Bassman 2204-style modding

Discussion in 'Shock Brother's DIY Amps' started by sizzlemeister, Sep 1, 2019.

  1. sizzlemeister

    sizzlemeister TDPRI Member

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    Hola! I took inspiration on a re-design of the Bass channel of my Bassman from @robrob, Hasserl, the amp-guy who used to write for Premier Guitar and this guy:



    If anyone is interested in going through my chicken-scratching to check the concept, I would appreciate it.

    Basically, the objective is to revise the Bass channel on a 68 SF Bassman (I can't remember the circuit variant, but I had already worked on Normal channel several years ago so it's not quite stock).

    The goal is a 2204-esque channel utilizing the unused part of V2 as a Cathode Follower and re-routing the tone stack after it ala Marshall.

    Marshall schematics have 470k mixer resistors between stages but Hasserl left them out in a schematic I have from him. Curious what people think of that, specifically the omission of a 470k feeding V1b after the 1M gain pot (I am intrigued at the possible extra gain).

    Also, sort of torn whether to add a PPIMV or not. If I do it, I wouldn't want to do anything but the Type 3. I guess it's easy enough to test, but I am interested to know what people think of the Type 3.

    My particular amp was a "rescue" so it does have several holes drilled out in the back that I covered up when I un-modded it. I can use those for Gain, Presence and PPIMV pots.

    Fortunately, the front has been left unmolested and I'd like to keep it that way, so don't worry.




    BassmanModsSizzlemeister.jpg
     
    robrob likes this.
  2. sizzlemeister

    sizzlemeister TDPRI Member

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    Hmm, apparently not a popular approach, or I nailed the schematic first time.

    If anyone could answer this question, though, I would appreciate it:

    Is it worth lifting the 470k that creates the local feedback for the last gain stage, more like a mixing stage before the PI of an AB165? How much of a clean-up job is that feedback loop doing? I left that out of my schematic, but it's in the original circuit I'm working with.

    AB165: http://www.prowessamplifiers.com/schematics/fender/bassman_ab165_schem.pdf
     
  3. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I tried something similar and it went sideways.

    I’m thinking about rewiring the entire thing with a tweed or blonde Bassman in one channel and a BF Bassman in the other with a PPIMV at the end of the preamp.

    So I’m following this thread. Maybe it will inspire me to get back at this project.
     
  4. sizzlemeister

    sizzlemeister TDPRI Member

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    In what way?
     
  5. sizzlemeister

    sizzlemeister TDPRI Member

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    I guess I'll just keep posting and at some point someone will take pity on me and respond.

    I have a question about Valve Wizard's proposed mod to cathode followers.

    http://www.valvewizard.co.uk/dccf.html

    At the bottom, Valve Wizard states:

    "It is very common to see a DC-coupled cathode follower in a guitar amp, often before a tone stack. The traditional arrangement suffers from the problem that on start up, the grid will immediately rise to HT potential while the cathode is still cold, at ground potential. It is not unknown for this to cause arcing between the electrodes, and rapid destruction of the valve, even when a standby switch is used..."

    And then proposes adding a resistor and diode to prevent this.

    In my build-up to a Bassman modification and looking at various CF/tone stack circuits, mainly in Marshall and Marshall-inspired schematics, I see nothing like this pair added.

    The question is: why? Is the issue that Valve Wizard describes overblown?

    Ain't no thing to add it to the circuit, but since I see it nowhere but on his page, and it doesn't seem to be discussed at all beyond this paragraph on that page, I'm curious why.
     
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