francais:)

    When you ground the centre tap you are cutting the input voltage in half.

    The bridge gives input voltage X 1.4 and uses two diodes at a time.

    You have drawn a fullwave rectifier which also gives input X 1.4 but the input is halved. Incidentally it uses one diode at a time and is not as efficient as the bridge. This won't matter to you here because bias current is very small.

    The bridge will give you a nice low impedance bias supply.

    Hope this helps...:)
     
  16. nasdak

    nasdak Tele-Meister

    226
    Mar 22, 2013
    france
    why not simply that ?
    if it's safe to leave the CT unconnected and ground one lead of the secondary, , the other should give me 50vAC ? that's 65DC rectified :
    [​IMG]
     
  17. Sean Mac

    Sean Mac TDPRI Member

    20
    Mar 2, 2016
    Dublin. Ireland.
  18. gusfinley

    gusfinley Tele-Holic

    504
    Jun 11, 2014
    SLC, UT
  19. gusfinley

    gusfinley Tele-Holic

    504
    Jun 11, 2014
    SLC, UT
    If you're going to do half wave recitfication, added some more filter in to smooth out the power supply. Take a look at the Marshall 50W model 1987 schematic.
     
  20. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    807
    Apr 8, 2016
    Oakland, CA
    I finally came across a photo showing an alternative to Marshall-style multisection capacitors outside the chassis. Instead of cutting the standard 1.375" or 1.5" holes to clamp the caps with their terminals in the same plane as the tube sockets, this HiWatt chassis shows that if you have enough space, the same style of multisection cap and clamp can be mounted inside laterally with the terminals floating in space. Is the JCM2000 chassis very large, like a JCM900? This might fit.
    HiWatt_chassis.jpg
     
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