Building a new amp from Hammond AO-39 amp! Schematic posted.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by V2Racing, Sep 9, 2019.

  1. V2Racing

    V2Racing Member

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    I've been kicking around what I'm going to build out of the 4 organ amps I have. I found a schematic online drawn by Steve Luckey of a AC15 normal channel he built from an AO-39 organ amp.

    I wanted something different and wanted to play with some different ways of tone shaping, so since the AC15 has a treble cut between the PI and power tubes and a volume bright cap, that was almost different enough from the amps I have.

    I decided to use the schematic as written from the PI on through the power section, but do my own preamp with a 6U8A or one of it's close cousins with a triode and a pentode in it. I went triode first into the pentode. I am going to have DPDT switches on the cathode resistors to change gain and voicing along with what some call a squish control off grid 2 on the pentode to ground.

    I may change some of the resisters in the power section for C and D to up the voltage to the 6U8A and the PI to push the EL84's harder.

    The amp will have no NFB and will be cathode biased.

    What think you all?

    Mike

    [​IMG]
     
    Last edited: Sep 10, 2019
    Vaibhav Joshi likes this.
  2. Tone_Terrific

    Tone_Terrific Supporting Member

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    Coupling caps missing? I would not want B+ on the vol pot.
     
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  3. V2Racing

    V2Racing Member

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    Right you are! I forgot to draw the .01uF cap back in the circuit. That's why I put it up so fresh eyes are looking it over.

    All fixed now!

    Mike
     
    Last edited: Sep 10, 2019
  4. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    There is no ground reference for the pentode's grid. Also, you might want to add large value resistors between each of the selectable cathode bypass caps and their respective switch common connection, to allow them to pre-charge and reduce switching pop.
     
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  5. kidmandude

    kidmandude Supporting Member

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    Very nice gets me thinking I have 2 or 3 vintage hammond chassis laying around too.. Hmmmmm :)
     
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  6. Rockinrob86

    Rockinrob86 Member

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    I built an 18 wattish amp out of one of these that is awesome.

    And then I fixed the hammond and kind of wished I still had the reverb! But the AO39 amp is a go to around here, so it could be worse
     
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  7. V2Racing

    V2Racing Member

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    Thanks Jeff! A 1M ground reference should work shouldn't it? The high value resistor on bypass caps is a good idea. I have a reverb pedal that needs a cap on it's switch to charge the cap. It sounds like a gunshot when you switch it on.

    Mike
     
    Last edited: Sep 10, 2019
  8. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Sure. Or maybe even a 470k.
     
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  9. kidmandude

    kidmandude Supporting Member

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    I have some older Reverb units if anyone is looking? Spring and or Oil units.
     
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  10. KramerPacerCarreraGuy

    KramerPacerCarreraGuy Member

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    I totally forgot I have a pristine AO35 waiting to be a Dr Z
     
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  11. V2Racing

    V2Racing Member

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    I have made changes to the schematic that Jeff suggested and went through my parts and made some changes to use what I have and also to up the voltages to the 6U8A. Also added a switch to the bright cap.

    Mike
     
  12. Fatboy666

    Fatboy666 Supporting Member

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    I was gonna comment, "hey, isn't this how Dr Z started?"

    Heard that on Tone Talk last week
     
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  13. V2Racing

    V2Racing Member

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    I have an off topic question? How could this thread have gotten over 22,000 visits since Monday when the stickeys don't have that many hits???

    Mike
     
  14. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    It's me. I keep coming back to see if you have it built yet. :D
     
  15. Travst

    Travst Supporting Member

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    I have a '59 A-100. I had a guy mod the amp with an effects loop so that I could plug pedals like my Leslie pedal into it. It's a fantastic mod and Curt Granger does excellent work.

    https://grangeramp.com/ufxloop.php
     
    Last edited: Sep 12, 2019
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  16. HotBluePlates

    HotBluePlates Member

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    Someone has a bot that keeps reloading the pages that have the text "AO-39."

    I bet they have a warehouse of 300 AO-39 amps bought for $50 each, and they're just waiting for the forum-craze to hit so they can charge $400 for them.
     
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  17. Baxtercat

    Baxtercat Member

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    It got to the top of Trending Topics rt away.
    Weird how many hits.
     
  18. Tone_Terrific

    Tone_Terrific Supporting Member

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    Amp tech is generally an under-utilized forum, imo.
    Extra traffic may raise its profile..amongst robots.
     
  19. HotBluePlates

    HotBluePlates Member

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    1,000+ hits in the past 12 hours.
     
  20. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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