>

Marshall Mods - What are your Favourite Mods?

Discussion in 'The Workbench' started by clusterstudios, Aug 2, 2021.

  1. clusterstudios

    clusterstudios Member

    Joined:
    Nov 10, 2008
    Messages:
    43
    Likes Received:
    5
    Hey Guys,

    I built a platform amp based on a JTM45 (KT66) and now I want to experiment with Mods to shape the tone of the amp. More like "Voicing" Mods. I'm curious as to what are your favourite mods. So far I've split the cathode at V1 to make it more plexi like. 0.1µf coupling caps at the output to fatten the tone a bit. 50/50µf power cap and solid state rectifier to tighten the feel. 32/32µf PI and 16µf preamp power cap. 500pf/33k at the tone stack. Put an 12AT7 at PI to tame the harshness. Not really interested in a PPIMV or Cascading the preamp. This amp has more than enough gain for what I do (70's Classic Rock). I looked at the Friedman mods he's done, like the C45 mod (works best with the extra preamp tube HBE). Not sure if it will work with this amp which doesn't have the extra gain stage and isn't cascaded? The Sat mod never sounded good to me. His Fat switch is nice for single coils (easy to do). The one I'm interested in is his structure switch mod. Does any know what that does?

    Mods I'm considering ATM:

    1) Cathode switch at V2 with different Resistor/Bypass Cap combos
    2) Some kind of presence feedback mod on a switch.
    3) ??

    So, I'm pretty slim on ideas. Can anyone chime in on other ideas on ways to voice the amp?

    Thx..
     
  2. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    6,264
    Likes Received:
    5,117
    I am still puzzled why your JTM45 is harsh needing a 12AT7 in the pi.
    Might be worth the extra 10k dropper to v1 and dual preamp filter cap.
    I really like my Dickinson 45/100 with the lead tone stack
     
    _Steve likes this.
  3. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    6,264
    Likes Received:
    5,117
    But the split cathode is not good with harsh amp
     
    NewReligion likes this.
  4. clusterstudios

    clusterstudios Member

    Joined:
    Nov 10, 2008
    Messages:
    43
    Likes Received:
    5
    I can put that on a switch and try it. The harshness is getting less the more I play it. Maybe it's the components burning in or my ears adjusting. Or maybe I'm just use to EL34s instead of KT66s?
     
  5. junk notes

    junk notes Well-Known Member

    Joined:
    Dec 10, 2018
    Messages:
    2,884
    Likes Received:
    3,247
    Mods!? Just wait until I make an SV20H into a 1992! :lalala:
     
  6. _Steve

    _Steve Well-Known Member

    Joined:
    Apr 29, 2020
    Messages:
    522
    Likes Received:
    436
    Location:
    Los Angeles
    I'd recommend changing the preamp filtering from 1x16uF to 32uF-10K-32uF, also like the Dickenson 45/100 @neikeel mentioned above. I actually put mine on a switch, but it just stays 100% in the 32uF-10K-32uF configuration as it's hands-down just better to my ear. Also the lead-spec tonestack @neikeel mentioned I also feel is generally an improvement, although i am glad I put that one on a switch as I quite often switch back to the JTM45/Fender spec for a more vintagy feel now and then. FWIW I also much prefer the shared V1 cathode vs the split one.

    Personally I would stay away from all the Friedman, Bogner, Jose etc mods. IMHO they're all geared towards more, and/or 'smoother' gain which to my ear just make the amps sound more American vs British. I just don't understand the motive?

    Here is the Dickenson 45/100 schematic fyi. This is my favorite amp:

    HendrixAmpMod.gif
     
    Stephen H and neikeel like this.
  7. clusterstudios

    clusterstudios Member

    Joined:
    Nov 10, 2008
    Messages:
    43
    Likes Received:
    5
    Thanks so much. I will do that. I already have 2 Dickenson 45/100s in my collection. One is the official Marshall Hendrix Stack and the other is one I made myself about 7 or 8 years ago when Metroamps was supplying all the right parts. I has the higher Voltage PT.

    This one could be a 50 watt version of that running the 2 KT66 tubes. Attached is my edits as to how my amp is now compared to the Dickenson version. The only changes I need to make are the 470k->270K mixing resistors; 16µf -> 32/32µf preamp cap and then to a single cathode at V1. I'll keep the 50/50µf power cap. My tone stack matches the Dickenson already.

    Question: What is the reason for using 6 diodes in the power section (3 on each side)? Are these back up diodes?

    HendrixAmpMod-with-edits.gif
     
  8. XTRXTR

    XTRXTR Well-Known Member

    Joined:
    Feb 14, 2021
    Messages:
    343
    Likes Received:
    304
    • Tone cut switch - lifts the stack from ground creating a boost.
    • You could place a three way switch on V1, center would be as you have it, then either side play with resistor values, even an LED on one side(if the cathode voltage is right it can bypass the center) The Center will be in parallel to what ever you put on either side.
    I have a 3 way switch on V1b and another on V1a for all kinds of fun on my 2204 - I know its a different amp but you wanted ideas. Rob Robinette has some screen caps and resistor values that I plan on toying with when I'm done with all my voicing mods, just to see whats what. Also may try messing with a NFB switch.
     
    clusterstudios and _Steve like this.
  9. _Steve

    _Steve Well-Known Member

    Joined:
    Apr 29, 2020
    Messages:
    522
    Likes Received:
    436
    Location:
    Los Angeles
    I think so yes. If they intended to share the load across them I think they would have needed resistors across them.

    Another mod you might be interested in is adding reverse biased Zener diodes in series between the screen resistors and the B+ to drop the screens by around 40-60V. I haven't tried it on KT66s, but on EL34's it makes it more punchy, less squishy. I don't fully understand why to be honest.
     
  10. clusterstudios

    clusterstudios Member

    Joined:
    Nov 10, 2008
    Messages:
    43
    Likes Received:
    5
    You mean between pin 4 and 3?
     
  11. _Steve

    _Steve Well-Known Member

    Joined:
    Apr 29, 2020
    Messages:
    522
    Likes Received:
    436
    Location:
    Los Angeles
    No, the zeners just insert in-between the screen resistors and the B+.

    I think you are using pin 6 as a terminal for the screen resistors so it would look like this:
    20210803_153355.jpg
     
  12. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,306
    Likes Received:
    1,255
    Location:
    flo rida
    are you talking about about the rectifier? there's three per transformer secondary tap because the peak voltage would/could exceed the rating for a single diode
     
    neikeel and _Steve like this.
  13. clusterstudios

    clusterstudios Member

    Joined:
    Nov 10, 2008
    Messages:
    43
    Likes Received:
    5
    Thanks..
     
  14. NewReligion

    NewReligion Well-Known Member VIP Member

    Joined:
    Nov 17, 2009
    Messages:
    11,655
    Likes Received:
    9,036
    Location:
    Paradise
    Hello.

    You spoke of a Presence/NFB mod.

    1.) You can run the Violet NFB wire out to the center of a SPDT On/On switch. On one side add nothing (or a 47k) then to the 4 or 8 ohm impedance tap then place a 220k on the other side of the switch to 16 ohm impedance tap. This will tighten, brighten and add more NFB increasing volume as well. Try disconnecting the NFB from the presence. This will reduce the over all volume but give you a great idea of just how significant NFB is. Ken Fischer (RIP) of Trainwreck Amplification often used no Presence/NFB especially when using certain types of PPIMV’s as this essentially removes NFB from the circuit anyway.

    2.) Consider adding a 1/2 watt CF 1.5k - 5.6k (stock on Super Leads) swamp resistor on power tubes pin 5 for more low end in power section. (NOTE: any higher it may begin to sound more like a Fender or Boogie)

    3.) Perhaps paralleling another 47pf to the cap between the PI 82k & 100k (aka fizz cap).

    4.) Presence Cap between .1uf to .68uf.

    5.) Parallel a .68uf across the V2a Cathode resistor.

    6.) Experiment with Grid Leak & Grid Stop resistors.

    The possibilities are endless and all ideas listed on your thread are great for learning how to manipulate the circuit.

    Hope you have fun. And keep us apprised of what you tried and your thoughts as will are interested in your findings.

    David Hopkins aka New Religion
     

Share This Page