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Tube Design Gurus - Alembic F2B - Circuit Question

Discussion in 'Amps and Cabs [BG]' started by svtb15, Mar 24, 2013.


  1. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    Is there are way to incorporate or is there an existing design for a DIY Direct Out for an Alembic F2B Preamp.

    I love my old 70s F2B.. It has had a few mods over the years and i rebuilt it on a new PCB from Alembic years ago. Recapped at the same time etc.

    I would like to add a DI out following the first tube gain stage pre EQ if possible..

    I am great with soldering iron and homemade PCB fabrication and circuit building.. I just don't know how to design the circuit .:(

    Dont know if there are any plans out there for this. I don't want to go to the F1X pre.
    Since i had one years ago and it really does not sound the same as the F2B to me. As different as the Mesa walkabout to the Mpulse line.. Not night and day but still different.
    Thank you in advance.
     
  2. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Not that this answers your question, but I always ran the output of mine either directly into a studio console or via a Jensen passive transformer.
    IMO, a transformer out would preserve a lot of the vibe (as opposed to some other active design) of this unit but is there enough room in the housing?
     
  3. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    Without having to totally rebuild/redesign everything to be fully optimized... if the schematics that show up on the web are correct, I would take your signal feed right after the 250pF (C3) coupling cap from the first stage. If the pots are not mounted to the PC board, the input lug of the treble pot would be a convenient spot. Feed that to a 6.8K and then into Jensen JTDB-E.

    http://www.jensentransformers.com/datashts/dbe.pdf

    You can skip the ground lift if you want to keep things simple.

    Connect the ground for the transformer DI circuit to the ground (-) of C8 22uF filter or maybe R7 1.5K cathode resistor depending on how the circuit is laid out. That should do it. Shoot me an email if you have any questions.
     
  4. xk49w

    xk49w

    Apr 13, 2008
    A simple unbalanced buffer could be added before the tone stack, though it would be somewhat affected by the tone stack adjustments. Check this note from R.G. Keen. I am using this technique in a homemade F2B clonish preamp.
     
  5. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    Thank you very much.. I will make this a project this week... Great info.... And i think i will go for the Ground Lift... Just in case....


     
  6. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    If the schematics that pop up on the net are correct the pre appears to have two identical and separate channels. That being the case Jonathan's version will only work on one channel. Is there a known to be correct version available??
     
  7. chadds

    chadds

    Mar 18, 2000
    Get a Countryman and move on with your life.
     
  8. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    Get a countryman?.. I have a REDDI. why would i want a countryman,,,
     
  9. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    yeah, you're trying to get a direct out of the the alembic before its EQ, right?
     
  10. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    Yes... I think the one option given above is a good one... Does a transformer need to be used or can a tube circuit be used instead.

    Years ago...... maybe mid 80s, i was friends with a guy that worked at Alembic he sketched a circuit for a Cathode follower that I made and inserted into my F2B case..
    It needed another transformer to power the additional tube..

    I dont even remember why i asked about it, why he suggested it, why he sent it to me or why i made it and then installed it.. and then returned the Preamp to stock.. HAHA..

    IIRC i needed to boost the output for some reason on a gig that i was doing and that was his suggestion... Anyone that has free time and likes to type can comment... LOL.. Thank you all in advance...
     
  11. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    That circuit leaves a lot do be desired IMO, especially the output which would greatly benefit from a cathode follower to improve its ability to properly drive a power amp.
     
  12. are you sure you want it before the tonestack? Most of the sound of these units comes from the way the tonestack colours the tone
     
  13. chadds

    chadds

    Mar 18, 2000
    Sorry!!!
    Just trying to help you alleviate the problem you don't have, obviously. :)
     
  14. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    ;)
     
  15. This is the simplest workable solution. The only potential pitfall might be that the transformer primary might load down the gain stage a little, decreasing the available gain. I think the nominal input impedance of the tone stack is around 100k, and the output impedance of the gain stage is around 40k, so the 140k primary impedance of the xfmr (assuming 1k mic preamp input impedance) in parallel with the tone stack would have some effect. It's probably worth trying and will probably be fine, though.

    Sticking a cathode follower in front of the transformer would basically negate that effect, but it's a fair bit more work, though if you still have the parts around from the one you put in before.

    A cathode follower has very high input impedance (<1Ohm) and very low output impedance (and ~unity gain) which means it wouldn't load down the gain stage in front of it like the 140k of the transformer will in parallel with the tone stack.

    EDIT/clarification: I don't consider myself a guru but I know a few things about tubes. Probably just enough to be dangerous. :D
     
  16. http://www.frontiernet.net/~jff/SonOfAlembic/SonOfAlembicF2B.html

    This guy might have what you are looking for, but you are right, it will take a beefier power trans to make it work.

    A white cathode follower is key if you want tubes to do the job, It will also still take a 15k-600 transformer, Edcor USA makes them for $11.

    I am planning on building a single channel version of this in the near future.
     
  17. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    Thank you all.... Im going to hash it out and perhaps do it this week...
     
  18. svtb15

    svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    That is a nice piece of work.. Art! Would love to hear it..
    I only wish Alembic made a F2B with one channel. yet add an all Tube DI out with a vari mid added to the tone stack ... in a 1 U rack..

     
  19. Passinwind

    Passinwind I am Passinwind and some of you are not. Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    The solution Megas posted is by far the most attractive. I have a Spice model of the tone stack and can try some sims of various loading effects like Mark mentioned, but I'm pretty strapped for time this week as I'm trying to get my first customer tube preamp build happening. If you can drive LTspice yourself PM me and I'll send you the model.
     
  20. http://www.duncanamps.com/tsc/

    tone stack simulator, lets you monkey with part values of most common tone stacks. (Fender, Marshall, Vox, etc...)
     

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