Lunchbox “PBJ” (Pro Blues Junior) Handwired

Discussion in 'Shock Brother's DIY Amps' started by BobSmith, Dec 7, 2019.

  1. BobSmith

    BobSmith Tele-Meister

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    People out there definitely have love-hate feelings for the blues juniors. I’m part of the love group! I have 2 of them and think they are an incredible sounding amp clean to raw. (I will even use the reverb on it, but not my favorite part.)

    Recently, one of my stock BJ died. I tracked down the problem to an open 100k plate resistor, but in the process of troubleshooting, I was dismayed about the flimsy construction of the board, tube attachment, etc. What a shame. It will definitely cause more headaches to come, so I will rewire it!

    Hoffman sells a turret board, with more or less drop-in fashion to convert it, minus reverb. But I decided to go a step further and put it all together in a new lunchbox. (Same Hammond ones that Lil Dawg uses). I’m calling it a Lunchbox PBJ (Pro Blues Junior) because the pro junior did not come with reverb (and I like peanut butter of course).

    The chassis is of course the challenge for me. I lack experience and tools to do this, plus it can be really expensive to do it right. It makes those ready made chassis from Mojotone and the like look like total steals for the money.

    I decided to go with the black powder coated steel, I’m hoping it will be more rigid than aluminum and less work being paint already. Looking back, aluminum may be better for conductivity, and going unpainted may not be a difficult problem to overcome.

    I think the label-less look on controls looks a little cheesy, especially with so many knobs, so I know I want labels. I struggle with the lettering and looked into screen printing, laser etching/engraving, but all fairly expensive (something in the $200+ range for one off simple lettering. ). So I’m going to try Datak dry transfer rub on letters, one at a time, and coat with clear matte finish (this is why I now kinda wish I went unpainted, I have to paint it now anyway!)

    Below are some of the layout placement mockups on the painters tape covered chassis. As you can see I also am adding a choke to the design. Why not...

    I may also line up the output and choke as I have seen on countless examples, rather than going 90 degrees. It seems to be far more important to make this consideration for the PT and OT than the choke and OT.

    oh, and ignore the fuse holder, that is just a place holder while I wait for a switch. An actual fuse holder will be mounted next to the power cord on the back.


    5C5B2169-223F-4391-8984-5E1C2470FE3E.jpeg 5402113C-CE23-41C9-B2DD-C1C28FA80F9E.jpeg 376CF3B9-389E-468B-8F44-F57A29E9984F.jpeg
     
  2. Mongo Park

    Mongo Park Tele-Holic

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    It will be a beauty when you are done
    Looks like a total rebuild. Are you going to add in some mods.
     
  3. sds1

    sds1 Tele-Afflicted

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    You can't build an amp like that and make payroll at $600 per unit without cutting a few corners. Fender cut them all when they made these Blues Jr's.

    They are super popular amps with the working guys around here, what an easy amp to lug around town. I just finished an amp design inspired by the physical dimensions of the Blues Jr. It's hard to get much amp in that little space.

    Dare you to add solid state reverb back in to the Hoffman circuit! :)
     
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  4. tubelectron

    tubelectron Tele-Holic

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    I agree ! If I find one day a non-working BJ for peanuts, it's the route I would follow too...

    -tbln
     
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  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Go big or go home! Haha! Good luck with it and I'm waiting to see how it ends up. I've seen that Hoffman did the Blues Jr board, and I know he started a Pro Jr layout but don't know if it's completed and tested yet.

    I've never owned a Blues Jr, and probably only spent 10-15 minutes total time on a few. But I did a 2x12 for a guy recently and he put an Allen/BillM OT on it for 4 ohm use. I'll be honest, a 2x12 Blues Jr with two Cannabis Rex speakers, an Allen OT, and a big pine cabinet is a killer setup. It sounded incredible. I can only imagine that a complete build with quality parts would leave the stock version D.O.A.



    Sent from my moto g(6) using Tapatalk
     
  6. BobSmith

    BobSmith Tele-Meister

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    Since it is a lunchbox head, no room for a tank. I don’t absolutely need the reverb, but would be cool for sure. Others (like Rat Valve Amps in UK) have added a tube reverb to their conversion service. But he won’t do business in the US. Turns out there is one other boutique amp maker that I can’t talk about that is doing the same thing. Perhaps I can say more in the next few months... ;)
     
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  7. BobSmith

    BobSmith Tele-Meister

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    Well, I’ve done other amps, but this is my first foray into the chassis part of it. I’m not going to go too far with mods other than to upgrade the trannies: Allen TP24, Allen/Heyboer TO20. I will also add the Mercury Choke (FBLJRxC image.jpg )
     
  8. tubelectron

    tubelectron Tele-Holic

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    @BobSmith ,

    If you change your mind about reverb, here is a quite simple single tube reverb that works very well with a small tank 8AB3CIB from Belton/Accutronics - nothing to do with the ****ty reverb circuit of the BJ ! The reverb driver transformer can be the well known Fender 0220921.

    [​IMG]

    I use it on one of my (compact 10" 25W) DIY amps :

    [​IMG]
    [​IMG]

    -tbln
     
  9. BobSmith

    BobSmith Tele-Meister

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    That seems pretty simple, and very interesting. I didn’t know you could get by with 1 tube. What voltages should be going to +2 and +3? Interestingly, there is half of a 12ax7 that is unused in the stock design.

    If I get a design down, I may consider this for Version 2. (It turns out I have 2 sets of transformers). Maybe the tank could hang upsidedown from the top of the cover. Boy that could get weird!
     
  10. BobSmith

    BobSmith Tele-Meister

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    Clearly the challenge for me here is the chassis stuff. For a reasonable price I got a stock Hammond chassis (1441-26 chassis with 1451-26 cage cover). Lil Dawg amps uses the same one so I figure I’m in good territory. What puzzles me is how the fastening is supposed to happen.

    The cover comes with 2 simple holes, no corresponding holes on the chassis itself. The cover also comes with several screws that one would assume are for mounting. Do they intend for holes to be threaded into the chassis directly? For a cover that supports all the weight and comes off for routine tube changes, that doesn’t seem strong enough, does it?

    I considered PEM nuts but I don’t have a press, but even if I did, there is no room on the backside for the nut because they placed the holes so close to the edge of the chassis. Rivnuts wouldn’t work either because of the same reason, and there is no gap between the cover and chassis for the flange of the rivnut.

    I could try to insert a bolt from the outside and use a small acorn nut for aesthetics on the outside. (I’d have to go this way because I think the location near the edge wouldn’t have room for a nut, but a pan head screw would fit.). This solution would require opening the bottom to take off the cage, but access seems good to those locations and otherwise not a problem.

    Any suggestions? (besides give up). Anyone use this chassis before or seen how Li’l Dawg does it?

    thanks!
    0FF50699-0AB1-475E-9059-CF62D2831D21.jpeg 0EE64587-F01E-4984-85B8-4026238B37E5.jpeg
     
  11. tubelectron

    tubelectron Tele-Holic

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    Ah, sorry :oops: yes : +2 = 350V, +3 = 278V

    Fine : it may be possible to use it as the recovery stage, so you can add a classic Fender reverb driver using a supplementary 12AT7 both sections //.

    -tbln
     
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  12. BigDaddy23

    BigDaddy23 Tele-Meister

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    Perhaps those screws are for the base plate cover on the amp chassis component?

    How much overlap does the cage edge have down the side of the chassis? It looks to be about 1/2"?? A nice pan head screw (M5 or 5/32) with a lock nut on the inside of the chassis would do the job and have plenty purchase.
     
  13. BobSmith

    BobSmith Tele-Meister

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    Unfortunately the screws came with the cage cover (a separately purchased option), so it seems they intend for them to be used for that.

    The center for the corresponding hole is only 3/16” below the top surface of the chassis. I’d be happy if I could fit an M3 there...
     
  14. BobSmith

    BobSmith Tele-Meister

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    Ok, I got the PT cutout done. (Along with all the center drill marks for other holes.). As expected it was a PITA.

    I had to start by making a pattern to transfer to the chassis. (Would be nice if the transformer came with such a pattern...). I cut the clear plastic top of a strawberry carton and rough cut a hole for the transformer bell. I then used masking tape to create the pattern in situ. Then put the pattern on my chassis and traced it with a fat sharpie, making sure the sharpie bled to the edge of the pattern. Then I found a drill bit that matched the radius of the corners and drilled out the 4 corners. Using a cutoff wheel on my dremel, I removed any part that had sharpie on it. I destroyed a grinding bit smoothing and chamfering the final hole.

    It took a few hours but makes a good fit. E9F65AF2-06B2-40A2-8046-9DC789AF7A3E.jpeg 259A62D0-0F00-49DC-A351-AF8EE69C7938.jpeg 28AEACA3-40D5-4E15-BB2B-4D71CE555C52.jpeg 188F9377-8E24-426D-8A8D-73105B6C22E6.jpeg 2D9843F7-A193-4855-9094-7780A25C77AE.jpeg
     
  15. jrblue

    jrblue Tele-Afflicted

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    Thanks for this very interesting thread. I'm a Pro Jr guy -- I like them more than BJs -- and have a first-issue, USA-made PJ (bought new by me) deliberately rebuilt and modded over the years. Both PJs and BJs can be cleaned up, strengthened, and tweaked to become very reliable, quiet, wonderful-sounding amps. Your project looks excellent! My PJ gets raves from listeners for its sound and lots of love, sometimes jealousy, from other guitarists. BJs and PJs can sometimes be crappy and noisy and junky in terms of build, but all tht stuff is fixable and you get an amp that works at a much higher level. No wonder lots of pros use these.
     
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  16. Tonetele

    Tonetele Poster Extraordinaire

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    Too true, I sold mine as it didn't have that Fender sound.I blame it on the EL-84 tubes/valves, usually used in British sounding amps.
     
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  17. trev333

    trev333 Telefied Ad Free Member

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    I recently had an amp head converted back to transformers from mosfet power and there wasn't any room left in there for the tank... so we hung it off the back.... works fine..:)

    V33 reverb tank.jpg
     
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  18. BobSmith

    BobSmith Tele-Meister

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    That reminds me, I had an old Mesa Boogie that mounted the reverb tank against the back of the chassis.
     
  19. BobSmith

    BobSmith Tele-Meister

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    While waiting for some parts to arrive for the chassis...
    ...a little prep work...

    let’s play find the foil!
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  20. King Fan

    King Fan Friend of Leo's

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    Those are ODs, I assume; if so what type? IIRC some/many ODs are 'non-polar' as noted by Mr. Aiken: "Also, some higher-voltage film caps (typically the 1000VDC/450VAC and higher values, such as the Orange Drop 716P high-voltage units) use a "series-wound" technique that has two separate sections, side by side, with a common "floating" connection layer, usually at the bottom of the layer stack. These caps will have no inherent shielding...."

    Did you ever figure out the Hammond cage screw thing? Jim at LilDawg does it like this, I assume per Hammond convention, here on a 5F10. My 5e3 was the same -- I assume they all are. Jim's handle solution is pretty elegant too.

    upload_2019-12-13_10-10-46.jpeg
     
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