Modding my Blues Junior - Adding an Effects Loop

Discussion in 'Shock Brother's DIY Amps' started by mrboson, Mar 19, 2014.

  1. mrboson

    mrboson Tele-Afflicted

    Posts:
    1,426
    Joined:
    Aug 9, 2011
    Location:
    Brookings, OR
    So I've been wanting to do this for a while. The Blues Junior doesn't have a lot of clean headroom, and IMO sometimes it's hard for time-based pedals in front of the amp to sound good (again, IMO, probably it's just my preference). Personally, I struggle a bit finding a good use for my delay/verb/trem pedals when I use my Blues Jr. With a BJr, turning it up loud means you are going to get dirty. Nature of the beast.

    So... my theory is that I can put an effects loop before the master volume and my time-based pedals should work nicer with the amp when I turn it up loud. I wanted a tube-driven buffered serial loop with a switch so I can turn it on and off.

    The Blues Junior has an unused triode (V2A) and I could have used that. But I had already modded my amp and use V2A as a cathode follower to drive the tone stack. So I need to add a tube. Since my plan was to add a tube, I looked for circuits that used both sides of the tube I would be adding. One side for the buffer, the other side for a recovery stage. Some ideas for circuits:

    There was a schematic I found from a Weber kit but it didn't look like it would work.

    Valve Wizard - Cathode follower page has an idea for a buffered loop at the bottom

    http://www.blueguitar.org/new/schem/misc_mod/fx_loop.gif

    I actually tried both circuits, and settled on the Valve Wizard version. I also tried inserting the loop at various points: Before the amp's reverb. After the amp's reverb. Separate from the amp's reverb. My amp's circuit board looks like it took a beating from all that :oops: but I wasn't worried about it. I got my amp used for pretty cheap.

    Here is a sketch of the circuit integrated into the Blues Junior schematic (my version is the old Green Board):
    [​IMG]

    After trying different placements, I settled on putting the loop send and return between C16 (V2B coupling capacitor - C8 on the cream board version) and R40 (430K). Thus, the amp's reverb comes after the effects loop.

    Note: The reverb on Green Board versions is after the Master Volume, while Cream Board versions the signal for reverb is taken before that 430K (R18 on cream boards) resistor and returns after it. My Green Board was previously modded to handle the reverb just like the cream board does.

    So after the V2b coupling cap, the signal is sent to the loop buffer (1/2 of a 12AU7) which drives the send. Send goes out a switching jack so if nothing is plugged in then the signal goes to the return jack. From the return jack the signal goes through the recovery stage (the other half of the 12AU7) and the signal is returned to a point on my board where it is picked up at R40 and also sent into the reverb circuit.

    I don't have lots of pictures because a lot of this was trial and error. With trying different spots to get and return signal, and building the loop circuit several times with different components, there was no way to document the process. But I'll post a few pics of the final result in the next posts.
     
    Last edited: Mar 19, 2014
  2. mrboson

    mrboson Tele-Afflicted

    Posts:
    1,426
    Joined:
    Aug 9, 2011
    Location:
    Brookings, OR
    So the real challenge for this mod is finding a place to put a new tube!

    There's not really any room on the chassis. But I had Hammond enclosure lying around that I was going to use to build a pedal sometime. I ended up building the circuit in that, and put the tube there as well. I attached the enclosure to the back of the BJr chassis, with the tube socket on the bottom so that the 12AU7 can point down. It ends up being right behind the preamp tubes:
    [​IMG]

    Here's another shot:
    [​IMG]

    I have shielded Mogami hookup wire carrying the signal in from C16, then back to the Send jack. Then from the return jack into the enclosure, finally the recovered signal back to R40. The Mogami wire is the shielded double lead thick stuff.

    The twisted green wire is for the heater, the red wire is B+, and the black wire is ground sent back to the Bjr board.
     
  3. mrboson

    mrboson Tele-Afflicted

    Posts:
    1,426
    Joined:
    Aug 9, 2011
    Location:
    Brookings, OR
    The next hardest challenge is where to put send and return jacks. I thought about putting them down next to the speaker jack, but I also wanted a Send Level pot and a Return level pot. There's just no room on a little Blues Jr chassis to do anything fun.

    So I got a blank dish plate and tried to figure out if there was any place that I could put it on the back panel, where the jacks and pots would fit between the dish plate and the BJr circuit board. There is only one place on Green Board's layout, and it has to be placed precisely:
    [​IMG]

    Believe it or not (if you have ever seen the guts of a BJr, you might not believe it), this fits right there, with the send and return jacks just fitting in between some of the components on the board. There is not way for me to take a picture to show it, but inside there is about 3/8" clearance between the tip of a jack and the nearest component.

    Here's a shot of the back of the dish plate:
    [​IMG]
    Sorry it's fuzzy.

    Just to be safe from vibrations, I put those little round sticky pads on the bottom tip of the jacks. They don't touch anything, but just in case, they are there. I gooped up with hot melt glue the leads connected to the DPDT switch because that is kind of a weak spot, especially where the big thick mogami shielded wire hangs. The blob of goop acts as strain relief.

    When the control plate is in place, one pot is just above and to the left of the clean boost board (that's a billm mod) in the right upper corner. The send and return jacks hang down just above the Orange Drop caps (tone stack and coupling cap). The big mogami leads just kind fold around and fit.

    It's a tight fit, but it does just fit :cool:.

    At the bottom, next to the V1 ribbon cable, you see a red wire. That sends B+ to the fx loop enclosure. I tapped into B+ right next to R10. You can't see it in the picture, but the twisted green heater wires connect on the tube board. I tapped into the heater circuit by connecting those wires to pin 5 and pin 9 of V1.
     
    Last edited: Mar 19, 2014
  4. mrboson

    mrboson Tele-Afflicted

    Posts:
    1,426
    Joined:
    Aug 9, 2011
    Location:
    Brookings, OR
    Finally, how does it sound?

    Well, I wanted it to be as transparent as possible, but as you can imagine, running the signal through another tube will change the amp's tone some. It does. But not a great deal.

    With the switch, I can A/B the sound between stock (no loop circuit interrupting the signal) and the new tube added to the signal chain (no effects plugged in). With the loop circuit on, there is a slight amount of additional bottom. It also sounds like there are some additional harmonics. I didn't want to add any distortion, and the return level allows me to find a spot where there is no volume drop off with no additional distortion. Essentially, I can pretty much get close to equivalent sounds to the stock amp with the loop circuit on (adjusting the amp's tone controls brings it close to stock sound). I actually really like the sound with the loop circuit on and nothing plugged in. It's still the Blues Jr sound.... it's just... different :cool:

    With pedals in the loop, it sounds great! I have a Nova Repeater which has a Hi input (for FX loops). Plugged into the Hi input, the loop works great (pedals after the delay pedal operate normally). The loop's send plugged into the Lo (instrument) input sounds like crap, the signal is too hot. The 22K pot in my send level works to cut the signal down, but the tone suffers. Anyway I'm glad the pedal has a loop input :D. I think I also made a mistake on the return level pot. I'm guessing I put a 1M pot there due to the way it's range is working. I have a drawer full of pots, so I'll be trying different values on both send and return levels to see how that changes things.

    Oh, and the circuit is absolutely quiet. No additional noise like hiss or hum or anything. I was worried the heater wires might introduce some noise because they pass close to some of the V1 signal, but noise didn't happen.

    Overall, I am really happy with this mod.

    I will be rewiring my pedal board to use this fx loop. I'll play out with it and give an update.
     
  5. Mickyvai

    Mickyvai TDPRI Member

    Posts:
    4
    Joined:
    Jan 26, 2006
    Hi! I'm really interested in performing this mod to my cream board blues junior. Can you please repost the photos in your comments? The links are not working and they are not visualized correctly.

    Thanks in advance!
     
  6. roseblood11

    roseblood11 TDPRI Member

    Posts:
    9
    Joined:
    Oct 27, 2008
    Location:
    Germany
    @mrboson
    Please repost the photos or at least the schematic!
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.