Reverse mods on a 1979 Marshall JMP 100 watt head

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Hiatt1991, Mar 18, 2015.

  1. Hiatt1991

    Hiatt1991 Member

    Messages:
    224
    Joined:
    Dec 2, 2014
    Location:
    AZ
    Hi guys. I recently traded for a 1979 Marshall JMP. It's extremely clean except for a couple mods (half power switch and effects loop). The half power switch seems usable but I'm not happy with the effects loop and would like to take it out of the circuit. It uses 2 of the 4 speaker outputs so the only holes drilled into the chassis are the half power switch and the hole for the 12ax7. Can you guys tell if there is anything else not stock with the amp? I was told by a reliable tech that the added preamp tube is used to recover lost gain from the effects loop. Should I get the amp restored to full original condition and loose the half power switch? Also, how should I go about covering the hole that was drilled for the extra 12ax7 when I get rid of it. Thanks for the help.

    [​IMG]

    [​IMG]

    [​IMG]



    [​IMG]




    [​IMG]

    [​IMG]

    [​IMG]

    Half power switch

    [​IMG]
     
  2. justgtrs

    justgtrs Member

    Messages:
    108
    Joined:
    Mar 14, 2006
    Location:
    Orange, Calif
    just go back to completely stock



    BTW: nice score
     
  3. Hiatt1991

    Hiatt1991 Member

    Messages:
    224
    Joined:
    Dec 2, 2014
    Location:
    AZ
    That's what I'm thinking. I don't really like how cheap the half power switch is along with those thin wires.

    Thanks. I traded an 8 watt head plus $600. Not the best deal but it sounds sweet.
     
  4. wyatt

    wyatt Member

    Messages:
    4,136
    Joined:
    Oct 11, 2005
    Depending on how good the tube-buffer FX loop is (caveat), I would probably just leave it. Many are designed to switch out completely when not used and you would just be paying to money to remove something someone already paid a pretty penny to have added. If a good loop, it neither adds nor subtracts value to remove it. But there are those who need the placebo effect of it being gone.

    If the half-power switch is a pentode/triode switch, then IMHO it's useless anyway.

    As for covering the hole...there are two common choices...

    1) Plastic plugs that snap in from outside the chassis. Possibly available in metal or metallic finishes
    [​IMG]

    2) Metal covers that bolt on inside the chassis
    [​IMG]
     
  5. Hiatt1991

    Hiatt1991 Member

    Messages:
    224
    Joined:
    Dec 2, 2014
    Location:
    AZ
    Gotcha. Well I think I'm just going to get it put back to as stock as possible. I just have no need for the loop because I rarely if ever use effects.

    As for those metal covers, where can I find those? I'll check mojotone and the like places.
     
  6. wyatt

    wyatt Member

    Messages:
    4,136
    Joined:
    Oct 11, 2005
    The bolt ons?
    Mojo...
    http://www.mojotone.com/amp-parts/vacuum-tube-sockets/9-pin-socket-hole-cover-plate

    Weber CP9...
    http://www.tedweber.com/chassis/

    Snap-in? Not sure...but hardware store is your best bet. Something like this...
    http://www.stimpson.com/hole-plugs/standard-hole-plugs/
    http://www.stimpson.com/hole-plugs/electrical-knockout-plugs/
     
  7. Hiatt1991

    Hiatt1991 Member

    Messages:
    224
    Joined:
    Dec 2, 2014
    Location:
    AZ
    Got it. I'll most likely order those bolt on covers. Thank you for your help!
     
  8. wyatt

    wyatt Member

    Messages:
    4,136
    Joined:
    Oct 11, 2005
    You or a friend can probably cut and drill on for less than Mojo and Weber charge for shipping one.
     
  9. RussB

    RussB low rent hobbyist Silver Supporting Member

    Messages:
    11,071
    Joined:
    Jul 3, 2004
    Location:
    CT
  10. walterw

    walterw Gold Supporting Member

    Messages:
    33,373
    Joined:
    Oct 16, 2006
    i couldn't tell, is it an actual buffered loop, or just some sort of patch point wired in?

    a proper loop is damn useful in these amps! even if you don't want to use effects, do you want to play crunchy rhythm and then step out in the mix for a solo? if so, a loop makes that possible, by way of a clean boost pedal in there.

    otherwise, all the boosting in the world in front of the amp just makes the preamp more distorted instead of making the amp louder.

    i have a metro loop in my '78 2203, minimally invasive, totally transparent and bypassable, and makes all the difference for practical live use.
     
  11. Hiatt1991

    Hiatt1991 Member

    Messages:
    224
    Joined:
    Dec 2, 2014
    Location:
    AZ

    I'm honestly not sure. I was told by Lowell @ Hunt Amplification that it was a poorly designed loop. I know the loop is useful but I just don't use any effects. I play classic rock and love the sound of plugging strait into an amp. I might occasionally use a strymon flint Tremelo/Reverb pedal but that's about all.
     
  12. walterw

    walterw Gold Supporting Member

    Messages:
    33,373
    Joined:
    Oct 16, 2006
    me too, but if you love that awesome, naturally crunchy sound of going straight into the high input, you pay a price in flexibility. reverbs and delays in front sound kind of obnoxious and in the way, and like i said you can't boost your actual volume for solos.
    a perfect example of a pedal that would sound way better in a good loop, so it was after the preamp distortion.

    when it's in front of the crunchy preamp the trem will just cycle from clean to crunchy, and the verb will be gritchy and annoying; after the preamp (in a loop) the trem will really make the volume go up and down like it should, and the reverb will actually sound good, ambient and spacious.

    (still, if it's just some kind of half-assed patch point instead of a real buffered loop then yeah lose it, it's no good for anything.)
     
  13. RussB

    RussB low rent hobbyist Silver Supporting Member

    Messages:
    11,071
    Joined:
    Jul 3, 2004
    Location:
    CT
    Seeing as the tube socket is already there, I would make a great tube buffered loop out of that not-so-good loop.

    When you run that Flint in the good loop, you'll thank me (and Walter)
     
  14. walterw

    walterw Gold Supporting Member

    Messages:
    33,373
    Joined:
    Oct 16, 2006
    Oh, I totally missed that extra tube socket!

    Frankly that kind of skeeves me out too, I think I'd rather put it back to stock and get the much less invasive metro loop in there instead.
     
  15. Dave M

    Dave M Supporting Member

    Messages:
    3,246
    Joined:
    Aug 4, 2004
    Location:
    somewhere near Seattle
    But, at this point the chassis has been punched and a socket installed. Nothing wrong with a well designed (edit: and well implemented) tube buffered loop since the holes are now irreversibly there.
     
    Last edited: Mar 22, 2015
  16. Hiatt1991

    Hiatt1991 Member

    Messages:
    224
    Joined:
    Dec 2, 2014
    Location:
    AZ
    I might see if there is a way to improve the existing loop. What concerns me is how close the preamp tubes are mounted to each other. Im not sure if it creates unnecessary hum.


    [​IMG]
     
  17. RussB

    RussB low rent hobbyist Silver Supporting Member

    Messages:
    11,071
    Joined:
    Jul 3, 2004
    Location:
    CT

    That's my way of thinking as well
     

Share This Page