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Show me your IBAM/IBBA 2016

Discussion in 'Fisher' started by bhamham, Apr 24, 2016.

  1. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    1,990
    Location:
    Austin
    I remember a couple of years ago we did a thread like this, where a few folks shared their individual bias mods. I thought it was pretty cool and got a few ideas on building them.

    It seems there's an interest in making them so I thought I'd share my IBAM building method. There's a lot of ways to do these, but this works for me. I use the IBAM more b/c I like old stock 7868/7591s and find at times with the IBBA I have a hard time balancing a pair. If I get new production tubes then I'll use a IBBA - I think it's the better board as there's a fail-safe feature that protects the unit when there's a run-away tube. At least that's how I understand it. But, TBH I've been using the IBAM for around 6 yrs and never had a problem with one.

    First, you'll need the parts: 4 x 12k resistors 1/4-1/2w, 4 - 33uF/50v e-caps (anything close will work), 4 - 10k 1/4-1/2w trimmers (I like multi-turns, but singles are good too). Some solid core hook-up wire and a perf board. I mount the trimmers first and bend the leads as shown in the pic. One side of the trimmer on all four are connected to a buss that feeds the filament voltage to the preamp tubes. I use a yellow wire as that's usually the color that Fisher used on 400s to run that supply. After soldering the trimmer leads to the buss mount the 12k resistors. Mount them so one lead of the resistor is touching the other side of the trimmer - not the middle or wiper lead. Solder these. Now, mount the e-caps where the negative side touches the wiper lead on the trimmers. Bend the two leads at each junction like a teepee, your going to connect the lead that goes to each of grid resistors - 330k or 220k. I like to bend the end on these wires so you can stick a bit in the hole on the perf bd then bend the leads - the teepee - on the caps and trimmer wiper where there's a good connection, then solder them. I use a different color wire for each of these but it really doesn't matter. The alignment of the trimmers on the board are the same as the output tubes, i.e. the top trimmer works the top output tube, and so on. That's with the unit standing upright resting on the power transformer. So make sure that the wiper wires to the grid resistors go in the same order. You want to adjust the trimmer while your reading the voltage on pin 3 for a 7868. The last thing is to run your ground buss so that all the remaining leads: the other side of the 12k resistors and the positives of the caps are connected. I hope that wasn't too confusing but now your board is done.

    Testing: check resistance from each lead to the grid resistors, to ground and to the supply to the preamps. That's 8 checks. Adust the trimmers to make sure the resistance changes.

    Mounting the board: I like to mount mine on top of the new e-cans (1000uF) to the bias supply. I remove the clip and shoe-goo these cans to the chassis. The board gets an insulated backing that's tied to the board with plastic zip ties. Then the board is shoe-gooed. I use a 4 way terminal for the grid resistor leads. If you need to get to the board later, hopefully that won't happen, but just cut the plastic zips and you can mess with the bottom of the board.

    I'm sure there's better ways to this but this has worked for me. I liked to see how others do their IBAM/IBBAs, we can all learn from each other.

    ibam 003.JPG

    ibam 004.JPG


    ibam 005.JPG



    ibam 008.JPG

    ibam 009.JPG














    ibam 011.JPG

    ibam 012.JPG

    ibam 014.JPG

    ibam 015.JPG

    ibam 016.JPG
     
    Last edited: Apr 25, 2016
    The Fuxtor and notdigital like this.

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Location:
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    You pervert! If you think I'm gonna let you peek under my kilt you got another thing coming. :):p:D

    I build mine same as I did 4 years ago!

    100_4520.JPG
     
    eduarsan1 likes this.
  3. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    1,990
    Location:
    Austin
    Real men wear kilts! Good looking board Larry, I like how you mounted it plus it's closer to the action than mine

    Here's a shot of how I run the grid resistor wires from the 4 way terminal to the 330k resistors. Despite all the great advice from Larry I still use the factory .047uF and 330k resistors on old stock 7868s.



    ibam 018.JPG
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,552
    Location:
    Southern NJ
    An IBBA in my Sherwood S-5500II.

    [​IMG]
    Not a Fisher, but built from the same design and living in a comparably similar amplifier. I should have put the jumpers underneath. Not honestly sure why I didn't do that. Smaller trimmers would also be not a bad idea, but those are what I had on hand. I think they're the 3/8" square ones.
     
  5. Leestereo

    Leestereo Super Member

    Messages:
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    Location:
    Ottawa, ON
  6. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    1,990
    Location:
    Austin

     

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Lee, Did you take a post graduate course in soldering @ Lincoln Tech?:naughty::D
    If I did something like that my dad would have a hat attack (boston accent).
     
  8. xbart

    xbart AK Subscriber Subscriber

    Messages:
    974
    Location:
    So. Oregon
    Here is my 500C.
    Showing that with the help of this forum, even a novice can successfully rebuild one of these.
    It sound great!

    _DSC9171.jpg
     
  9. Rockyhill

    Rockyhill No marigolds in the promised land Subscriber

    Messages:
    3,158
    Location:
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    Any 500B IBBA/IBAM implementations? I'd like to see how you installed it.
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Rocky. Installation on the "B" series is basically identical to the "C" series and the 400. Locating it on the chassis might be a little different, but you've got as many different installations as you do owners and units'.

    Larry
     
  11. Rockyhill

    Rockyhill No marigolds in the promised land Subscriber

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    Roger that.
     

     

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  12. Leestereo

    Leestereo Super Member

    Messages:
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    Like most here, I learnt to solder the old school way: on an ancient wooden kitchen table!
     
  13. wcarroll

    wcarroll Active Member

    Messages:
    301
    Location:
    Baton Rouge, LA
    Just finished installing an IBAM into my 800C and thought I'd share a photo!

    20160503_193156-1.jpg

    View attachment 728993

    Second photo was taken before the wiring was completed to the output tube grids...

    20160430_181235-1.jpg
     
    Last edited: May 4, 2016
  14. wcarroll

    wcarroll Active Member

    Messages:
    301
    Location:
    Baton Rouge, LA
    Tried out my new IBAM this evening and found it to be working perfectly. My original Fisher 7591's are all now dialed in at 31mA. A check before I started upgrading showed one tube in each pair drawing about 28mA and the other two at 21 mA. I think I saw ice forming on the two colder one's! :)

    Thanks Drew Bolce for the IBAM design, it's great. I also scoured tons of old threads and have done all of the major recommended updates on my 800C. So, thanks to all of the contributors such as Larry Deroiun and Dave Gillespie.

    Thinking now about buying a new set of Tung Sol's and saving my nice original tubes...

    Wes
     
  15. ducati2

    ducati2 AK Subscriber Subscriber

    Messages:
    576
    Location:
    Oklahoma
    I sure would love to have someone build me an IBAM for my 500c ! I could install it, but not sure about building one...
     
  16. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
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    Location:
    Jupiter, Florida
    I have somewhat mastered building Dave's design and could possibly put one together for you, mine are built to mount on top so you can adjust and check bias without taking the bottom off.

    20170815_082011.jpg
     
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  17. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

    Messages:
    446
    Location:
    Bogotá, Colombia
    This is the IBAM in the Fisher 400 that I have

    [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG]
     
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  18. Rockyhill

    Rockyhill No marigolds in the promised land Subscriber

    Messages:
    3,158
    Location:
    Maine
    I started with this, a single pot to control bias:
    IMG_0049.JPG

    And replaced it with IBAM on my 500B:
    IMG_0760.JPG
     
    Last edited: Aug 30, 2017
  19. ducati2

    ducati2 AK Subscriber Subscriber

    Messages:
    576
    Location:
    Oklahoma
    I want to mount my IBBA on the topside of my late serial numbered fisher 400. To anyone that has successfully done this - I would love pix and tips on how to physically mount and where it will fit. My board measures 5x7cm.
     
  20. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Ducati; Check out posts 167 and 17 this thread!
     

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