Marshall Bluesbreaker clone in a Fender Bassman Cab

Discussion in 'Building the Classics' started by danfrank, Aug 12, 2020.

  1. danfrank

    danfrank Well-Known Member

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    I bought this Bassman cab on CL a while back and it sat around while I decided what to make it into. I finally decided to make a JTM45/BB combo but with a 15" Celestion Fullback.
    The cab was originally cut for 2 10" speakers but the cab just seemed too big for that. I always wanted to make a 15" combo and this would be perfect for that, so out came the jigsaw!
    Power transformer is an old mil-spec unit with correct specs. OT is from an old 60s PA amp, also with correct windings for a BB. I used WMF film/foil caps which are about the only polyester film/foil types still made. KT66 tubes and a GZ34 rectifier.
    The neat thing about this amp is that since the BB is basically the exact circuit as the '59 Bassman, I put 2 speaker jacks on the back so I can have it BB spec or Bassman spec. Only difference between the 2 amps is OT primary impedance and NFB resistor.
    I also put a standby switch on the front where I can select between tube or SS rectification. IMG_20200812_153745.jpg IMG_20200812_210930.jpg IMG_20200812_153657.jpg
     
  2. RCM 800

    RCM 800 Well-Known Member

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    sweet!
     
  3. pleximaster

    pleximaster Well-Known Member

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    Very nice!!!!

    plexi
     
  4. Chris-in-LA

    Chris-in-LA Well-Known Member

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    No gut shots?
     
  5. danfrank

    danfrank Well-Known Member

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    The original one I took was too big a file size and I ain't opening up the amp again.
    Lol!
    Hopefully this cropped pic will do. IMG_20200812_132217_01~2.jpg
     
  6. StingRay85

    StingRay85 Well-Known Member

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    What kind of board did you use? Not the typical kind of kit build where every amp looks the same. Congrats with the amp it looks stunning
     
  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Yes, very unusual board construction, total custom job.
     
  8. danfrank

    danfrank Well-Known Member

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    The board is 0.125" FR4 fiberglass, basically PCB material but much thicker. Turrets look very nice but frankly, require way too much time and work to install and there really isn't any benefit to using them. I love using E6000 to secure the large components to the board, a little dab goes a long way and the added benefit is that it offers shock/vibration protection to the components. This amp could be put on a shaker table for days and nothing is gonna go anywhere.
    I love using mil-spec surplus, unfortunately it's a lot harder to come by these days. Mil-spec surplus is built better and will outlast anything manufactured today.
    So, in conclusion, there is more than one way to skin... er, I mean build a reliable and great sounding amp.
    Lol!
     
    Last edited: Aug 15, 2020
  9. danfrank

    danfrank Well-Known Member

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    Ok, so I did open it up again. There's always "one more thing" I can do to my builds. Lol!
    A better picture of the "guts" and another pic of the topside of the chassis. The opaque stuff over the power tubes is silicone sheeting. This REALLY helps with protecting the power tubes from breaking. I have seen so many power tubes have their glass crack because of these metal type tube retainers, especially in combo amps because of the vibrations. IMG_20200815_223227.jpg IMG_20200815_223012.jpg The sheeting is cheap and it can withstand the heat from the power tubes without melting. Cheap insurance for power tubes.
    Oh, and a "hidden switch" for changing the PT from 120v mains to 240v mains
     
    Last edited: Aug 16, 2020
    tce63 and Dogs of Doom like this.
  10. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    that one would be good for a video demo, showing how the differences between the 2 different amp styles come out...
     
  11. South Park

    South Park Well-Known Member

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    Good job . So what is the next build
     
  12. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Never mind about the silicone sheeting, that’s minor compared to that big old honkin’ power transformer. What was that originally designed for?
     
  13. neikeel

    neikeel Well-Known Member

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    That looks a great way to use up odds and ends up.
    Very economic method of wiring up the board. Particularly interest me as I have a Vamp 100 (well it's a Johnson made by Triumph but more commonly seen as a Vamp) to restore and it should have cheap paxolin PCBs that are unobtanium. I was going to make new boards and drill to original pattern and hardwire under the board with bus wire?
     
  14. danfrank

    danfrank Well-Known Member

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    Hi everyone, and thanks for the comments.
    I'm going to try to get together with a friend next weekend and do some recordings in his practice space. I want to record a few amps I've built recently plus I have some pedals that I'd like to get on tape. The guy has good mics and has been doing a lot of trial and error to get recording techniques. Hopefully he won't cancel!
    Next build is finishing up a Superlead I've had sitting around that I put to the side for who knows why... This one is courtesy of Pleximaster that he calls "The Beast". I've added a few switches to have more EQ possibilities.
    IMG_20200816_103527.jpg
    IMG_20200816_103504.jpg

    I don't know what the PT was originally off of, probably a modulator of some sort from the military. I can't remember when I came across it. It does have the correct specs though 150ma HV @ 690v, plus several filament windings. A neat thing about it is the primary has taps for 8 different input voltages, anything from 100 volts to 260 volts.
    @neikeel: Yes, I used the component leads to make the connections underneath the board. Hardwired. There's this stuff in the US called E6000, which is basically a type of rubber cement that will glue anything with the added bonus of offering vibration protection to the components when it's cured. I don't know if it's available in Europe but I'm sure there's an equivalent available.
     
  15. neikeel

    neikeel Well-Known Member

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    I see you have gone radical on this one with your transformer rotations and choke position, does having the choke out there help?
     
  16. danfrank

    danfrank Well-Known Member

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    I placed the choke way out on the other end to (sort of) balance the weight in the amp. I did it the same way with a JTM45/100 I built long ago and have had no noise or hum issues. The way the choke is positioned and being far enough away from the first tube and circuit board, doesn't bring up any noise problems.
    What I've learned is that bunching up all the iron in a small space usually creates hum/coupling issues in an amp. Another thing is that the main magnetic flux lines in a transformer radiate parallel to the core orientation. If the PT holes weren't already drilled in this chassis when I bought it, I would have rotated the PT 90 degrees, so it would be oriented like the OT. I've had really good results with the transformers oriented this way. On this chassis the two transformers are far enough apart that they won't cause hum problems.


    Oh, and this is a general observation for anybody who's interested...
    If someone wants to build a low hum/noise amp, DON'T EVER use the chassis as a ground return path. That is absolutely terrible amp building etiquette. Lol! I know Marshall did it this way (and Fender and many others) but it really is bad amp design. The proper way to tie audio ground with the chassis is at ONE location.
     

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