Working on a Blues Junior

Discussion in 'The Squawk Box' started by Lyle Caldwell, Jul 25, 2011.

  1. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Trying to make a friend's old Blues Junior into a good sounding amp. I'll post more later when I finish, as I don't mind "giving away" mods for inexpensive amps like this.

    But for now, jeebus. I don't think there's a single rule of good design Fender didn't entirely negate here. Do they have a random circuit generator or something?

    Got a lot of stupid to undo...
     
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  2. hipofutura

    hipofutura Senior Member

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    These amps severely suffer from an undersized cab and dog crap speaker. Definitely want to consider insulating it. I had one for a brief period and swapped several speakers. My favorite was an old JBL D120, but it all depends on the voicing one's after.
     
  3. Lyle Caldwell

    Lyle Caldwell Senior Member

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    They suffer from worse than that. Take a look at the schematic:

    http://ampwares.com/schematics/blues_jr.pdf

    Look at the resistor in parallel with lugs 2 and 3 of the volume pot and then look at the resistor ahead of the master volume and then look at the value of the master volume pot.

    See why this is such a weird weird thing Fender did? Meanwhile there's an unused triode...
     
  4. Lyle Caldwell

    Lyle Caldwell Senior Member

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    OK, rather than try to fix the stock amp I just redid it.

    Preamp is now the Bright side of a 5F6-A with the added cascaded gain stage from the JCM800 (this extra gain is completely bypassable - I repurposed the "FAT" switch), so the preamp goes from Clean Bassman to Marshallesque, though I shaped the "Marshall gain" for this specific amp (220K plate bypassed by a 1000pf cap for example).

    Reverb is now pre-MV, and the MV is bootstrapped to the PI.

    PI and output section are essentially JTM-45, though with the EL84s.

    New Switchcraft input and speaker jacks.

    Stock speaker was just nasty. Bright and nasal and boomy. I had a slightly used Vox/Wharfdale cheap clone of a G12H30 sitting in my closet for a few years, so I tried it in this. Great match. Best speaker ever? Of course not, but it sounds very good in this amp and the price is perfect (who doesn't like free?).

    I beefed up the output tube screens from 100R to 1K but now the highs aren't quite where I want them. Sure, I could bring those back down to 470R or lower, but EL84s are hardworking little dudes and it's good to make their lives easier. I'll decrease the cathode bypass cap at one of the preamp stages and see if that brings the balance back where I want it.
     
  5. Lurko

    Lurko Senior Member

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    50k M/V? Ummmm.....okay.

    I'll give them a break on the unused triode. That amp is not supposed to be a gain machine and it would just add noise into the circuit. (Although I guess they could've at least paralleled the two triodes in that tube.)
     
  6. Lyle Caldwell

    Lyle Caldwell Senior Member

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    But that's the thing! They massively ramp up the gain, then they bring it way way back down with those voltage dividers. 130K in parallel with lugs 2 and 3 of the volume pot, then the 430K resistor before the 50K MV.

    Insanely bad gain staging. They could have made a better sounding version of what they were aiming for with just two 12AX7s (one for preamp, one for PI).
     
  7. Lyle Caldwell

    Lyle Caldwell Senior Member

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    OK, finished with the exception that I need to get two new chicken head knobs as I replaced two of the stock pots (bass and MV).

    [​IMG]

    [​IMG]


    Added a presence control:

    [​IMG]

    [​IMG]

    The stock Fender/Groove Tube EL84s hummed terribly. I put in a pair of Sovteks I pulled from an amp months ago, hum gone now. My friend should definitely replace all the tubes with something better though. The amp is worth it now.
     
  8. Lurko

    Lurko Senior Member

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    Okay, you know that in 2032 so dude is going to be on an Internet (or whatever replaces the Internet) forum going, "man, some IDIOT drilled a hole in the chassis; totally botched the resale value of this Fender classic!":laugh2::laugh2:
     
  9. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Nah, this is an ultra rare highly sought after Psionic Audio modded amp, back when people could afford them.
     
  10. hipofutura

    hipofutura Senior Member

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    I remember, back in 2011 I think it was, when Caldwell was this up and coming amp designer. Who would have guessed?! In those days a guy named Dumble, or some such thing, was all the rage. You could buy a caldwell amp for next to nothing. Why, if I had kept the Psionic pedal, I could retire on it now.


    [​IMG]
     

    Attached Files:

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  11. Bluz

    Bluz Junior Member

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    Hey, I was looking at this old thread about your modding a Blues Jr. I have a BJRIII that I've modded will Billm mods. I am interested in your mod. You stated the gain section was like a JTM45.....I've looked and read severa lforums for this very type of mod. Would you mind sharing yours mod with me? Thanks, Bluz.
     
  12. Thomas Madden

    Thomas Madden Junior Member

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    Lyle, I have followed you over the years and kept pretty much quiet. I enjoyed the over the top of screening crazy questions of the Marshall class 5 and I did mine without the headphone or bright switch. I must say it is a fantastic sounding amp that increased it's worth and sound tremendously. Would it be too much to ask if you have a schematic for what you did for the Fender Blues Junior. I've acquired a Blues Junior III brand new in the box and nobody wants it with me offering savings of over $200!!! So I've decided to keep it and see what mods are available to make it a better sounding amp. If you have a schematic with your changes on it that can be uploaded, I would greatly appreciate it. I think I can distinguish some of your work with what you have said as well as the pictures, but I would like to see your finished product. If so I would greatly appreciate it, if not I understand that you are a busy man. Again many thanks for your fortitude over the Marshall Class 5 and thank you for your expertise in the area of fixing these companies screw ups. Thomas
     
  13. Mr Insane

    Mr Insane Senior Member

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    Miss my old Blues Jr
     
  14. chasenblues

    chasenblues Senior Member

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    You might be waiting quite awhile for a response. The OP/Lyle Caldwell hasn't posted here in 3 years.
     

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