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Blackface Bassman Build

Discussion in 'Shock Brother's DIY Amps' started by Phrygian77, Apr 9, 2017.

  1. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Well, a blackface Bassman isn't exactly what I was thinking that I was going to do for my next build, but it's an amp that I've always wanted to have and use as pedal platform for traditional and modern country. I'm probably going to build it as a 1x12 combo. I know vintage AB165s are relatively inexpensive, but this way I get what I want out of it and I'm not modding a vintage amp.

    I'm leaning towards using a ClassicTone 40-18029 PT and using the 610v taps SS rectified with 5881s. The real question is what to do with the bass channel... Liverpool, Plexi, 5F6A?

    IMG_20170407_172756.jpg
     
    Last edited: Apr 9, 2017
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  2. Colors

    Colors Tele-Holic

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    I'm partial to the plexi mod myself.
     
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  3. zook

    zook Friend of Leo's

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    I'd go with a Cathode follower tone stack using the other 1/2 of the unused 12AX7.
     
  4. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I'm leaning towards doing a JCM800 2203/4 master volume preamp. I was a little concerned about putting a .1uf cap between the 1M master volume pot and the 220k mixing resistor going into the PI. With the wiper at ground, that creates a voltage divider with the clean channel mixing resistor. I thought it would probably cause a significant change in frequency response, but when I tried it out in LTSpice, it doesn't seem to have much effect at all, other than -6 dB of overall attenuation.
     
    Last edited: Apr 15, 2017
  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Well this is a bit of a problem...

    1492733956188.jpg

    At least the cap pan matches up.

    1492733985802.jpg
    1492733996660.jpg
     
  6. Bill Moore

    Bill Moore Tele-Afflicted

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    If you flip the cap board will the through holes line up with the holes for the grommets?
     
  7. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Eyelets would be on the wrong side, but yes, it would line up. The screw holes are also slightly off. I could drill the fiberboard, but I'm thinking about just making one, which is really what I wanted to do to begin with. I think I'll throw the fiberboards into the scanner to use as the basis for the layout.
     
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  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The AA/AB165 cap board should be longer to accommodate the extra cap. I don't understand why Mojotone based their main blackface Bassman fiberboard off of an AA165, but their large cap board doesn't work for the layout. I'm not sure what they were thinking. There's a lot of room under that cap pan. If they're only going to have one version of the large cap board, why not add the extra length and eyelets.
     
    Last edited: Apr 22, 2017
  9. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I decided to go with a Trainwreck preamp in the bass channel. I couldn't find a good example of a schematic, so I decided to make one. This is based on the Ceriatone 'Liverpull'. The only differences are the fixed mid resistor and the value of the grid leak resistor for the last gain stage.
     
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  10. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Forgot to attach the schematic...
     

    Attached Files:

  11. D'tar

    D'tar Friend of Leo's

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    Phrygian77, have you been to ampgarage.com? there's a lot of info available in the trainwreck files. You will have to register and log in to have access to.
     
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  12. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    My copy of the original is right on the money, so my 6 cap 6.375" board layout should be good also. I ordered a 12"x12" sheet of phenolic today.
     
  13. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    1493252278321.jpg
     
  14. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Got my phenolic board from McMaster Carr yesterday. I also ordered a bunch of stuff from Hoffman. I went crazy and decided to go for Jupiter caps for this build!
     
  15. keithb7

    keithb7 Poster Extraordinaire

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    Dang. There was a 1967 original Bassman PT tranny on Ebay. Ended today. I was watching it. 2 hours to go was $10. Would have been a perfect fit here.
     
  16. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I haven't really thought about using vintage iron. A 67 PT would probably yeld a much higher B+ than I want. I'd like to get close to 6g6-b voltages.
    I'm aware of that site. For a long time, it was flagged for malicious software. That seems to have changed fairly recently.
     
  17. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Well, in addition to the Mojotone filter cap board being upside down, some of the chassis holes for the cap board are also off. Specifically, the mounting screw hole that is on the front side of the chassis and the through holes for the wires are off by 1/8" towards the rear of the chassis. Pictures here are worth a 1000 words... don't get distracted by the colorful blanket underneath. [emoji14]
    Here are the Mojotone boards (flipped over) with the rear side mounting screw in place. The cap pan almost fits like this (the board needs angled corners like the vintage ones), but the wire holes don't line up. Note the position of the mounting holes for the the cap pan.

    IMG_20170502_152541.jpg


    Now, this is with the font side mounting screw in place. The wire holes line up perfectly, but the whole board is offset by 1/8" relative to the cap pan, so there's no way it's going to fit.

    IMG_20170502_152850.jpg
    IMG_20170502_152820-2.jpg


    It's not that big of a deal since I'm making my own board. Although, I think I'm going to contact Zachmdhunter about the chassis and ask him WTF is going on here. On the layout for board that I'm going to build, I guess I'll just move those holes by 1/8" ... maybe even the eyelets, and shorten the width of whole board by 1/8".
     
    Last edited: May 2, 2017
  18. Bill Moore

    Bill Moore Tele-Afflicted

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    I've done a couple of builds using Weber chassis, and there is plenty of room to run the wires through the grommets, even if they are off a little. (Both were!)
    Edit:
    I remembered I have a 79 Twin chassis exposed, the fish paper to grommet holes don't line up perfectly either!
     
    Last edited: May 2, 2017
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  19. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Wow, got a quick response from the chassis maker!

    "Thanks and will recheck that against our original!! We'll note your comment on the "facts" part of the presentation as well. Carl and Zach"
     
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  20. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    1493775710591.jpg
     
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