Blues Junior hand wired conversion with tube reverb

Discussion in 'Shock Brother's DIY Amps' started by galaxiex, Aug 11, 2018.

  1. galaxiex

    galaxiex Tele-Holic

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    I built this some time ago and posted a few pics here and there in various threads.

    I’ve been asked to do a build thread, so here we go….

    Design Goals

    1. Keep the looks of a stock BJ and the function of all the front panel controls.

    I didn’t want to have to re-label controls or commit to memory the altered function of a control.

    2. Tube driven reverb.

    3. Have the “FAT” function continue to be foot-switch able and, make the reverb foot-switch able as well.

    That’ll do for a start.

    I have some small experience “working on” but not building, tube amps.

    Since I had never built anything like this before, I decided to practice making an eyelet circuit card.

    I decided to use eyelets since the amps I am familiar with working on are Fenders and Traynors from the 70’s.

    This is a generic card layout for practice.

    100_2398.jpg

    Eyelet press rig.

    100_2395.jpg

    100_2402.jpg

    Once I got that out of the way I had to decide on a circuit.

    I didn’t want to just duplicate the stock Blues Junior circuit, and anyway, I wanted tube driven reverb.

    So I worked up a mish-mash of the AB763 circuit front end (minus the tremolo) with the Blues Junior circuit from the master volume to output.

    BLUES JUNIOR Conversion.jpg

    Now comes the slightly more difficult part. Making a layout.

    I tried just drawing by hand on paper… but that didn't go so well.

    Searched around for layout programs and found DIYLC. Do It Yourself Layout Creator.

    So now there is a learning curve for the program.
    Fortunately it is fairly intuitive.

    After about 2 dozen iterations I came up with this.
    Please note this is NOT complete.
    Some wires and off board parts not shown.


    BJ BOARD 2 STAR.png


    Also there are the mechanical considerations.

    I’d need to add another 12A_7 tube socket, so cut a hole in the chassis.

    And there is the problem of the existing holes in the chassis being too big to mount tube sockets.

    I looked at various adapters but nothing appealed to me.

    So I went the hard way and machined some aluminium flat stock to cover the existing holes, and of course add an extra hole.

    Careful measuring and patience are required. :)

    It took me 3 tries to get something I liked.

    On the far left of the aluminium plate are the heater hum balance control and the speaker jack.

    100_2388.jpg


    The tube mount plate was made with nothing more than some drill bits, hole saws and a drill press.

    More later... :)
     
    Last edited: Aug 11, 2018
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  2. King Fan

    King Fan Friend of Leo's

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    Subscribed. The finished pic in a recent BJ conversion thread by @JuneauMike knocked me out.
     
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  3. JuneauMike

    JuneauMike Friend of Leo's

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    Thanks for doing this, @galaxiex. Such an impressive accomplishment. You've said it got a huge bass. It looks like your cathode bypass caps were pretty supersized, especially at V1. You think that's the reason?

    Also, could you sketch out your grounding scheme? Why are the electrolytics oriented so the ground wires are generally towards the tube sockets and not the pots (ala: tweeds). You mentioned distributed filtering in an earlier conversation. Thats a new one on me? How many grounding points do you have?

    And you had some issues taming the reverb, correct. How did you resolve that?

    Why did you select the tone caps that you chose? 0.047uf bass cap would be somewhat dark, right? Or do I gave that backwards.

    This is the most creative use of a Blues Jr I've ever seen. I'm inspired.
     
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  4. galaxiex

    galaxiex Tele-Holic

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    Not sure why it has such huge bass.
    I was expecting to still have some "boxy" stock BJ type of sound.
    Indeed, you can sorta dial that in if you want it. ;)
    The bypass caps may have something to do with the huge bass, but if you look at stock Fender diagrams they are pretty normal.
    The exception in my schem is the 0.68uf on V1A.
    I used that value to try to eliminate "flubby" bass, and it has worked well on my other 3 Silverface Fenders.

    Re the grounding, if you carefully examine the layout diagram you will see it. :)

    Example; follow the reverb power and ground in the layout.
    Notice that the reverb driver is fed V+ from the "Z" power node to the reverb transformer,
    and from the transformer to the reverb driver tube. (red and blue wires near the bottom right of the layout)

    Now look at the reverb driver tube V2 ground. Black wires on V2. They go to the cathode resistor and cap.
    And from there, where do the R and C go to ground?
    Why directly to the "Z" node ground. :) Follow the green wire from the cathode R and C.

    Another example, look at the "X" power node.
    Follow the cathodes from each tube that the X node feeds.
    Where are the grounds?
    Why right at the X node ground of course. :)

    Really, its all there, just look at each power node and what it feeds and where are the grounds.
    This is what I meant by "wired grounds" and "distributed filtering".
    The Kevin O'Connor book I mentioned in our conversation explains it quite well.
    Plus its a great book for all the other info that is in it.

    Taming the reverb, I removed the cathode bypass cap on V3A.
    That helps, but you can still dial up tons of reverb.

    Tone caps were chosen from "normal" Fender values depending on what vintage stock Fender circuit you look at.
    I forget which one I used as a reference. I think the values are right out of the AB763 circuit?
    I fully expected to have to tweak those tone cap values, but so-far am happy with it the way it is.

    Thank you! :)
     
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  5. galaxiex

    galaxiex Tele-Holic

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  6. galaxiex

    galaxiex Tele-Holic

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    I need to give huge thanks and appreciation to Doug Hoffman for his website and making so much information available.

    I should have done this in the first post but will add it here.

    I absolutely could not have done this without the huge resource and information that is available on his site.

    So... a big THANKS to Mr Hoffman! :)

    Also Thanks to robrob for his site, I learned lots from him as well. Great resource!
     
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  7. galaxiex

    galaxiex Tele-Holic

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    Here is how I kept the front panel FAT switch.

    100_2407.jpg

    100_2408.jpg
     
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  8. JuneauMike

    JuneauMike Friend of Leo's

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    Amen to all that you said.

    (You know I'm going to drive you crazy with a million questions, I appologize in advance.)

    I will take your schematic and trace the layout when I get some time. Thats a new grounding scheme for me, but I think I understand what you are saying.
     
  9. JuneauMike

    JuneauMike Friend of Leo's

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    Couple of interesting observations.

    Billm recommended changing the .47uf filter cap to 100uf for power stiffening and noise. Neither you or Hoffman did that. Just an observation really.

    Bandmaster, PR, and your tone caps are:
    T 250pf
    M. .1uf
    B .047uf

    Hoffman left them stock

    Deluxe AA763 uses
    T .00025uf
    .1uf
    B .033uf
     
  10. galaxiex

    galaxiex Tele-Holic

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    Ahhhh, I probably copied the PR cap values.

    I was working from/getting ideas from, a couple of Fender and Hoffman circuits.
     
  11. JuneauMike

    JuneauMike Friend of Leo's

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    No, they're the same. In any case all three Fender examples use 2-band EQ (bass, treble) and no master volume. I'm not knowledgeable enough to know how that changes the balancing act of the tone shaping circuit. But fun to chew on.
     
  12. galaxiex

    galaxiex Tele-Holic

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    Oh, I see what I did.

    I took the Deluxe AB763 tone stack values and added the mid pot instead of the 6k8 mid resistor.
     
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  13. galaxiex

    galaxiex Tele-Holic

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    Here is a board template in case anyone wants to build this. ;)

    The grid is .25".

    It "should" print to scale.

    In case it does not, I'll try to attach a pdf.

    BJ BOARD 2 STAR eyelets only.png
     

    Attached Files:

  14. galaxiex

    galaxiex Tele-Holic

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    I did some infrastructure wiring on the back of the board, to keep the top side wiring neater.

    You may or may not want to do this... if something goes wrong under there it will be... trouble to get to.

    100_2409.jpg
     
  15. galaxiex

    galaxiex Tele-Holic

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    I've posted this pic in other threads... but for the sake of completeness I'll put it here as well.

    This is the finished product.
    I'll also post more construction shots later. :)

    100_2443.jpg


    I made a new back panel to allow access to the fuse and tubes etc.

    You can also see the MOD reverb tank I used instead of the stock tank.

    100_2493.jpg
     
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  16. galaxiex

    galaxiex Tele-Holic

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    Here's some heater wiring.

    100_2432.jpg
     
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  17. JuneauMike

    JuneauMike Friend of Leo's

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    Where did you use shielded signal wire? I see a couple places on V1, but is the yellow wire in the background with shrink tube also coax shielded?


    Your build looks very clean, btw.
     
    Last edited: Aug 13, 2018
  18. galaxiex

    galaxiex Tele-Holic

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    Thank you. :)
    Shielded wire at V1, reverb, and input jack.
    I'm at work now so I'll post more detail later.
     
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  19. Wally

    Wally Telefied Ad Free Member

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    Nice work, galaxiex. Commendations!
     
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  20. D'tar

    D'tar Friend of Leo's

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    Excellent job! I think you have a Blues Senior now!
     
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