Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

BF Bassman Marshall mod?

Discussion in 'Glowing Bottle Tube Amp Forum' started by JohnnyCrash, Aug 9, 2017.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Silver Supporter

    Mar 12, 2005
    Fullerton, CA
    I'm going back to rebuild a Bassman head and want to do the "Marshall" mod to the Bass channel (and a AA864 circuit's Normal channel).

    Any schematics for this?

    PS
    I think it's also called the "(Dave) Allen mod" or the "Trainwreck mod."
     

  2. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    No insights here, but I remember @Phrygian77 talking about the Trainwreck mod in his recent Bassman thread. When I was looking up his thread I came across his version of a trainwreck schematic -- I'll stick it here in case it's relevant. No idea if this is actually what you're looking for...
     

    Attached Files:


  3. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Silver Supporter

    Mar 12, 2005
    Fullerton, CA
    I'm looking for the mod that makes the Bass channel into a high gain channel with a Gain AND a Master Volume control.

    The Gain control replaces one of the Bass channel's input jacks.
     

  4. Phrygian77

    Phrygian77 Tele-Holic

    655
    Apr 30, 2016
    Crawfordville, FL
    You're talking about doing a pre-PI master for the bass channel only, right? You'd just insert .1uf cap before the 220k mixing resistor, and then a 1MA volume pot before that.
     
    JohnnyCrash likes this.

  5. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Silver Supporter

    Mar 12, 2005
    Fullerton, CA


    Will I need to increase the cap size if both channels are high gain with two MVs?
     

  6. Phrygian77

    Phrygian77 Tele-Holic

    655
    Apr 30, 2016
    Crawfordville, FL
    By the way, I really like my Trainwreck channel. Although, I have no idea how well it would work with a master volume. It drops right into the bassman layout, where as the 5F6A/Marshall tone stack is after the 3rd (or 4th on a 2204/JCM800) gain stage.

    Also, you'll lose about 6dB in the clean channel with the pre-PI master at 0 because of the voltage divider.
     

  7. Phrygian77

    Phrygian77 Tele-Holic

    655
    Apr 30, 2016
    Crawfordville, FL
    If each channel has a tone stack preceding the master volume(s), then you don't need it.
     

  8. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    I think you'd be better off putting a Lar Mar master volume. Then it can work for both channels (and there seems to be agreement that it's pretty much out of the circuit [tonewise] when dimed).

    I uploaded a preamp schem and a layout of my Bassman 10 rebuild which put a Marshall JCM800 channel on one side. Lar Mar MV, too.
    And Kingfan and Phryg, I dowloaded that wreck mod PDF for future use. Thx! :)
     

    Attached Files:


  9. Phrygian77

    Phrygian77 Tele-Holic

    655
    Apr 30, 2016
    Crawfordville, FL
    I'll post the final version of that schematic, since that version was basically a draft.
     

  10. bigben55

    bigben55 Friend of Leo's

    May 19, 2010
    Cincinnati, OH
    I'm not a tech, and couldn't begin to tell you the ins and outs of the mod, but I can say, having had it done to the normal channel of my 66 Pro Reverb, it's BADASS. I keep my controls as such: bright switch off, vol=3, treble=9, bass=7. Think about that. Bass at 7 on any BF Fender I've ever had turns it to mush. On mine, the bass stays tight, evening diming the amp. And it crunches like no BF Fender. I can use it, at 3 with a tele, as a plug and play and ride the gtr vol knob between clean and crunch. Clean at 6-7, overdriven past that. I use an ABY box to switch channels. Makes my amp 1. A plug and play on the normal channel, 2. A pedal platform on the vibrato channel, and 3. A usable channel switcher at gig volumes. I had the verb and trem put on both channels as well.
     

  11. cousinpaul

    cousinpaul Friend of Leo's

    Jun 19, 2009
    Nashville TN
    As far as voicing the amp goes, I used to replace the resistor to ground at the bottom of the tonestack with a 25k mid pot and increase the 250pf treble cap to taste for more upper mid content. IIRC, I also replaced the 820 ohm NFB resistor with a 22k.
     

  12. Phrygian77

    Phrygian77 Tele-Holic

    655
    Apr 30, 2016
    Crawfordville, FL
    A 22k with a 47 or 100 ohm voltage divider going to the 2nd PI input would basically be like having no NFB at all.
     
    robrob likes this.

  13. cousinpaul

    cousinpaul Friend of Leo's

    Jun 19, 2009
    Nashville TN
    Yeah, it was a couple of decades ago... I think that mod may have been replaced with a "soul control".
     

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