marshall mod info needed using cathoder follower as a gain stage | The Canadian Guitar Forum

marshall mod info needed using cathoder follower as a gain stage

Discussion in 'Amp Building/Technical/Repair' started by collector, Sep 22, 2019.

  1. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    hello all , loking for some help via diagram of a gain mod for a plexi circut using the cathode follower as a gain stage with a plate driven tone stack . there is info on it online all over but no one got the wiring schematic for it , was told that bogner has something in there line that uses this setup, i know doug aldrich has his amp modded this way , alot of people just add another tube and use 1/2 its triode to add the stage , but wanted to do this this way as i hear its quite creamy to the ear , 4 stages of gain
     
  2. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    btw for your info the amp is a 50 watt plexi clone 4 holer with el34s , 480v on the plates cascaded input , ppimv master volume , split cathode on v1
     
  3. Jim Jones

    Jim Jones Gold Member

    Joined:
    Sep 18, 2006
    Location:
    Winnipeg
    If you're set on modding the amp that's up to you but with the tinkering I've done over the years I've come to the conclusion that the classic "Plexi" circuit is about as perfect as it's gonna get. If you want more jam there are a million different stompboxes you can goose the front end with. A good chunk of the classic Marshall charm is that clean-but-not-totally-clean sound and I'm pretty sure you'll lose that with an extra 12AX7 stage.

    Again, basically "old man yelling at cloud" unsolicited advice. ;)
     
  4. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    No concerned about the marshall tone as such , but trying new ideas on gain. My amp is cascaded , with a split cathode , , wanted more smoother controlled gain , increased the fizz cap to 100pf, switched the 470 pf on bright cap, have 3 way neg feedback taps 16 ohm/25k—8 ohm/47k—-100k/4 ohm , Also have a 4 position snubber across v1 plates 22pf/47pf/100pf/200pf that works very as do the neg feedback switching . I have a hopkins modded jcm800 with 4 stages of gain with jose modded diode clipping in a 3 pos switch Low diode clipping/ high diode clipping/ 4 stages of gain using the 3 existing tubes and plate driven tone stack , his mods are concealed pretty good , cannot seem to wrap my head around what he has done . Lol
     
  5. Jim Jones

    Jim Jones Gold Member

    Joined:
    Sep 18, 2006
    Location:
    Winnipeg
    Alrighty then, you're more of a gain guy that I am I see. :) I've done lots of messing around with the classic Bassman/Plexi circuit but never in the search for that kind of gain. My idea of lots is getting the voltages on the first tube down between 160-180! ;)
     
  6. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    Thnx for the tip Cheers
     
  7. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    Havent seen a handwired layout of thiis mod anywhere online , i know its possible amd has been used by splawn/ bogner/ . Its a handwired jtm45 that i Want to use for this mod , I have no problem to convert ithe preamp stage to a jcm800 style if necessary .The tone from this is killer 80’s rock plugged straight in ,If anyone can draw up a handwired layout for the basic .
     
  8. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    Anyone wired up a marshall jtm45 this way . Would like some insight on this if possible
     
  9. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    Since you're dealing with a hard wired amp, this may be possible. I can't guarantee results but a simulation shows it should work. Gain will be through the roof though.
    FYI - I've not done this, just simulated the circuit.
    You will need a coupling capacitor, and three resistors. You also need to make sure the tone stack capacitors can handle the voltage. If they aren't at least 400V caps you may need to replace them as well.
    I don't know what your skill level is, so if I ask obvious questions, please don't be insulted. Can you follow a schematic?
    Also are you familiar with safety aspects of this? Voltages are lethal in this amp.
     
  10. jb welder

    jb welder

    Joined:
    Sep 13, 2010
    Location:
    Melonville, Canada
    Dan, just wondering if you simulated driving a tone stack? My understanding is that the cathode follower is used because there is such a great loss of signal when the stack is plate driven. So I would think while the distortion character may change, there should not be much gain increase to be had.
    The marshall circuits that have cathode follower driving the stack do not need the extra recovery triode that is seen after the stack like in circuits such as the 1966. So the marshall circuits that plate drive the stack need the extra triode after the stack to provide the signal increase necessary to drive the PI stage with enough signal to give full output.
     
  11. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    And you would be correct up to a point. The cathode follower versions drive the tone stack much more linearly but there is still a 20dB loss because of the stack. The gain in Marshall amps is very high to compensate before the stack and less gain is needed after it. What suffers the most is frequency response. The amp will become considerably more mid-range oriented in sound quality, and distortion rises both from the added stage, and from trying to drive the stack. The loading caused by the stack is actually not that high because of the 1M pots used.
    I've posted the schematic of the sim. A couple of notes. Ignore the output stage. I took that from another sim to save time, and is irrelevant here. Also you will notice the pots are made up two resistors as pots are impossible to do in simulation software.
    It's basically the JTM45 modified.
     

    Attached Files:

  12. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    Yes i can follow a handwired type layout , like ceriatones’s . Willing to give it a go on what you have in mind
    , love the learning part , , lately on this amp I have tried various cathode cap/ res combo’s with increases in v1a plate res .
     
  13. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    BTW The attached file has nothing in it
     
  14. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    Be careful there. 12AX7 tubes don't like a lot of current. Marshall already runs that a bit hot. They also needlessly use a rather large bypass cap on the V1 cathode.
     
  15. collector

    collector

    Joined:
    Feb 11, 2017
    Location:
    hamilton
    Hey dan , hope all is well with you . Yes i have it cascaded , split on v1a/b 4k5/5uf on v1a And 2k7 /.68uf on v1b , nos pre tubes Phillips 12ax7 in v1/ v2/v3 has nos electrohome organ grade , sounds pretty good in the hvy gain , 27k nfb , but the sound is not as smooth as a 4 gain stage
     

Share This Page