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IBAM for 800c

Discussion in 'Fisher' started by The Fuxtor, Jun 9, 2018.

  1. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    I built one for a 400 receiver, but I think the install procedure for an 800 is slightly different? The bias resistor R119 is removed as well as two caps ( can't remember offhand ) are replaced with new board caps. Is there a 0.01 uf cap that should be removed? Or is this left causing no issues?
    I was gonna try building an IBBA this time, but the tubes in my unit are old stock RCA 7591 which so far bias fairly close ( -18 v equates to around 28ma via pin3) with one tube being slightly stronger at 32ma. From what I have read the IBAM will be more helpful with bringing these more similar. I think for the sweet spot I look for achieving 34ma?
    Charles
     

     

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  2. thornev

    thornev Well-Known Member

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    When one talks about sweet spot, I believe that refers to setting the bias so the output wattage is in the 70% to 80% range of the max power rating of the power tubes. But sweet spot really should mean what sounds best to your ears without blowing up your rig. Thorne
     
  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Actually the IBBA board will balance the out of spec tube with it's mate, then you bias the pair. You come off the BIAS cap DIRECTLY for POWER. C-92-R119-R122 are removed. The IBBA board replaces the cap and resistors with it's own distribution network. It's basically the same as on the 400.
     
  4. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Same for IBAM,?
     
  5. thornev

    thornev Well-Known Member

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    IBAM biases each tube individually. IBBA biases pairs of tubes so that each pair is biased to match. Thorne (oh, you were probably asking about the components, not the functions)
     
  6. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Yes, understood.
    I have one tube approx 5ma higher then the other 3, so I figured the IBAM would be more beneficial to my needs. If using a closer matched quad then the IBBA would be the way to go.
    Anyway, the 800c procedure is remove R119, R122 and clip out C92?
    I remember the hook up but I forget what is removed.... Just double checking, I read something about a 0.1 uf cap that was removed but later it was discovered it wasn't necessary??
     
    Last edited: Jun 9, 2018

     

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  7. thornev

    thornev Well-Known Member

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    IBAM & IBBA are needed not just for biasing tubes that are not matched as I'm sure you also understand. They are used also to adjust to find that sweet spot you mentioned earlier.
     
  8. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    well it turns out, i managed to get 34ma for one channel and only 29ma for the other. My bias voltage is right on -17v , so i'm assuming my old stock rcas have seen better days?
    She certainly sounds good tho! Thanks Larry for the IBAM info. This is the first 7591 tube rig ive had. Is there really much difference with these and the 7868's?
    Oh ya, these rcas have been dormant for probably 15 - 20 years. does that make any difference regarding the strength of them? Kinda hoping some voltage will pump them up a bit maybe??
    I plan on running the unit for a few days and then recheck the bias and readjust if necessary.
     
    Last edited: Jun 10, 2018
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    When you bias using the IBAM/IBBA, DISREGARD the PIN6 VOLTAGE!!!!!!! Your goal is to set all the tubes to the same CURRENT in ma on PIN 5 to ground (provided you installed cathode resistors (10 ohm) on pin 5. Note that the settings on each tube are interactive with the others, so you'll do them 1 thru 4 a few times until the othertubes don't change when you do a tube. The PIN 6 voltages will be spread a little with the current set to equal. Try for a nominal 75% of max dissipation.
     
  10. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Maybe I need a different set of resistors , I have (12k) on my board? That was the best I could get per channel?
    I found that on this unit , thst as you mentioned the tubes interact with each other when adjusted.. Setting the one channel to 34 ma would leave me with a tube either biased way high or low ( depending which pair was adjusted) on the opposite channel. Managed to get the happy middle with 29ma (left channel) and 34ma (right).
    Pin 6 was only checked afterwards, but I couldn't get that voltage more negative anyway? If I did, the balance between channels would be all screwy....
     
    Last edited: Jun 11, 2018
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If your IBAM POTS are TWO BLOCKED against the stops with those readings, yeah change the IBAM Resistors. 15K would probably get you back into the center of rotation on the pots.

    What's your voltage on pin3 and pin4 of the 7591's?
     

     

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  12. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    I'll have to report back tonight on that request, I had it all buttoned up last night for the first listening session!
    Voltages were slightly higher then schematic but all was looking good!
    I will open her up and change my ibam resistors anyway, try to dial in that other channel a bit better...
     

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