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Bassman Micro - My first build

Discussion in 'Shock Brother's DIY Amps' started by recoveryroom, Nov 8, 2018.

  1. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    Hi All:

    I joined recently after deciding to build a Bassman Micro head as a winter project. https://robrobinette.com/Bassman_Micro.htm



    Since there are several build threads here and the designer participates, it should increase my chances for success. I've built a few pedals, done minor amp mods, and switched pick ups/electronics on guitars, but this is obviously beyond my experience.


    One of the first decisions I need to make is the placement for the transformers and choke. Here are a few pics with different layouts. Since this is a low watt amp, placement may not be a big deal, I'm not sure.

    [​IMG]

    [​IMG]




    I'm using a marshall chassis, so the inverted circuitboard will be tricky for me. I'll post pics as I go along, and I'm hoping anyone seeing anything concerning chimes in.


    I'll follow robrob's instructions as closely as possible, and use his first start up procedure.


    This will be a nice piece of equipment for me, I can run it through my computer's audio interface, a cab or frfr at home, and run it into a pa or a larger cab for a performance.



    Thanks in advance for your help, I wouldn't be comfortable doing this without a resource like tdpri for help/advice.
     
    bftfender likes this.
  2. sds1

    sds1 Tele-Holic

    Age:
    42
    811
    May 4, 2017
    Orlando, FL, USA
    So is that a PT / OT / and choke? I think it is but I'm just confused by the PT cutout. Are you repurposing a chassis here or something?

    Anyhow, typically the choke would immediately follow the PT. I reckon that works out better layout-wise for a number of reasons.

    However, in this case it is a little concerning to see your OT so close to the preamp. Like you said, maybe for a low power amp this is less of concern.

    That cutout is bummer, it's eating up a lot of that layout space!
     
  3. aerhed

    aerhed Tele-Afflicted

    Age:
    59
    Dec 24, 2016
    Boulder, WY
    Yup. I'd place the PT way over left near the corner where everybody puts them (for a reason). Stick a cover plate over the original cutout.
     
  4. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT

    I went with the Marshall chassis primarily because there’s lots of room. I was wondering about using the cutout, too.
     
  5. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT

    Is there any reason to be concerned about grounding?

    I get the impression I’ll need to take some precautions with an aluminum chassis.
     
  6. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    Nothing to worry about. I'm pretty sure aluminum is a better conductor than steel anyway. The only issue I can see would be if you were planning to solder your ground points to the chassis like an old Fender, but most people these days just use a solder lug on one of the transformer bolts (or a dedicated mounting point if you want to be conscientious about it).
     
  7. aerhed

    aerhed Tele-Afflicted

    Age:
    59
    Dec 24, 2016
    Boulder, WY
    As stated. If going for a separate ground near the inputs I'd use a bolt, star washer and nut. Then a kep nut for the grounds on that.
     
    awasson likes this.
  8. gusfinley

    gusfinley Tele-Meister

    477
    Jun 11, 2014
    SLC, UT
    Get yourself an adapter plate and mount the power transformer near your AC line voltage parts (lamp, power switch, fuse, etc.)

    Here is how I did it on my 50w plexi build:

    [​IMG]
     
  9. sds1

    sds1 Tele-Holic

    Age:
    42
    811
    May 4, 2017
    Orlando, FL, USA
    The adapter plate @gusfinley suggests seems like the right move to me!
     
  10. gusfinley

    gusfinley Tele-Meister

    477
    Jun 11, 2014
    SLC, UT
    In case you couldn't tell this is just an AC double outlet box cover that I picked up at the local hardware store.

    https://www.lowes.com/pd/RACO-2-Gang-Square-Metal-Electrical-Box-Cover/3129183
     
    Nickadermis likes this.
  11. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    yes, all good advice.. exactly what I was hoping for. I'll get a plate and put the pt there.



    Looking at the circuit diagram, it looks like the design has the choke between the pt and ot. On rob's bassman micro page, there are build examples with the choke between the pt and ot, and with the pt/ot next to each other (and close).

    Since these aren't huge transformers, location may not matter much, but I can use the existing holes and do something like this. It looks like the choke lead should be long enough.

    Or I can move the ot over a little (using one of the existing holes), and get the choke in the middle.





    [​IMG]
     
  12. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    And here's the diagram


    [​IMG]
     
  13. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    I picked up a plate for the pt, I think I'll plan to drill with this layout. It'll utilize some of the existing holes.


    [​IMG]
     
    sds1 likes this.
  14. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    Here's a pic of the board placement.


    [​IMG]
     
    robrob and sds1 like this.
  15. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    Got the plate drilled and here's the layout, I'll need to clean up the holes running wire through and the ot holes may need to be opened to get it to sit right.

    That plate on the inside looks like it was done by someone doing their first build on a sunday while watching football and drinking beers. :eek:




    [​IMG]
     
    Tsquared, robrob and Nickadermis like this.
  16. John How

    John How Tele-Meister

    Age:
    67
    374
    Mar 22, 2016
    Auburn, Ca
    And some grommets in those holes...
     
    awasson, Kirshman and sds1 like this.
  17. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    Yes, I was going to wrap some black tape but this is better.
     
  18. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    I'm going to start to prepare for the board work, and this part looks confusing to me.


    There are two circuit layouts, the one in post 12, and this one below, for a marshall chassis. I assumed I'd use the marshall chassis circuit board layout for the 18 watt marshall chassis I have.


    But it looks to me like the post 12 layout (which I assume is for the fender chassis of the period) aligns all the switches/inputs/pots for my chassis.


    Here's a pic.







    [​IMG]


    [​IMG]
     
  19. recoveryroom

    recoveryroom TDPRI Member

    Age:
    41
    72
    Sep 28, 2018
    CT
    Here's the circuit on the 9" board, 1/8" glass epoxy. I initially intended to include the tube/ss rectifier mod. It looks like a tight fit and one of the screwheads on that side is in the way, I guess I could shift it a little.

    I'm thinking maybe sticking to the basic design and getting it stable first, then add this mod and maybe the nfb mod.







    [​IMG]
     
  20. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    recoveryroom, the Fender layout definitely works best with your chassis.
     
    King Fan likes this.
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